Kaplan Micro Flashcards

Question

25. An elderly man develops a vesicular rash localized to a narrow circumferential band on one side of his chest. The rash is very painful and the vesicles are confluent with some ulceration. No other significant findings are demonstrated on physical examination. Which of the following diagnoses is most likely to be correct? A. Chicken pox B. Herpes simplex I infection C. Herpes simplex II infection D. Measles E. Shingles

Answer 1

The correct answer is E. This is shingles, the recurrent form of herpes zoster infection, which is usually (except in the case of immunosuppressed patients) confined to a single, unilateral dermatome. Isolated vesicles may be seen outside the dermatome. The primary herpes zoster infection (chicken pox) precedes the development of shingles by years or decades; the prevalence of shingles rises steadily with age, to the point that 1% of people older than 80 years of age have the condition. Shingles lesions are infections and should be considered an infectious hazard in the hospital setting. Acyclovir can ameliorate the condition. Varicella, or chicken pox (choice A), is the primary form of herpes zoster infection and affects face and trunk diffusely. It is generally seen in young patients. The macules evolve to papules and vesicles then crust over; all stages are simultaneously present. Herpes simplex I (choice B) affects oral and perioral sites. Herpes simplex II (choice C) primarily affects genital sites, producing a painful set of beefy red lesions. Measles (choice D) causes a blotchy rash, rather than a dermatomal one. Patients present with a maculopapular rash that is brick red. It begins on the head and neck and spreads downward and outward. Koplik spots often appear on the buccal mucosa.

Answer 2

The correct answer is C.Entamoebahistolytica is the usual cause of intestinal amebiasis, and has the microscopic features described in the question stem. A particularly helpful (but not always present) feature of this organism is the presence of ingested red blood cells within the amoebae. 10 These amoebae cause flask-shaped ulceration of the intestinal mucosa and submucosa, with a particular propensity for involving the cecum and ascending colon. The disease manifestations range from none (asymptomatic carriers) to mild chronic diarrhea, to severe, purging dysentery. In symptomatic cases, the liver may develop destructive amoebic liver abscesses that tend to become secondarily (and potentially life-threateningly) infected by bacteria. Patients initially present with recurrent diarrhea and abdominal cramps. As the condition worsens severe colitis may be present. Then hepatic involvement may occur in fulminant cases. Isospora belli (choice A), Cryptosporidium parvum (choice B), and Microsporidia spp. (choice E) are commonly seen in AIDS patients. These organisms can cause severe diarrhea. None of these organisms are amebae. Giardia lamblia(choice D) is a small intestinal protozoa with a distinctive pear-shaped morphology that appears to have a "face."

Answer 3

The correct answer is A. Streptococci are usually initially speciated by their hemolytic capacity on sheep blood agar. Beta-hemolytic streptococci include groups A, B, and D. S. agalactiae is the classic group B streptococcus. The non-beta-hemolytic streptococci consist principally of the pneumococci and the viridans group. These groups are both alpha-hemolytic. Streptococcus is a genus of grampositive facultative cocci occurring in pairs or chains. The genus is separable into the pyrogenic group, viridans group, enterococcus group, and lactic group. Because streptococci are so common, it is essential that the dental student understand how to differentiate all the different species. Note that alpha hemolysis produces a green halo around the colony in RBC culture, whereas beta hemolysis produces a clear halo. Both S. agalactiae and pneumococcus have a carbohydrate capsule (choice B), an important virulence factor and means of subtyping streptococcal species. None of the streptococci use cytochrome enzymes (choice C). They derive all of their energy from the fermentation of sugars to lactic acid. Neither pneumococcus nor S. agalactiae can grow in bile (choice D). This ability is specific for the enterococcus group (group D) of streptococci. Both pneumococci and S. agalactiae are usually treated with penicillin-type antibiotics, although group B streptococci require a penicillinase-resistant type such as oxacillin (choice E).

Answer 4

The correct answer is D.Pseudomonas aeruginosa causes malignant otitis externa, which is a severe necrotizing infection of the external ear canal. Infection tends to spread to the mastoid bone, temporal bone, sigmoid sinus, base of the skull, meninges, and brain. Patients at increased risk include the elderly, those with diabetes, and the immunocompromised. Haemophilus influenzae(choice A) produces a variety of clinical syndromes. H. influenzae is the third most common cause of meningitis in children aged 1 month to 18 years. It is the most common cause of acute epiglottitis, the most common cause of purulent bacterial conjunctivitis, and the second most common cause of otitis media. Patients at risk include those with COPD and cystic fibrosis, alcoholics, splenectomized patients, and young patients. Klebsiella pneumoniae(choice B) is a gram-negative organism that produces a necrotizing pneumonia in diabetic patients and alcoholics. Patients typically present with an abrupt onset of fever, shaking chills, and purulent, foul-smelling sputum. Mucor (choice C) is a fungal infection that is particularly severe in the diabetic or immunocompromised patient. In the acidotic diabetic, the fungus produces a life-threatening, invasive rhinocerebral infection. The infection begins in the nasal passages, extends into the paranasal sinuses, and 11 spreads through the cribiform plate to the frontal lobes of the brain. Patients typically complain of headache, facial pain, and orbital swelling. Streptococcus pyogenes(choice E) causes bacterial pharyngitis, otitis media, and sinusitis. It is also associated with toxin-related diseases and skin infections.

Answer 5

The correct answer is E. Although hepatitis E is not generally seen in this country, it is essential that the student have an understanding of all forms of hepatitis. Hepatitis E is an important, and until recently, unrecognized cause of epidemics of enterically transmitted (fecal-oral) acute hepatitis. Hepatitis E is caused by a virus that occurs primarily in India, Asia, Africa, and Central America. Infection is associated with a 10-20% mortality among pregnant women. This type of hepatitis needs to be considered in patients traveling to and from endemic areas. CMV (choice A) can cause acute hepatitis, but the disease is usually mild and often goes unrecognized, except in profoundly immunosuppressed patients. Hepatitis A virus (choice B) is the major cause of epidemics of enterically transmitted viral hepatitis, especially in the United States, but is not a significant cause of mortality in pregnant women. Hepatitis C virus (choice C) is usually transmitted parenterally rather than enterically and is not a significant cause of mortality in pregnant women. Herpes simplex (choice D) usually causes significant hepatitis only in profoundly immunosuppressed patients.

Answer 6

The correct answer is A. Bacillus cereus produces a self-limited diarrhea caused by ingestion of the preformed enterotoxin in contaminated fried rice (especially when reheated) and seafood. The incubation period is typically approximately 4 hours. The degree of vomiting is usually greater than that of diarrhea. B. cereus is also associated with keratitis, producing a corneal ring abscess. Clostridium botulinum (choice B) produces a neurotoxin that blocks the release of acetylcholine, resulting in a symmetric descending paralysis that may lead to respiratory complications causing death. Symptoms include blurred vision, photophobia, dysphagia, nausea, vomiting, and dysphonia. Most cases are associated with the ingestion of contaminated home-canned food. Clostridium perfringens (choice C) produces a severe diarrhea with abdominal pain and cramping (sometimes called "church picnic" diarrhea). The incubation period is 8-24 hours after ingesting contaminated meat, meat products, or poultry. The meats have usually been cooked, allowed to cool, and then warmed, which causes germination of the clostridial spores. EHEC, enterohemorrhagic escherichia coli (choice D), produces a bloody, noninvasive diarrhea caused by the ingestion of verotoxin found in undercooked hamburger at fast food restaurants. Some patients develop a life-threatening complication called hemolytic-uremic syndrome. Staphylococcus aureus (choice E) produces a self-limited food poisoning syndrome with nausea, vomiting, and abdominal pain followed by diarrhea beginning 1-6 hours after ingestion of the enterotoxin. The organism is found in foods such as potato salad, custard, milk shakes, and mayonnaise.

Answer 7

The correct answer is B.Chlamydia, unlike the other choices, is a type of sexually transmitted disease, as is gonorrhea. Gonococcal infections typically produce profuse urethal discharge, especially in men, yielding a positive smear. Fever, rash and arthritis can occur with disseminated disease. Chlamydia, Mycoplasma, and Ureaplasma are not effectively treated by penicillins and cephalosporins, and are important causes of post-gonococcal urethritis. Chlamydial urethritis can be diagnosed by using fluorescent antibodies to identify inclusions in epithelial cells. Actinomyces(choice A) is a mouth commensal that rarely causes a deeper oral infection. Mycobacteria(choice C) cause chronic granulomatous diseases such as tuberculosis and leprosy. Nocardia(choice D) can cause necrotizing pneumonia and disseminated disease. Rickettsia(choice E) cause typhus and Rocky Mountain spotted fever.

Answer 8

The correct answer is E. Enteritis is an inflammation of the intestine, especially the small intestine. Yersinia enterocolitica is an important cause of mini-epidemics of pediatric diarrhea. Adults can also be affected, but less commonly than children. Some diarrheal cases are severe (and occasionally fatal) and may be complicated by severe dysentery, appendicitis, or chronic relapsing ileocolitis that may require antibiotics to shorten the course. Yersinia is a non-motile ovoid or rod shaped, nonencapsulated gram-negative bacteria. Helicobacter jejuni (choice A) is an important cause of hospital-acquired diarrhea, especially in immunocompromised individuals. Salmonella typhi (choice B) causes typhoid fever. It is a gram-negative bacteria. Shigella species (choice C) causes epidemics of dysentery in military camps and other close quarter areas. It is a gram-negative bacteria. Vibrio cholerae (choice D) causes cholera. It is a gram-negative bacteria. Gastroenteritis can occur in the United States upon eating improperly cooked seafood from other countries.

Answer 9

The correct answer is E. Spore formation usually occurs during the stationary phase, when cell growth ceases because of a developing lack of nutrients or accumulation of toxins. During the phase of decline (choice A), the lack of nutrients and the accumulation of toxin become so severe that any viable organisms usually die before they can form spores. The exponential phase (choices B) and log phase (choice D) are descriptors for active growth occurring after the lag phase and before the stationary phase. Many antibiotics are most effective in this period. The lag phase (choice C) is the initial period of adaptation, prior to growth, which occurs when organisms are introduced to a new environment.

Answer 10

The correct answer is D. The disease in question is leprosy, or Hansen disease. The key feature in the description is the fact the organism is acid-fast. These acid-fast bacilli are commonly found in skin lesions or nasal scrapings. Both of the mycobacteria, M. avium-intracellulare and M. leprae are strongly acid-fast, that is, they retain the carbol fuchsin dye in the face of acid-alcohol decolorization. M. leprae has a predilection for the skin and cutaneous nerves, thereby producing the symptoms of depigmentation and anesthetic cutaneous lesions. This loss of peripheral nerve function leads to many of the disfiguring features of the disease; because patients do not have normal pain sensation, they sustain repeated injuries. In addition, the organism attacks cartilage and causes granuloma formation in the skin, leading to some of the facial disfigurement. Patients often have a history of residence in an endemic area in childhood. Bartonella henselae(choice A) is a very small, gram-negative bacterium that is closely related to the rickettsia, although it is able to be cultured on lifeless media. It is the cause of cat-scratch disease, a local, chronic lymphadenitis most commonly seen in children. Listeria monocytogenes(choice B) is a ubiquitous microbe that causes disease in more than 100 animal species. Although it is best known as an agent of meningitis in the newborn, it is a cause of multiple other diseases. A characteristic feature of these infections is the development of granulomas at the site of the infection. The organism is not acid-fast. M. avium-intracellulare(choice C) causes tuberculosis-like pulmonary disease in the immunosuppressed, primarily in HIV infected individuals. Nocardia asteroides(choice E) primarily produces pulmonary infections in humans. The organism is considered to be "weakly" acid-fast.

Answer 11

The correct answer is A. The most common early form of oral lesions in HIV infected individuals includes thrush, hairy leukoplakia, and aphthous ulcers. Candida albicans produces oral thrush, an AIDS-defining lesion, which is common in acute HIV disease, and becomes increasingly common as the CD4 + cell count falls, especially as the CD4 decreases to less than 300. As the CD4 count decreases to less than 100, the esophagus, trachea, bronchi, or lungs may be infected. The lesions are usually painless. Diagnosis is by demonstration of pseudohyphae using a wet smear with confirmation by culture. Although cytomegalovirus (choice B) is associated with numerous clinical scenarios in the AIDS population, including odynophagia (painful swallowing), it would not produce white plaques on the oral mucosa. Herpes simplex I (choice C) produces vesicular lesions occurring in clusters in the oral cavity. There is an increased risk for herpes infections in the AIDS group, but the lesions do not resemble those described in the question. These lesions are commonly red and extremely painful. Human herpesvirus 8 (choice D) is the causative agent of Kaposi sarcoma, a malignancy arising from endothelial cells that appears as hemorrhagic nodules in different organ systems. It is the most common cancer in the HIV infected population. Human papilloma virus (choice E) is associated with a variety of lesions, including warts and intraepithelial neoplasias of the vulva and cervix. It is associated with anal condyloma, which can occur in the AIDS population.

Answer 12

The correct answer is C. Herpes simplex often causes recurrent grouped vesicles on an erythematous base. The Tzanck smear is positive for multi-nucleated epithelial giant cells. Herpes simplex is an enveloped, linear DNA virus that is a very common infectious agent; most adults will have anti-Herpes simplex antibodies in their serum, although many may never have had any clinical signs of disease. The hallmark of this disease is these painful skin vesicles, often called "cold" sores, or "fever" blisters to denote the precipitating event that preceded the appearance of the lesions. The virus has a propensity to become latent in the host's nervous tissue. Activation of the infection occurs following mild trauma (e.g., a visit to the dentist), hormonal changes (e.g., menses), immunosuppression (e.g., following organ transplantation), or may follow minor infections, stress or sun exposure. Other, more serious, manifestations of disease include encephalitis, pneumonia, and hepatitis; these are particularly likely to be seen in immunodeficient patients such as those with AIDS. Adenoviruses (choice A) are naked, linear, double-stranded DNA viruses that cause acute, usually self-limiting, influenza-like illnesses occurring in the fall and winter. The symptoms include pharyngitis, fever, cough, and general malaise. Epidemic pharyngoconjunctivitis and pneumonia can occur in closed populations such as military installations. Coxsackie viruses (choice B) are naked, single-stranded, polycistronic viruses with an RNA genome. They are divided into groups A and B based on their virulence. Coxsackie A causes herpangina and hand-foot-and-mouth disease, while Coxsackie B is seen in patients with pleurodynia, myocarditis, and pericarditis. Both groups cause upper respiratory infections, febrile rashes, and meningitis. Herpes zoster (choice D), the varicella virus, is an enveloped, double-stranded DNA virus that is a very common infectious agent in children. Chickenpox is a mild, self-limiting illness in children that is evidenced as a fever followed by a macular rash that progresses to papules, then vesicles of the skin and mucous membranes. Shingles is a recurrence of a latent varicella infection in which the virus has taken refuge in sensory ganglia of spinal or cranial nerves. Various factors that decrease the immune status of the patient contribute to the exacerbation of the infection. Severe dermatomal pain occurs with a vesicular eruption, fever, and malaise. Papilloma viruses (choice E) are members of the Papovavirus family. They are non-enveloped and possess a double-stranded, circular DNA genome. They cause skin, plantar, and genital warts.

Answer 13

The correct answer is E. The disease is postinfectious glomerulonephritis, which is usually related to prior sore throat or skin infection by Streptococcal species; Staphylococcus also causes some cases. The prominent deposits are also known as subepithelial humps, and consist of immunoglobulin and complement. Postinfectious glomerulonephritis commonly occurs after impetigo. Onset occurs within 1-3 weeks after infection (average 7-10 days). Other causes of postinfectious glomerulonephritis include bacteremic states, such as systemic staphylococcus infections. 15 Gram-negative rods such as Escherichia(choice A), Klebsiella (choice B), and Pseudomonas (choice D) have not been implicated as significant causes of postinfectious glomerulonephritis. The gram-negative cocci Neisseria(choice C) have not been implicated in postinfectious glomerulonephritis.

Answer 14

The correct answer is A. IL-1 is produced by macrophages and other antigen-presenting cells. It has several actions, including stimulation of T cells to secrete IL-2, chemotactic activity for neutrophils and monocytes, increased expression of intercellular adhesion molecules (ICAMs) on vascular endothelial cells, and activation of macrophages and natural killer (NK) cells. It is also pyrogenic (fever inducing) because of a direct effect on the hypothalamus. IL-2 (choice B) is produced by activated T helper cells. It stimulates the proliferation of other T cells as well as activated B cells. It also activates NK cells and stimulates lymphokine secretion. IL-3 (choice C) stimulates all stem cells to produce hematopoietic cells; it is also known as multilineage colony stimulating factor (CSF). IL-4 (choice D) is produced by T helper 2 (Th 2) cells and mast cells. It has several functions, including inducing cells to express class II major histocompatibility complex (MHC) antigens and stimulation of B-cell proliferation. It is involved in the induction of atopic allergies by its mitogenic activity for mast cells and its enhancement of immunoglobulin class switching to IgG and IeE. IL-5 (choice E) is secreted by activated T helper cells. It promotes B-cell proliferation, production of eosinophils, and stimulates B-cell class switching to IgA.

Answer 15

The correct answer is D. Although this is a clinically-based question, it is essentially asking about the microbiology of each bacteria. Pseudomonas aeruginosa is a very common opportunist in burn patients and patients on ventilators, in whom it classically causes secondary wound infections and septicemia. It may also cause cystitis in patients with urinary catheters and pneumonia in patients with cystic fibrosis. The organism is found in water and usually gains access to the body as a contaminant in the water used in respirators or in water baths, etc., used to cleanse wounds. This organism is a nonfermenter, that is, it does not metabolize sugars by classic pathways. It produces a blue-green, water-soluble pigment (pyocyanin), and has a fruity odor when grown on laboratory media. It has a propensity for developing antibiotic resistance; current therapy uses the synergistic combination of an aminoglycoside, such as amikacin, with a cell wall synthesis inhibitor (carbenicillin, ticarcillin, or piperacillin). Candida albicans (choice A) is a normal flora yeast that appears as large, weakly gram-positive, spherical to ovoid organisms with budding daughter cells in Gram-stained preparations. Candidiasis is an opportunistic infection in individuals with a compromised immune system. The fungus usually causes mucocutaneous lesions, but in severely compromised individuals like patients with AIDS, systemic disease may occur. Oral candidiasis appears as creamy white patches of exudate that can be scraped off an inflamed tongue or buccal mucosa to reveal a painful lesion that may bleed. 16 Clostridium perfringens(choice B) is a gram-positive, spore-forming, anaerobic rod. It is a common cause of gas gangrene when introduced into a wound. The organism produces a variety of toxins and enzymes that enable it to destroy muscle tissue and spread through the soft tissues of the body. Escherichia coli(choice C) is a lactose-fermenting, gram-negative rod seen as normal flora of the intestine. It is the most common cause of urinary bladder infections, pyelonephritis, and sepsis in patients with indwelling urinary catheters. It is also the major cause of traveler's diarrhea and is a very important pathogen in neonates, who become infected during passage through the birth canal.

Answer 16

The correct answer is D. This patient has osteomyelitis due to a cat bite that penetrated the periosteum. Whenever you see dog or cat bites in a question stem, consider Pasteurella multocida as a primary cause of wound infection. This organism is a short, encapsulated, gram-negative rod demonstrating bipolar staining. Rapidly arising cellulitis is particularly indicative of this organism. Brucella melitensis (choice A) is a small, acapsular, gram-negative rod that causes brucellosis (undulant fever) and is associated with contact with goats or sheep. Brucella abortus and Brucella suis are variants associated with contact with cows and pigs, respectively. The organisms enter the body through the skin or through contaminated dairy products, such as unpasteurized imported goat's milk or cheeses. Eikenella corrodens(choice B) is commonly found in human bites. It is a gram-negative rod that is part of the normal flora of the human mouth. Francisella tularensis(choice C) is a small, pleomorphic, gram-negative rod that causes tularemia (rabbit fever). It occurs most commonly in rural areas. In the U.S., rabbits are the main reservior for this organism, which is transmitted to humans by the Dermacentor tick or by contact with infectious animal tissues. Yersinia pestis(choice E) is responsible for bubonic plague, which has been known to occur in the western U.S. Its main reservoir is the prairie dog, and its vector is the rat flea.

Answer 17

The correct answer is choice A. The patient has Candida esophagitis. Any time a patient presents with dysphagia or odontophagia, along with whilte plaques in the oropharynx (thrush), you can assume that the Candida is affecting the esophagus as well. The fact that the patient is an IV drug user make an opportunistic infection such as Candida more likely. Candida appears as budding yeast with pseudohyphae in vivo. The other answer choices represent the morphology of other important opportunistic fungi: Cryptococcus are encapsulated yeast (choice B). You should think about Cryptococcus neoformans when you're presented with an immunocompromised patient with neurologic symptoms. The classic clue is the presence of encapsulated organisms observable in an India ink preparation. Mucor and Rhizopus are molds with nonseptate hyphae (choice C). You should think about Mucor when you are presented with a diabetic (especially ketoacidotic) or a leukemic patient with a severe sinus infection. Aspergillus is a mold with septate hyphae (choice D). In immunocompromised patients, aspergillosis can present with acute pneumonia, often with a cavitation (aspergillomas = fungus balls in the lungs).

Answer 18

The correct answer is A. This patient has the symptoms of a urinary tract infection (UTI). Escherichia coli is the leading cause of community-acquired UTIs. The proximity of the urinary tract to the anus facilitates colonization of the tract by fecal flora. Other gram-negative rods causing UTIs include Enterobacter cloacae, Klebsiella pneumoniae, Serratia marcescens, Proteus mirabilis, and Pseudomonas aeruginosa. None of the other choices listed cause UTIs.

Answer 19

The correct answer is D. All of the organisms listed are protozoa. There are two intestinal protozoa specifically associated with AIDS that can cause transient diarrhea in immunocompetent individuals but can cause debilitating and potentially life-threatening chronic diarrhea in AIDS patients. These organisms are Isospora belli, treated with trimethoprim-sulfamethoxazole (or other folate antagonists) and Cryptosporidium parvum (no treatment presently available). Chlamydia psittaci (choice A) is associated with the development of fever, chills, cough, and atypical pneumonia with slightly delayed appearance of signs of pneumonitis. Entamoeba histolytica (choice B) and Giardia lamblia(choice C) are both causes of diarrhea, but they are not specifically associated with AIDS. Giardiasis, for example, can cause mild to severe bulky, greasy, frothy, malodorous stools, free of blood and pus. Microsporidia (choice E) are a protozoan cause of diarrhea, but produce spores rather than oocysts.

Answer 20

The correct answer is B. Hepatitis B is a hepadnavirus, which is enveloped DNA. Hepatitis A (choice A) is a picornavirus and is a naked-capsid RNA. Hepatitis C (choice C) is a flavivirus, which is RNA enveloped. Hepatitis E (choice D) is a Calicivirus, which is a naked-capsid RNA.

Answer 21

The correct answer is E. Rotavirus is the major cause of diarrhea in infants and children under the age of 2. It is a major cause of diarrheal morbidity worldwide due to dehydration. The virus replicates in the intestinal mucosa, producing a profuse, watery, non-bloody diarrhea, often coupled with nausea and vomiting. Transmission is by the fecal-oral route. Treatment is symptomatic with fluid and electrolyte replacement. Coronaviruses (choice A) usually cause cold-like illnesses. Lymphocytic choriomeningitis virus (choice B) can cause headache, malaise, myalgia, conjunctivitis, and, occasionally, meningitis. Norwalk agent (choice C) can also cause diarrhea, but usually affects patients older than 2 years. This causes profuse vomiting and diarrhea and is often transmitted by food, especially shellfish. Orbivirus (choice D) is the cause of Colorado tick fever, which is the only tick-borne viral disease in the United States.

Answer 22

The correct answer is C. For NBDE Part 1, it is important to know general properties of antibiotic families. Ceftriaxone is a third generation cephalosporin commonly used in the treatment of a variety of life-threatening infections caused by gram-negative organisms. The agent has moderate grampositive activity. A general rule to remember about the cephalosporins is that when progressing from first generation to third generation, the gram-negative activity is increased and gram-positive activity is decreased. In other words, first generation cephalosporins have the most activity against grampositive organisms and third generation cephalosporins have the most activity against gram-negative organisms. Metronidazole (choice D) is an antibiotic with excellent anaerobic bacterial coverage. Vancomycin (choice E) is an antibiotic with excellent gram-positive coverage; it is generally used in the treatment of life-threatening infections.

Answer 23

The correct answer is E.Chlamydiae are obligate intracellular parasites. Studies in the United States demonstrate that 5-25% of pregnant women have C. trachomatis infections of the cervix. In these women, approximately one half to two thirds of infants are exposed and if untreated can develop inclusion conjunctivitis. The lesions begin with formation of lymphoid follicles in the conjunctiva. With disease progression, there is tissue necrosis, granulation tissue deposition, and scar formation, leading to lacrimal duct obstruction and distortion of the eyelids. With the loss of an adequate tear system, the cornea becomes vulnerable to dehydration and opacification. Also, the vigorous inflammatory response can directly involve the cornea, with resulting opacity.

Answer 24

The correct answer is E. Most of the features listed are those of gram-negative bacteria, which have a complex cell envelope consisting of a cytoplasmic (inner) membrane, a periplasmic space containing peptidoglycan (choice D) and lipoprotein (choice B), an outer membrane (choice C), and sometimes a capsule. The outer membrane contains lipopolysaccharide (choice A) which is a major component of endotoxin. The peptidoglycan cell wall of the gram-negative bacteria is thin, while that of the gram-positive bacteria is thick. Other features of gram- positive bacteria include a fairly simple surface of cytoplasmic membrane, peptidoglycan, cell wall, and sometimes, an outer capsule. The cell wall contains lipoteichoic acids. Gram-positive bacteria retain the stain or resist decolorizationby alcohol in Gram's method of staining. This is primarily characteristic of bacteria whose cell wall is composed of peptidoglycan or teichoic acid.

Answer 25

The correct answer is choice A. Mucormycosis, to which diabetics are particularly vulnerable (perhaps because of the high glucose content of the diabetic nasal secretions), typically involves the nasal sinuses. It is especially dangerous because the organism can erode into the bones of the cranium, causing life-threatening meningitis and/or encephalitis that is very difficult to treat. Mucor can cause pneumonia (choice B), but this is not the most immediate threat. Mucor can cause septicemia (choice C), but this usually occurs in more seriously ill patients and is not an immediate risk in this patient. Mucor can cause a skin infection (choice D), but this generally occurs in conjuncition with severe burns. Mucor does not usually cause urinary tract infections (choice E).

Answer 26

The correct answer is choice D. Note firstly that ethyl alcohol and glutaraldehyde are chemicals that can only high level disinfect, and cannot be used for sterilization. In particular, they are not sporicidal. Choices A, B and D are the three most common methods of instrument sterilization. However, the steam autoclave operates at 121 C, and the dry heat sterilizer at 160 C, both too hot for the material which melts at 115 C. So we can only use a non-heat sterilization method. The only non-heat method is ethylene oxide, at 8-12 hours of contact time.

Answer 27

The correct answer is A. Borrelia burgdorferi can be maintained in nature indefinitely by a tick vector. This organism is the tick-transmitted spirochete that causes Lyme disease. The tick, Ixodes dammini, can infect the white-footed mouse and large mammals such as deer during its life cycle, making them reservoirs. The tick itself is a reservoir, however, because it acquires the disease through transovarial passage of the organism. Together, these factors make Lyme disease an endemic infection with little hope for eradication. Lyme disease is characterized by erythema migrus, a flat or slightly raised red lesion that expands with central clearing. Headache or stiff neck are common. Arthritis is common, chronic, and recurrent. Borrelia burgdorferi is not resistant to antibiotics and disinfectants (compare with choice B). The spirochete can be successfully treated with penicillins, tetracycline, and ceftriaxone. Borrelia burgdorferi is a delicate spirochete and is not resistant to environmental stresses (compare with choice C). Brill-Zinsser disease (choice D) is the reactivation of epidemic typhus infection caused by Rickettsia prowazekii. It can occur many years after an infection that was not treated with antibiotics. Humans are incidental hosts, rather than the primary reservoir, for Borrelia burgdorferi(choice E). The primary reservoirs are ticks, mice, and large mammals.

Answer 28

The correct answer is E. Cellulitis is an acute infection involving the epidermis and dermis. Cellulitis can lead to the development of an abscess, as seen in this patient. The most common types of skin infections (cellulitis or abscess) usually involve gram-positive organisms such as staphylococcus and streptococci. The chef had a staphylococcal abscess on his finger. S. aureus, produces enterotoxin A, which was likely transferred to the pastries. When ingested, the toxin causes severe nausea and vomiting within a few hours (the average incubation time is 3-6 hours). There is little diarrhea associated with this type of food poisoning outbreak. Bacillus cereus(choice A) is a gram-positive spore-forming rod that is associated with food poisoning outbreaks following the ingestion of fried rice. The time of onset and symptoms would mimic staphylococcal disease; the major differentiating feature is the food involved. The organism survives the boiling of the rice because it is a spore-former. It germinates as the rice cools, grows, and elaborates an enterotoxin that is responsible for the nausea and vomiting characteristic of the disease. Clostridium perfringens(choice B) is a gram-positive spore-forming anaerobe that can cause a longer incubation (18-24 hour) food poisoning, typically with marked diarrhea. Once again, the spores allow the organism to survive the heating process used in the preparation of the food. Both Clostridium perfringens and Clostridium botulinum are associated with home-canned vegetable and sausages. 21 The symptoms of Escherichia coli(choice C) food poisoning are usually watery diarrhea (traveler's diarrhea) with minimal nausea and vomiting, or a bloody diarrhea caused by enteroinvasive strains of the agent. Also, E. coli would be an unlikely cause of the primary infection in the chef. Shigella sonnei (choice D) causes enterocolitis characterized by fever, cramps, and diarrhea after an incubation period of one to four days. Transmission is fecal-oral, associated with poor hygiene. A wide range of foods has been implicated.

Answer 29

The correct answer is E. Any previously healthy person in the southwestern United States who develops septic shock or severe pulmonary disease should be evaluated for plague. Plague is not an extinct disease, but is still encountered in sporadic cases in various places, including Asia, Africa, parts of Europe, and the American Southwest. The causative organism is Yersinia pestis, which is endemic in many wild animal populations, and can be transmitted to humans either by direct contact or by arthropod bite. Human plague may take many forms, including pestis minor (mild lymphadenopathy); bubonic plague (prominent lymphadenopathy); pneumonic plague (as described above); and septicemic plague. Antibiotics are most effective if given within the first 24 hours, which can be problematic if medical staff do not suspect the disease. Because plague is rare in the United States, a high degree of clinical suspicion is required to make a rapid diagnosis and to institute timely treatment. If the diagnosis is missed, the mortality rate is high. Poliomyelitis virus (choice A) is an enterovirus that causes muscle weakness, headache, stiff neck, sore throat, fever, nausea, and vomiting. Lower motor neuron lesions (flaccid paralysis) can be seen as well as deep tendon reflexes and muscle wasting. Clostridium perfringens(choice B) causes gas gangrene and gastroenteritis. This is the most common cause of gangrene. Paramyxovirus (choice C) is the causative organism of the mumps. In this condition, exposure to paramyxovirus occurs 14-21 days before onset of symptoms. Patients present with painful swollen salivary glands, usually the parotids. Frequent involvement of the other tissues is common, such as the testes, pancreas, and meninges. Listeria monocytogenes(choice D) causes listeriosis. Infection during pregnancy may result in sepsis, abortion, or premature delivery. Infection in the neonate may produce meningitis. In immunocompromised adults, either meningitis or sepsis may occur.

Answer 30

``` The correct answer is C. The cardiac lesions described are characteristic of acute bacterial endocarditis. Intravenous drug users are among those susceptible to acute endocarditis. The fact the valvular vegetations are large and penetrating strongly suggests that they are caused by a virulent, pyogenic pathogen. Staphylococcus aureus is the most common cause of acute bacterial endocarditis. Brain abscess, which produces ring-enhancing radiologic lesions, is a known complication of bacterial endocarditis, occurring when the vegetations fragment and release septic emboli. The essential diagnostic characteristic for infective endocarditis includes a preexisting organic heart lesion, fever, new or changing heart murmur, evidence of systemic emboli, positive blood culture, and evidence of vegetation on endocardiography. 22 Herpesvirus (choice A) can cause encephalitis, but is not a cause of vegetative endocarditis. Mycobacterium tuberculosis(choice B) can cause brain masses, but does not usually cause endocarditis. Streptococcus pneumoniae(choice D) can cause acute pyogenic meningitis, but does not usually cause endocarditis. Treponema pallidum(choice E), the causative agent of syphilis, can cause aortic aneurysms and a variety of neurologic problems, but does not usually cause endocarditis or brain abscess. ```

Answer 31

The correct answer is D. HIV virus is notoriously easy to kill on most environmental surfaces. Many disinfectants and other common physical and chemical exposures will kill the virus fairly easily. Therefore, ability to kill HIV is NOT an important criterion in choosing disinfectants. Mycobacterium tuberculosis is chosen as the "benchmark organism" (the standard against which the disinfectant is compared). It is very resistant to surface disinfectants, partially because of its waxy cell wall. Another highly resistant pathogen on environmental surfaces is Hepatitis A virus.

Answer 32

The correct answer is choice B. This patient has lymphogranuloma venereum caused by Chlamydia trachomatis (type L1, 2, or 3). Chlamydia exhibit distinct infectious and reproductive forms. The extracellular infectious form is known as the elementarly body (EB), which cannot reproduce. It attaches to the host cell and enters through endocytosis. Once inside the cell, the EB is transformed into the reticulate body (RB) within the endosome. The RB is capable of binary fission and divides within the endosome; fusion with other endosomes occurs to form a single large inclusion. Eventually, the RBs undergo DNA condensation and disulfide bond bridgings of the major outer membrane protein, forming EBs. The EBs are then released. Note that C. trachomatis is responsible for several sexually or perinatally transmitted diseases, including ocular trachoma (types A, B and C), neonatal conjunctivitis, nongonococcal urethritis, cervicitis, and pelvic inflammatory disease (types D-K). Calymmatobacterium granulomatis (choice A) is a gram-negative rod that causes superficially ulcerated genital or inguinal papules that coalesce to form substantial lesions. The appearance of Donovan bodies in histiocytes is diagnostic of this infection. Haemophilus ducreyi (choice C) is a gram-negative rod that causes a soft, painful penile chancre, unlike that of a chlamydial or syphilitic lesion. This infection is common in the tropics. Neisseria gonorrhoeae (choice D) is a gram-negative diplococcus responsible for gonorrhea. Patients typically present with purulent penile discharge, not genital lesion. Treponema pallidum (choice E) is the spirochete responsible for syphilis. It may cause a firm, painless ulcer as a manifestation of primary syphilis, but the organism does not exist in distinct extracellular and intracellular forms as does Chlamydia. Secondary syphilis is associated with the appearance of condyloma lata -- flat, gray wart-like lesions.

Answer 33

The correct answer is A. This is another question in which knowing the microbiological characteristics is essential. Campylobacter jejuni is a pathogen causing an invasive enteric infection associated with ingestion of raw or undercooked food products, or through direct contact with infected animals. In the United States, ingestion of contaminated poultry that has not been sufficiently cooked is the most common means of acquiring the infection. The patients typically have bloody diarrhea, abdominal pain, and fever. The presence of fecal leukocytes indicates an invasive infection. Campylobacter are microaerophilic, motile, gram-negative "comma-shaped" rods.E. coliare not comma-shaped. Enterotoxigenic E. coli(choice B) causes the classic traveler's diarrhea. The infection is noninvasive and is acquired by way of the fecal-oral route through consumption of unbottled water or uncooked vegetables. The major manifestation is a copious outpouring of fluid from the gastrointestinal tract presenting as explosive diarrhea. This is caused by the action of one of two types of enterotoxins on the gastrointestinal tract mucosa. Shigella sonnei(choice C) produces a syndrome very similar to C. jejuni. The organism, however, appears as a gram-negative rod on Gram's stain. It does not have a comma shape. Transmission is from person to person by way of the fecal-oral route. Infection requires a low infective dose because the organism is fairly resistant to gastric acidity. Staphylococcus aureus(choice D) produces food poisoning through the ingestion of a preformed enterotoxin. The organism is present in food that is high in salt content, such as potato salad, custard, milk shakes, and mayonnaise. The patient presents with nausea, vomiting, and abdominal pain, followed by diarrhea beginning 1-6 hours after ingestion of the enterotoxin.S. aureus are seen as clusters of coli and are not comma-shaped. Vibrio cholerae(choice E) produces a secretory diarrhea caused by increases in cAMP in the intestinal cells. The organism is comma-shaped, but is not invasive. The patient presents with the sudden onset of painless, watery diarrhea that becomes voluminous, followed by vomiting. The stool appears nonbilious, gray, and slightly cloudy with flecks of mucus, no blood, and a sweet odor.

Answer 34

The correct answer is C. Salmonella spp., including S. enteriditis and S. typhimurium, produce a gastroenteritis or enterocolitis. Patients with decreased gastric acidity, sickle cell disease, or defects in immunity, and children younger than 4 years of age have a more severe course of disease. Salmonella spp. are carried in nature by animal reservoirs such as poultry, turtles, cattle, pigs, and sheep. The incubation period is 8-48 hours after ingestion of contaminated food or water. Chlamydia psittaci (choice A) produces an interstitial pneumonitis accompanied by headache, backache, and a dry, hacking cough. A pale, macular rash is also found on the trunk (Horder spots). Patients at risk include pet shop workers, pigeon handlers, and poultry workers. Entamoeba histolytica (choice B) produces a diarrhea (frequently bloody or heme-positive), right lower quadrant crampy abdominal pain, and fever. Patients frequently have weight loss and anorexia. There is usually a history of travel outside the United States. Most cases are chronic. Complications include liver abscesses. Staphylococcus aureus (choice D) produces a self-limited gastroenteritis caused by the production of preformed, heat-stable enterotoxins. The incubation period is 16 hours. The toxins enhance intestinal peristalsis and induce vomiting by a direct effect on the CNS. Yersinia enterocolitica (choice E) usually produces a chronic enteritis in children. These patients have diarrhea, failure to thrive, hypoalbuminemia, and hypokalemia. Other findings include acute right lower quadrant abdominal pain, tenderness, nausea, and vomiting. The infection mimics appendicitis or Crohn disease.

Answer 35

The correct answer is E.The presence of hepatitis B surface antigen (HBsAg) along with hepatitis B core IgM antibody (HBc IgM Ab), and the absence of hepatitis B surface antibody (HBsAb) indicates the presence of the early stages of an acute infection with Hepatitis B. The presence of antibody to Hepatitis C (HCV) only indicates exposure, but not a specific time of exposure; however, 85% of patients who are infected with HCV develop chronic infections, indicating that this patient has an 85% chance of having a dual infection with HBV and HCV. A dual infection of HBV and HAV (choice A) is not plausible since the IgM anti-HAV serology is negative. Hepatitis A does not cause chronic disease (choice B). Chronic HBV infection (choice C) is unlikely because the patient has HBc IgM Ab, which is characteristic of an acute infection, rather than a chronic infection. Hepatitis C infection (choice D) is not confirmed by these data because the presence of HCV Ab only indicates exposure to the virus, and not the state of infection. This could be caused by exposure at some earlier time (the elevated serum transaminases might be due to HBV infection). An active or chronic HCV infection can only be confirmed by PCR.

Answer 36

The correct answer is D. When you see Chlamydia psittaci, one word should come to mind: birds! Infection with this organism is an occupational hazard for anyone who works with birds (e.g., veterinarians, pet store employees), including poultry farmers. Individuals who work with cats (choice A) would be at an increased risk for infection with Pasteurella multocida (which is acquired primarily through cat bites), Bartonella henselae (cat-scratch fever), and the protozoa Toxoplasma gondii (which can be acquired from ingestion of food contaminated with cat feces). Florists (choice B) are at increased risk for infection with the fungus Sporthrix schenckii, a primary pathogenic fungus acquired by inoculation (e.g., a rose thorn puncturing the skin). A person who works in a homeless shelter (choice C) would be at increased risk for infection with Mycobacterium tuberculosis, because this organism is spread through the air and is prevalent in the homeless community. Slaughterhouse workers (choice E) are at an increased risk for infection with Brucella, a bacterium that is acquired by handling infected animals.

Answer 37

The correct answer is D. This is an example of a two-step question. Step 1: what is the organism that causes pneumonia in cystic fibrosis? Then, this organism is commonly seen in cases of...? Anytime you see pneumonia in a cystic fibrosis patient you should suspect Pseudomonas aeruginosa. The Gram stain revealing aerobic, gram-negative rods confirms your suspicion in this case. Now the question is: which of the diseases listed is also caused by Pseudomonas? The answer is otitis externa. P. aeruginosa is often found in the external ear, especially if the conditions are moist ("swimmer's ear") and there is any sort of inflammation. External otitis is usually a benign process with the only symptoms being an itchy, painful ear. If, however, the organism penetrates the epithelium and invades the soft tissue, cartilage, and cortical bone, the process becomes malignant otitis 25 externa, which can progress to osteomyelitis leading to cranial nerve palsies. This condition is most common in diabetics. Croup (choice A), also called laryngotracheobronchitis, is a respiratory disease of children that presents with a characteristic "barking" cough. Croup is caused by parainfluenza virus. Epiglottitis (choice B) is a potentially fatal infection in children, caused by H. influenzae, which presents with drooling, difficulty breathing, and stridor. The incidence of this disease has dropped dramatically with the introduction of the H. influenzae type b (Hib) vaccine. Meningitis (choice C) is caused by numerous different bacteria, depending on the age of the patient. The most common causes include S. pneumoniae (elderly patients), H. influenzae (unvaccinated children), Group B strep and E. coli (neonates) and N. meningitidis (1 month to adult). Although P. aeruginosa can cause meningitis, it is not a common cause. The most common causes of otitis media (choice E) include S. pneumoniae and H. influenzae. Even in cases of external ear infections with P. aeruginosa, the middle ear is typically spared.

Answer 38

The correct answer is C. Besides the steam autoclave, dry heat autoclave, and ethylene oxide sterilizer, there is a heat/chemical combination known as a Chemiclave or heat/chemical sterilizer. Its conditions are formaldehyde and alcohol at a temperature of 132 C for periods of 20-30 minutes at 20-40 psi (pounds per square inch). The chemical mixture is provided by the manufacturer and is recollected for disposal.

Answer 39

The correct answer is E. This is varicella (chicken pox), which is the primary form of infection by the herpes zoster (varicella-zoster) virus. Recurrence due to virus harbored in neurons tends to be dermatomal in distribution and is called shingles. Fever, malaise, headache, and myalgia may also be present, particularly in the prodromal phase. The rash is pruritic, papular, changing to vesicular ("dew drops on a rose petal"), pustular and finally crusting. Tzanck smear of the base of a vesicle may demonstrate multinucleated giant cells. Immunocompromised patients can be treated with acyclovir to prevent dissemination. Chicken pox may be complicated by secondary bacterial infection, pneumonia and systemic spread (immunosuppressed patients). Herpes simplex I (choice A) causes oral vesicles and ulcers. The lesions are often very painful. A rash is not seen with herpes simplex I. Herpes simplex II (choice B) causes genital vesicles and ulcers. The lesions are described as being beefy red and very painful. Measles (choice C) causes a blotchy, nonvesicular rash. Prodrome consists of fever, coryza, malaise, photophobia and Koplik's spots. The rash is brick red, irregular and maculopapular. Shingles (choice D) is the recurrent form of herpes zoster infection and usually is localized to a single dermatome. It is typically seen in the elderly.

Answer 40

The correct answer is E. The most common bacterium implicated in community-acquired pneumonia is the pneumococcus Streptococcus pneumoniae. When community-acquired pneumonia occurs in elderly patients or patients with comorbidity, aerobic gram-negative bacilli and Staphylococcus aureus are added to the list. The organisms listed in choices A, B, and C are important causes of communityacquired pneumonia without comorbidity in patients younger than 60 years of age, but are not the most frequent causes. Staphylococcus aureus (choice D) is an important cause of communityacquired pneumonia (particularly in the elderly and in patients with comorbidity) but is not the most frequent cause.

Answer 41

The correct answer is B. This patient has Goodpasture syndrome, which affects both the renal and pulmonary systems. In the kidney, it causes a rapidly progressive glomerulonephritis associated with antibodies directed against a collagen component of the glomerular basement membrane (anti-GBM antibody a classic clue to this diagnosis). They are also active against the basement membrane of respiratory alveoli, accounting for the pulmonary component of the disease. These antibodies create a linear pattern on immunofluorescence. Autoimmune reactions such as those found in Goodpasture syndrome, certain drug allergies, blood transfusion reactions, and hemolytic disease of the newborn, are classified as Type II hypersensitivities (antibody-mediated cytotoxicity). IgG or IgM antibody reacts with membraneassociated antigen on the surface of cells, causing activation of the complement cascade and, ultimately, cell destruction. Type I (choice A) reactions (immediate, atopic, or anaphylactic) require an initial (sensitizing) exposure to an antigen. On re-exposure to the antigen, cross-linking of IgE receptors occurs on the surface of basophils and mast cells. The mast cells then release a variety of mediators, including histamine. Clinical syndromes include asthma, atopic dermatitis, eczema, hives, and allergic rhinitis. Type III (choice C) hypersensitivity (immune complex-mediated hypersensitivity) is caused by antibodies to foreign antigens. Immune complexes of IgG or IgM with the antigen activate complement. This results in the generation of C3b, which promotes neutrophil adherence to blood vessel walls. The complexes also generate C3a and C5a (anaphylatoxins), which lead to inflammation and tissue destruction. The hallmark signs of Type III sickness, which occur 7-14 days after exposure to the offending antigen, include urticaria, angioedema, fever, chills, malaise, and glomerulonephritis. Clinical syndromes include serum sickness (e.g., penicillin, streptomycin, sulfonamide, phenylbutazone hypersensitivity) and the Arthus response. Immune complexes are also observed in systemic lupus erythematosus (SLE). Type III glomerulonephritis (e.g., poststreptococcal glomerulonephritis) is characterized by a lumpy bumpy appearance on immunofluorescence using labeled antibody specific for immunoglobulin or complement. Type IV (choice D) is also known as delayed-type hypersensitivity (DTH). Unlike the other types, which are mediated by antibody, DTH depends on TDTH cells that have been sensitized to a particular antigen. T cells react with antigen in association with MHC class I gene products and release lymphokines. Examples include tuberculin skin sensitivity and contact dermatitis (e.g., poison ivy rash).

Answer 42

The correct answer is D. This question requires you to appreciate the key structural difference between gram-positive and gram-negative microorganisms. Most gram-negatives (other than exceptional microorganisms, such as Mycoplasma, that lack a cell wall) have a more complex cell envelope than gram-positive microorganisms. It includes both a cytoplasmic membrane and an outer membrane. Between these two membranes is the periplasmic space, which contains enzymes such as phosphatase and penicillinase, binding proteins for the transport of various nutrients, and peptidoglycan, as well as a portion of the lipoprotein that firmly anchors the outer membrane to the peptidoglycan. Gram-positives do not have outer membranes. Choice A, the cell envelope, is incorrect because both gram-positive and gram-negative microorganisms have this structure, which is defined as all the layers that enclose the cytosol of the bacterium. It is the composition of the envelope that differs between gram-positive and gram-negative microorganisms. Choice B, exotoxin, is not exclusive to gram-negative microorganisms, but is also found in some gram-positives. By contrast, endotoxin (lipopolysaccharide; LPS) is found exclusively in gram-negatives. Choice C, peptidoglycan, is found in the cell walls of both gram-positive and gramnegative microorganisms. Note that there is a larger amount of peptidoglycan in gram-positive microorganisms. Choice E, teichoic acids, are found exclusively in gram-positive organisms.

Answer 43

The correct answer is A. The child has whooping cough, or more accurately if one goes by the chronology of the symptoms, "coughing whoop," as the patient is subject to a series of coughing episodes that are followed by a rapid inspiration of air through a narrow airway (the "whoop"). The infection of the respiratory tract by Bordetella pertussis causes a hyperreactivity of the respiratory apparatus; even the slightest stimulus will trigger a coughing episode. The offending pathogen excretes adenylate cyclase and also produces an exotoxin that inactivates the inhibitory subunit of the G-protein complex, thus activating adenylate cyclase within the cells of the respiratory system. The symptoms of pertussis last about 6 weeks and are divided into 3 stages: 1) Catarhal stage: Insidious onset with lacrimation, sneezing, anorexia, coryza and a hacking cough at night 2) Paroxysmal stage: Characterized by rapid bursts of consecutive coughs followed by a deep, high-pitched inspiration (whoop) 3) Convalesent stage: Begins 4 weeks after onset and is associated with a reduction in frequency and severity of paroxysms of cough Haemophilus influenzae type b (choice B) was the major cause of infant meningitis before the Hib conjugate vaccine nearly eradicated this pathogen from the United States. Nonencapsulated strains cause otitis media in children and pneumonia in adults. Klebsiella pneumoniae (choice C) is a gramnegative, encapsulated rod that is a significant pulmonary pathogen in individuals with a compromised respiratory apparatus. It is a common cause of aspiration pneumonia and pulmonary abscesses in alcoholics and patients with chronic obstructive pulmonary disease. Legionella pneumophila (choice D) is another fastidious, gram-negative, respiratory pathogen that may cause either a fulminating disease or a mild "walking pneumonia-like" condition (i.e., an atypical pneumonia). Mycoplasma pneumoniae (choice E) is the most common cause of primary atypical pneumonia. The disease is considered "atypical" because the patients have a very mild disease with low-grade fever, little in the way of constitutional signs, and a non-productive cough. These organisms are fastidious and are difficult to grow in the laboratory.

Answer 44

The correct answer is C. The appropriate stain is methenamine silver, since routine hematoxylin and eosin does not adequately demonstrate the organisms. The cysts, when stained with methenamine silver, have a characteristic cup or boat shape; the trophozoites are difficult to demonstrate without electron microscopy. It is also worth knowing that sputum samples are not nearly as effective as bronchial washes in demonstrating the organisms. ELISA (choice A) is enzyme-linked immunosorbent assay. It is the primary test for HIV antibody. It is not a stain. Hematoxylin and eosin (choice B) is the routine tissue stain used in pathology laboratories. Prussian blue (choice D) is good for demonstrating iron. The western blot (choice E) is the confirmatory test of choice for diagnosing HIV infection. The western blot test is generally performed once the ELISA test has been shown to be positive.

Answer 45

The correct answer is E. The rash described is that of secondary syphilis, caused by Treponema pallidum. Involvement of palms and soles by a rash is unusual, and secondary syphilis should come to mind. Not all patients with secondary syphilis have a severe form of the rash, and consequentially some cases are missed. Primary syphilis takes the form of a painless, button-like mass called a chancre. Tertiary syphilis, which is now rare, has a propensity for involving the aorta and central nervous system and can also cause "gummas" (granulomatous-like lesions) in many sites, notably including liver and bone. The mucous patches are described as being painless silvery ulcerations of mucous membrane with surrounding erythema. Other symptoms include cranial neuropathies (II - VIII), iritis, glomerulonephritis, arthritis and periositis. Herpes simplex I (choice A) usually causes perioral vesicular lesions. Herpes simplex II (choice B) usually causes genital vesicular lesions. HIV (choice C) does not itself cause a rash, although co-infection with other organisms can result in a rash. Neisseria gonorrhoeae(choice D) does not typically cause a rash but does not cause urethral discharge and dysuria.

Answer 46

The correct answer is A. The patient has Burkitt lymphoma. This high-grade B-cell lymphoma occurs endemically in Africa (it is the most common neoplasm in children in an equatorial belt that includes Africa and New Guinea) and sporadically in the U.S. and Europe. The sporadic form is often in an abdominal site and occurs in young adults. The African form of Burkitt lymphoma has been strongly associated with antibodies directed against Epstein-Barr virus; the association is weaker in sporadic cases. Hepatitis B (choice B) is associated with hepatocellular carcinoma. Herpesvirus (choice C) type 8 is associated with Kaposi sarcoma. HIV (choice D) is linked to AIDS. Some patients also develop primary lymphomas (not usually Burkitt). Human papillomavirus (choice E) is linked with common warts, genital condylomata, and genital cancers.

Answer 47

The correct answer is A. The child has infant botulism (floppy baby syndrome)that is caused by germination of Clostridium botulinum spores (found in honey) in the baby's gastrointestinal tract. Patients improve when honey is removed from the diet. This disorder is most common in children under the age of 6 months; older children and adults do not seem to be vulnerable to this form of botulism, but are susceptible to botulism caused by ingestion of preformed toxin. Patients usually present with a sudden onset of dry mouth, diplopia, dysphagia, dysphonia, and muscle weakness progressing to respiratory paralysis. Pupils are often fixed and dilated. Clostridium difficile(choice B) causes pseudomembranous colitis, especially after antibiotic therapy. It causes severe diarrhea through the production of toxin from these bacteria. Clostridium perfringens(choice C) causes gas gangrene and gastroenteritis, and it is not associated with ingestion of honey. Clostridium tetani(choice D) causes tetanus and does not cause a food-borne illness in infants. Corynebacterium diphtheriae(choice E) causes diphtheria in susceptible individuals.

Answer 48

The correct answer is B. Escherichia coli is a very common pathogen associated with urinary tract infections and is a common cause of cystitis. In fact, more than 90% of all "first" infections are caused by Escherichia coli. It is part of the normal flora of the gastrointestinal tract. Patients with cystitis can develop bacteremia and subsequent septic shock and adult respiratory distress syndrome (ARDS).E. coli is a gram-negative rod. The key to this question is knowing that most urinary tract infections are caused by E. coli. Gram-negative diplococci (choice A) might be Neisseria spp. or Moraxella catarrhalis. Neisseria gonorrhea is the cause of gonorrhea, a sexually transmitted disease that presents with urethritis or may be asymptomatic. Patients are usually young, sexually active males. Moraxella spp. are gramnegative cocci that can cause a wide variety of infections, usually implicated as a cause of otitis media and sinusitis in children, or as a cause of purulent tracheobronchitis and pneumonia in a population of patients who are over 50 years of age and have underlying obstructive lung disease. Gram-positive cocci (choice C), such as Staphylococcus aureus, S. epidermidis and Streptococci, rarely cause cystitis. Staphylococcus saprophyticus causes urinary tract infections, but the patients are typically young, sexually active women. Gram-positive diplococci (choice D) would be a description of Streptococcus pneumoniae, which is the most common cause of community-acquired pneumonia. This is rarely a cause of UTIs. Gram-positive rods (choice E) would include members of the following genera: Clostridium, Bacillus, Listeria, and the coryneform bacteria. The only significant member of this group to produce urinary tract infections is Corynebacterium urealyticum.The organism creates an alkaline urine environment with the potential for stone formation. Patients are usually immunocompromised or have had recurrent urinary tract infections.

Answer 49

The correct answer is choice D. Periodontal disease pathogens are usually anaerobic or capnophilic. The term capnophilic means "carbon-dioxide loving". In the anaerobic environment deep in the sulcus, the following genera are often said to contribute to the tissue destruction of periodontal disease: Bacteroides, Porphyromonas gingivalis, Borelia, Spirochetes,Capnocytophaga, Fusobacteria and Eichenella. Streptococci (mutans, mitis, sanguis, salivarius) are all gram positive facultative anaerobes that do not require anaerobic conditions to survive. They are Viridans streptococci, generally non-pathogenic, and not linked to periodontal disease, although S. mutans is clearly linked to caries.

Answer 50

The correct answer is B. The concomitant administration of bacteriostatic and bactericidal antibiotics would result in antagonism of the antibacterial effects of both agents. The reason for the antagonism between the two is that each agent interferes with the mechanism of action of the other agent. By definition, a bacteriostatic agent is one that prohibits the growth and development of bacteria and a bactericidal agent is one that directly causes the death of a microbe during the growth and development stages. An extension of bacterial coverage between two agents (choice A) is typically seen when two bacteriostatic or two bactericidal agents are given together. The administration of a macrolide and a cephalosporin may cause some discomfort for the patient, such as nausea or stomach cramps; however, profound toxicity for the patients taking both agents (choice C) is very unlikely. Synergism (choice D) is the correlated action of two agents that results in a final effect that is greater than that of each agent acting separately. A classical example of synergism is seen when an aminoglycoside, such as gentamicin, is administered concomitantly with extended spectrum penicillin, such as piperacillin, for the treatment of a pseudomonas infection. The appearance of mild antifungal activity (choice E) would not be seen when bacteriostatic and bactericidal antibiotics are given together, if neither have any antifungal activity

Answer 51

The correct answer is E. Most RNA viruses (eg, poliovirus) replicate in the cytoplasm and therefore can replicate in enucleated cells. Poliovirus belongs to the family Picornaviridae. These viruses are nonenveloped and have an icosahedral nucleocapsid that contains positive-sense RNA. The exception to the rule regarding RNA viruses is the family Orthomyxoviridae, the influenza viruses (choice C). Orthomyxoviruses undergo transcription and RNA replication in the nucleus of the host cell because they need to cannibalize the capped 5' termini of cellular RNAs for use as primers for viral mRNA transcription. Adenoviruses (choice A) are nonenveloped and have an icosahedral nucleocapsid that contains a double-stranded linear DNA genome. Cytomegalovirus (choice B) is a member of family Herpesviridae. It is an enveloped virus with an icosahedral nucleocapsid that contains a double-stranded linear DNA genome. JC virus (choice D) belongs to family Papovaviridae. It is nonenveloped and has an icosahedral nucleocapsid that contains a double-stranded circular DNA genome. For most DNA viruses, transcription and DNA replication occur in the nucleus of the host cell. The exception to this observation is the family Poxviridae, which carries out its replication in the cytoplasm. Poxviridae includes variola virus, vaccinia virus, molluscum contagiosum, and orf virus.

Answer 52

The correct answer is E. The patient in this question has pneumococcal pneumonia. It is caused by Streptococcus pneumoniae, an alpha-hemolytic, gram-positive coccus that grows in chains. It can be 31 easily distinguished from other alpha-hemolytic streptococci because it is exquisitely sensitive to bile and bile-like compounds, such as optochin. It is the most common cause of community-acquired pneumonia and the most common cause of community-acquired meningitis in adults older than 30 years of age. The only recognized virulence factor of S. pneumoniae is its carbohydrate capsule (which contains acidic polysaccharides). Antibody to a specific capsule type is necessary to overcome infection. More than 80 capsule types have been recognized. The 23 types that most commonly cause disease are contained in a vaccine (pneumovax) that is recommended for high-risk groups, including the elderly and those undergoing splenectomy. Increased susceptibility is also found in patients with Hodgkin disease, chronic lymphocytic leukemia, and myeloma. The vaccine should still be given to patients with these conditions, but it is less successful. The alternative complement pathway (choice A) is important in clearing Neisseria infections. Individuals with deficiencies in C5 through C8 are at increased risk for disease from Neisseria. The fimbria of Streptococcus pyogenes (Group A beta-hemolytic streptococcus) is composed of an alpha-helically coiled M protein. Antibody against a specific M type (choice B) will prevent infection. Raising antibodies to M proteins, however, can lead to rheumatic fever, so strep throat infections are routinely treated with penicillin to prevent an antibody response. The C carbohydrate is an antigen of beta-hemolytic streptococci used to divide them into different groups. Antibody against C carbohydrate (choices C and D) is not protective.

Answer 53

The correct answer is A. Acute mastitis, frequently complicated by breast abscess formation, typically develops in the postpartum period when the nipples are first subjected to the physical stresses of breast feeding. Bacteria enter the breast by way of cracks in the nipple and flourish in the microenvironment of the lactating breast. Acute mastitis causes redness, pain, and swelling in the affected breast; S. aureus is the most common pathogen. There is a very apparent cellulitis and fever may be present. Penicillin antibiotics are generally administered because they are effective and safe to use during breast-feeding. Although endocarditis (choice B) could send septic emboli to the breast, S. aureus endocarditis is more commonly associated with bacteremia and fevers, proliferative glomerulonephritis, valvular dysfunction, and emboli to the brain, kidneys, heart, and gut. Inflammatory breast carcinoma (choice C) does not invoke an inflammatory response, and it is not associated with an infection. Inflammatory breast carcinoma is characterized by tumor spread into the dermal lymphatics, producing diffuse induration and skin tenderness with the typical peau d'orange (orange peel) appearance. Mastitis arising in the perimenopausal period (choice D) is usually a chronic mastitis caused by obstruction of ducts from inspissated secretions. Chronic mastitis is sterile. The breast tissue shows lymphocytes and plasma cells surrounding dilated ducts filled by cellular debris. Paget disease (choice E) presents as skin changes on the breast or nipple in association with ductal carcinoma in the underlying breast that is percolating out into the epidermis. Paget disease produces an eczematous skin lesion that may be crusted or weeping, but it is not related to bacterial infection.

Answer 54

The correct answer is D. This is impetigo, which is typically seen in preschool children with poor hygiene, particularly in the summer in warm climates. The characteristic lesion has a large golden crust. Most cases are caused by Staphylococcus aureus; Streptococcus pyogenes is occasionally 32 implicated. Impetigo is highly infectious, and mini-epidemics can occur in daycare settings. The initial treatment is typically with penicillins and topical preparations, primarily muciprocin ointment. Methicillin-resistant strains are presently rare in this setting, but can occur. Itching is the only symptom. The lesions consist of macules and vesicles. Aphthous ulcers (choice A), commonly known as "canker sores," are painful, shallow ulcers of the oral cavity. Erysipelas (choice B) is a different type of skin infection, often caused by Streptococcus pyogenes (also sometimes others including Staphylococcus), and is characterized by large erythematous patches. Herpes simplex I (choice C) causes tiny oral and perioral vesicles, but not large golden crusts. Measles (choice E) causes a blotchy erythematous rash on the face and all over the body. These lesions typically do not "weep" as seen with impetigo.

Answer 55

The correct answer is E. IgM immunoglobulin directed against Toxoplasma would provide evidence of infection in the newborn baby. IgM is the only antibody that a baby can form with an acute infection. It is the first antibody that is formed in an infection and it is also the antibody that is present on the surface of immature and mature B cells. IgA (choice A) is the antibody that is produced in response to mucosal infections. It cannot cross the placenta. The baby's immune system is still not well developed at birth and cannot produce IgA immunoglobulin. In adults or older children, IgA exists in serum in a monomeric form but is present in seromucous secretions as a dimer. IgE (choice B) is the antibody that is produced in response to an allergen. It cannot cross the placenta, and is not present in the baby at birth or for some time after birth. The baby's immune system is still not well developed at birth and can initially only form IgM immunoglobulin. IgG 1 (choice C) and IgG4 (choice D) immunoglobulins might be present in the baby, but would not indicate infection in the infant. The presence of these isotypes would indicate that the mother was infected with the organism and produced antibody that was transported across the placenta in utero. IgG is the only immunoglobulin that can cross the placenta, providing protection to the baby during the first few months of life.

Answer 56

The correct answer is B. All of the agents listed can cause devastating congenital infections with high mortality and often major organ malformation (the TORCH agents: Toxoplasma, other, rubella, cytomegalovirus, herpes simplex). It is herpes simplex type II, however, typically acquired during delivery, that causes the devastating neonatal encephalitis described in the question stem. The mortality rate for neonatal herpes is approximately 65%. Only 10% of these infants escape without neurologic sequelae. A distinguishing feature is the propensity to involve the temporal lobe, with mass effect on imaging studies and temporal lobe seizure foci on EEGs. A point worth remembering is that adult herpes encephalitis (in nonimmunosuppressed individuals) is usually caused by herpes simplex I, whereas neonatal and congenital herpes are usually caused by herpes simplex II.

Answer 57

The correct answer is D. Rubella, also called German measles or 3-day measles, is a disease caused by a Togavirus, which is a small, enveloped, single-stranded (+) linear RNA virus. Approximately 40% of patients are asymptomatic or have mild symptoms. In symptomatic patients, the clinical presentation typically consists of an erythematous rash that begins on the head and spreads downward to involve the trunk, and lasts for approximately 3 days. In addition, symptoms include fever, posterior cervical lymphadenopathy, and arthralgias. The greatest danger from rubella is to the fetus. If clinical rubella develops or seroconversion is demonstrated, there is a high risk for congenital abnormalities or spontaneous abortion. Women of childbearing age should be warned not to become pregnant within 2-3 months from the time of immunization. Mild arthralgias and other symptoms may develop in 25% of immunized women. Enteroviral rashes may mimic rubella and rubeola. Infectious mononucleosis (choice A) is caused by the Epstein-Barr virus, a herpesvirus. Classic findings include fever, exudative pharyngitis, generalized lymphadenopathy, severe malaise (most common complaint), and hepatosplenomegaly. A rash is not a characteristic feature unless the patient has been treated with ampicillin. Lyme disease (choice B) is caused by the spirochete Borrelia burgdorferi. The disease is transmitted by the bite of the tick Ixodes dammini. The initial lesion is an annular rash with central clearing and a raised red border (erythema chronicum migrans) at the bite site. Patients also have fever, malaise, myalgias, arthralgias, headache, generalized lymphadenopathy, and, occasionally, neurologic findings. Roseola (choice C) is caused by human herpesvirus 6. Other names include exanthem subitum or sixth disease. Children have a febrile period of 3-5 days with rapid defervescence followed by an erythematous maculopapular rash lasting 1-3 days. Rubeola (choice E), or regular measles, is a disease caused by a paramyxovirus. Patients present with an upper respiratory prodrome and characteristic oral lesions (Koplik spots) that precede the rash. The nonpruritic maculopapluar rash begins on the face and spreads to the trunk and extremities, including palms and soles. The incubation period is 10-14 days. Patients also have a posterior cervical lymphadenopathy. The virus is not associated with risk for a fetus.

Answer 58

The correct answer is choice D. The best drug treatment for CMV this infection of the retina is ganciclovir. Acyclovir (choice A) is not effective in CMV infections. It is used more for HSV type 1 and 2 infections. Amantadine (choice B) is used either therapeutically or prophylactically for the influenza A virus. Flucytosine (choice C) is an antifungal agent. Zidovudine (choice E) is a reverse transcriptase inhibitor and is used as a first-line drug for the treatment of AIDS. The drug by itself is ineffective against CMV retinitis.

Answer 59

The correct answer is B. The causative organism is Neisseria gonorrhoeae, which is a sexually transmitted gram-negative coccus that can live in phagocytic vacuoles in neutrophils and macrophages. The organism is the most common cause of septic arthritis in otherwise healthy, sexually active adults. Cervicitis in women with purulent discharge or women who are asymptomatic, yielding a positive culture, is a common charactristic. Fever, rash, tenosynovitis and arthritis occur with disseminated disease. Hyphae and spores (choice A) would be features of Candida, which usually causes arthritis as a rare complication of systemic candidiasis in immunocompromised patients. Furthermore, there would be the appearance of other classic characteristics, such as the appearance of creamy white lesions. Chlamydia is an intracellular organism that cannot make ATP 34 (choice C) and does not usually cause septic arthritis. Mycoplasma do not have true cell walls (choice D) and do not usually cause septic arthritis, but are commonly associated with respiratory conditions, such as mycoplasma tuberculosis. Syphilis is caused by a spirochete that stains with silver stains (choice E). Secondary syphilis can (uncommonly) cause an acute arthritis, but the interval between acquiring the infection and developing the arthritis is usually months to years.

Answer 60

The correct answer is C. Anaerobic bacteria make up most of the normal human oral flora. For example, prominent members of the normal microbial flora of the mouth include anaerobic spirochetes, fusobacteria, and prevotella. Anaerobic lung abscesses commonly arise from aspirated oral secretions in patients with compromised neurologic status (e.g., anesthesia, coma, intoxication), or in individuals with a depressed cough reflex. The most common anaerobic organisms isolated from lung abscesses include Bacteroides, Fusobacterium, and Peptococcus species, all of which are typical oral flora. Blood-borne pathogens (choice A) producing lung abscesses probably originate as emboli from vegetations on the right heart. The most likely pathogens in endocarditis are Staphylococcus and Streptococcus species; Fusobacterium would be very unusual. Fusobacterium usually is not pathogenic and lives in a commensal relationship with the host. Infective aerosols (choice B) generally contain organisms that are intrinsically pathogenic (e.g., mycobacteria, influenza virus). It is the acidity of the gastric aspirate that most seriously injures the lung, not the gastric bacteria. Fusobacterium do inhabit the gastrointestinal tract, but viable bacteria are much more likely to be transmitted from the oral secretions than from the stomach (choice D). The upper respiratory tract (choice E) is generally sterile, although COPD is frequently complicated by chronic bronchitis. Numerous bacterial and viral pathogens can be involved in chronic bronchitis; however, Fusobacterium is not an upper respiratory tract pathogen.

Answer 61

The correct answer is D. The woman has developed a pulmonary abscess, as evidenced by the chest radiograph. Of the organisms listed, Staph. aureus is the most likely cause of bacterial pneumonia complicated by abscess formation. Bacteria commonly producing pneumonia developing in the context of influenza include Staphylococcus aureus, Haemophilus influenzae, and Streptococcus pneumoniae, but Streptococcus pneumoniae is not a frequent cause of lung abscesses. Staph. aureus pneumonia/abscess are more common in patients in chronic care facilities, nosocomial infections, influenza epidemics and cystic fibrosis. Candida albicans(choice A) would not be a likely cause of this woman's abscess, or of the pneumonia that preceded it.Candida pneumonia in a nonimmunosupressed individual is rare. Note also that Candida is a fungus. Klebsiella pneumoniae(choice B) is more likely in men who are alcoholic or diabetic, or suffering from chronic obstructive lung disease. It is also a plump gram-negative encapsulated rod. Pneumocystis carinii(choice C) is associated with pneumonia in immunocompromised hosts, especially AIDS patients, but there is no indication of immune compromise in this patient. Furthermore, it would not show as a gram-positive organism, as it is a fungus. Streptococcus pneumoniae(choice E) is only rarely associated with lung abscess. It commonly causes pneumonia in chronic lung diseases. It is a gram-positive diplococcius.

Answer 62

The correct answer is E. Although inadequately cooked pork and trichinosis are commonly associated, the quality of purchased pork and the degree of cooking in this country are sufficiently high that pork-related trichinosis is uncommon. The more common scenario is actually the one described in the question stem, for two reasons. First, bear meat is not inspected, and it may often be served "rare," or relatively uncooked. The infected meat contains cysts that are released in the gastrointestinal tract after ingestion. The Trichinella spiralis larvae penetrate the intestinal mucosa and develop into adult worms in 30-40 hours. After reaching adulthood, they mate, and the females produce larvae that grow to maturity and seek out muscle in which to encyst (often in the orbital muscles) within approximately 5-8 days. If you didn't know the answer to this question, you still might have noticed the presence of eosinophilia, which should have clued you in to the presence of a parasitic infestation, making trichinosis the only plausible answer. Anthrax (choice A) after eating bear meat is very unlikely. In addition, anthrax would not produce the described symptoms; instead, cutaneous lesions (95% of cases) or respiratory disease culminating in death (5% of cases; wool sorter disease) would occur. Bacillus cereus food poisioning (choice B) and Escherichia coli gastroenteritis (choice D) might lead to similiar gastrointestinal symptoms but would not cause the eye findings. Botulism (choice C) causes flaccid paralysis, not the symptoms described.

Answer 63

The correct answer is A. Lobar pneumonia, in which an entire lobe of the lung becomes rapidly affected with pneumonia, is acutally a relatively uncommon pattern for pneumonia. Common causative organisms include Streptococcus pneumoniae (pneumococcus), Haemophilus influenzae, and Klebsiella pneumoniae. K. pneumoniae is specifically associated with alcohol abuse, diabetes mellitus, and nosocomial infections. Legionella spp. (choice B) can cause either a patchy or lobar consolidation but are not specifically associated with alcoholism. Instead, Legionella infection is associated with inspiration of aerosolized contaminated water. Mycoplasma pneumoniae (choice C) causes extensive patchy infiltrates rather than lobar pneumonia. It is the most common cause of pneumonia in young adults. Pneumocystis carinii (choice D) causes diffuse interstitial and alveolar infiltrates, typically in patients immunosupressed by AIDS, cytotoxic drug therapy, or cancer. Staphylococcus aureus (choice E) produces patchy infiltrates. Staphylococcal pneumonia may complicate influenza during epidemics, or it may be a nosocomial infection. It is associated with empyema formation.

Answer 64

The correct answer is B. The patient likely has candidal infection of the prosthetic aortic valve. That is why she did not respond favorably to antibacterial therapy, which is known to promote fungal infection. Note that Candida tends to colonize foreign bodies, such as intravenous and Foley catheters, prosthetic valves, and ventricular shunts. Actinomyces israelii (choice A) is known to cause cervicofacial infection in patients undergoing dental work. Histoplasma capsulatum (choice C) causes histoplasmosis, a pulmonary infection common in the midwestern river valleys. Multiorgan involvement is usually seen only in the immunocompromised. Transmission of the organism occurs through the inhalation of airborne microconidia (infectious) spores. The organism is found in bird and bat droppings and in the soil. Nocardia asteroides (choice D) is an actinomycete that causes a chronic lobar pneumonia that may metastasize to the brain. It is more common in immunocompromised patients. It is found in the soil and in aquatic environments. Trichophyton rubrum (choice E) is one of the organisms that commonly produces a variety of cutaneous mycoses, including tinea corporis (ringworm), tinea cruris (jock itch), and tinea pedis (athlete's foot).

Answer 65

The correct answer is D. This infant is exhibiting passive immunity acquired from his mother in utero. IgG is the only class of immunoglobulin that can cross the placenta. As such, IgG molecules diffuse into the fetal circulation, providing immunity. This circulating maternal IgG protects the newborn during the first 4-6 months of life. Note that IgG is also capable of opsonization and complement activation (a feature shared with IgM). IgA (choice A) functions in the secretory immune response. The secretory form of this immunoglobulin (sIgA) is found in tears, colostrum, saliva, breast milk, and other secretions. It is produced by the plasma cells in the lamina propria of the gastrointestinal and respiratory tracts. IgD (choice B) functions as a cell surface antigen receptor on undifferentiated B cells. IgE (choice C) is involved in the allergic response and immediate hypersensitivity reactions. The Fc region of IgE binds to the surface of basophils and mast cells. Antigen binding to two IgE molecules leads to mast cell degranulation and the release of leukotrienes, histamine, eosinophil chemotactic factors, and heparin. IgM (choice E) is the first antibody detected in serum after exposure to antigen. IgM circulates as a pentamer and thus has five Fc regions. This structure makes it especially effective in fixing complement.

Answer 66

The correct answer is B. Transformation is the uptake and integration of naked DNA by a bacteria from the environment. Transformation can be induced in the laboratory (a technique used to introduce gene-carrying plasmids into bacteria) or, relatively uncommonly, occurs naturally. Only a few medically important species undergo natural transformation: Haemophilus species, Streptococcus species, Neisseria gonorrhoeae, and Helicobacter pylori.

Answer 67

The correct answer is E. The history is characteristic for Lyme disease, caused by the spirochete Borrelia burgdorferi. The clues are deer, Connecticut, rash, knees, and acute arthritis followed by chronic arthritis. Patients with Lyme disease initially have erythema migrus, a flat or slightly raised lesion that expands with central clearing. Headache and stiff neck are also common. Arthralgias, arthritis, and myalgias are seen. The arthritis is often chronic and recurrent. Fungal arthritis (choice A) can be caused by coccidioidomycosis, blastomycosis, sporotrichosis, and candidiasis. N. gonorrhoeaeare gram-negative cocci (choice B) that can cause septic arthritis. Septic arthritis is associated with acute arthritis, usually monarticular, most often in large weightbearing joints. Previous joint damage or intravenous drug abuse are common risk factors. Haemophilus (in young children) and Salmonella (in sickle cell patients) are gram-negative rods (choice C) that can cause septic arthritis. Staphylococcus are gram-positive cocci (choice E) that can cause septic arthritis.

Answer 68

The correct answer is E. Antibiotic mechanism questions are NBDE Part 1 favorites. Ciprofloxacin and norfloxacin belong to a category of antibiotics called the fluroquinolones. They are bactericidal and work by inhibiting topoisomerase II (DNA gyrase). They are effective against gram-negative rods and are the only oral agents effective against Pseudomonas. Inhibition of dihydrofolate reductase (choice A) is the mechanism of action of trimethoprim, which is typically used in combination with sulfonamides which inhibit an earlier step in folate synthesis (dihydropteroate synthase). Trimethoprim-sulfa is used in the treatment of Shigella, Salmonella, recurrent urinary tract infections, and Pneumocystis carinii pneumonia. Inhibition of DNA-dependent RNA polymerase (choice B) is the mechanism of action of rifampin. Rifampin is used (along with other drugs) in the treatment of tuberculosis. Inhibition of the 30s ribosomal subunit (choice C) is the mechanism of action of two important classes of antibiotics - the tetracyclines (tetracycline, doxycycline, democlocycline) and the aminoglycosides (gentamycin, tobramycin, streptomycin, etc.). The tetracyclines inhibit the attachment of the aminoacyl-tRNA to the ribosome, whereas the aminoglycosides inhibit the formation of the intitiation complex. Inhibition of the 50s ribosomal subunit (choice D) is the mechanism of action of the macrolides (e.g., erythromycin), the lincosamides (e.g., lincomycin, clindamycin), and chloramphenicol. Chloramphenicol inhibits the 50s peptidyl transferase, whereas erythromycin blocks translocation.

Answer 69

The correct answer is E. Since lipopolysaccharide can be seen in gram-negative bacteria, it is important to understand how it will affect the body. Septic shock is a complex, multisystem organ failure that can be produced either by LPS (which is present in the cell wall of all gram-negative bacteria) or certain toxins released by gram-positive bacteria and fungi. LPS binds to a serum protein and stimulates CD14 receptors on endothelial cells and circulating inflammatory cells, eliciting a broad range of end-organ responses. Anaphylactic shock (choice A) is brought about by an exaggerated Type I hypersensitivity reaction mediated by IgE antibodies bound to mast cells and basophils. The resulting degranulation produces massive histamine and adenosine release, which produces constriction of the bronchi and pulmonary circulation. Cardiogenic shock (choice B) reflects the inability of the heart to maintain arterial pressure sufficiently to perfuse the systemic vasculature. Cardiogenic shock is intrinsic to the heart and usually a consequence of ischemia, arrhythmia, or obstruction. Hypovolemic shock (choice C) occurs when blood volume decreases to a point at which it is inadequate to maintain arterial pressure in the vital organs. Hypovolemic shock is due to hemorrhage, fluid loss from burns, or severe diarrhea and vomiting. Neurogenic shock (choice D) is an unusual form of shock that occurs in catastrophic nervous system injuries that cause diffuse vasodilation and hypotension.

Answer 70

The correct answer is C. The question is essentially asking "Which of the following is the most common cause of infective endocarditis (IE) in intravenous drug users?" Being an intravenous drug user places him at a very high risk for the development of IE. The factors that determine the clinical presentation of IE include the nature of the organism, the valve infected, and the route of the infection. Although the clinical characteristics of IE can vary, most patients present with a febrile illness lasting several days to 2 weeks. This illness is often accompanied by a variety of nonspecific signs and symptoms, such as chills, weakness, dyspnea, cough, arthralgia, diarrhea, and abdominal pain. Heart murmurs occur in approximately 90% of all patients, but may be absent in patients with right-sided infections. Other clinical signs include Osler nodules (purplish or erythematous subcutaneous papules or nodules on the pads of the fingers and toes), Janeway lesions (hemorrhagic painless plaques on the palms and soles), petechiae (small erythematous painless hemorrhagic lesions that may appear anywhere), and splinter hemorrhages (thin linear hemorrhages found under the nail beds of fingers and toes). The diagnosis of IE is dependent on positive blood cultures and echocardiographic evidence of "valvular vegetation" or valvular injury. Staphylococci, Streptococci, and Enterococci are possible pathogens; however, Staphylococcus aureus accounts for most cases of acute IE. Enterococci(choice A) cause a minority of cases of valvular endocarditis, but are not usually involved in endocarditis associated with intravenous drug abuse. Gram-negative organisms such as Pseudomonas aeruginosa(choice B) are rarely the cause of IE. They primarily cause pneumonia in cystic fibrosis patients and those on ventilators. Streptococcus pneumoniae(choice D) and Streptococcus pyogenes(choice E) are not common causes of IE. They are, however, common causes of pneumonia and pharyngitis, respectively.

Answer 71

The correct answer is C. The causative organism of tetanus is Clostridium tetani , which is a grampositive, spore-forming anaerobic rod. Bacteroides is gram-negative anaerobic rod (choice A). Examples of gram-positive cocci (choice B) are staphylococci and streptococci. The causative organism of anthrax, Bacillus anthracis , is a gram-positive, spore-forming aerobic rod (choice D).

Answer 72

The correct answer is E. The difficulty with developing a vaccine against influenza A arises because the influenza virus genome is composed of eight strands of single-stranded RNA. Minor shifts (antigenic drift) in surface antigens that occur as point mutations in the genes accumulate. Influenza A can also undergo larger, abrupt changes in antigen expression (antigenic shift), however, as a consequence of reassortment of some of the RNA fragments between human and nonhuman hosts. 39 Thus, last year's vaccine does not necessarily work against this year's virus. Polysaccharide coats (choice A) are a virulence factor of some bacteria, not of viruses. Influenza A can compromise the lungs sufficiently to predispose to secondary infections, producing a functional immunosupression (choice B), but this attribute does not make it difficult to produce vaccines against the virus. Unlike AIDS, influenza virus does not selectively target lymphocytes (choice C). Influenza A, bound to antibody and complement, can be effectively phagocytized (compare with choice D).

Answer 73

The correct answer is D. Hyaline cartilage contains very few, sparsely distributed, small blood vessels. The tissue consequently is very vulnerable to infection because the immune system (white cells and serum with clotting factors and antibodies) cannot access the cartilage in an efficient fashion. Chondroitin sulfate (choice A) is an important constituent of cartilage that can be attacked by bacteria, but this is not the cause of uncontrollable infections in the cartilage, because if the infection is controlled, chondrocytes will continue to synthesize chondroitin. Collagen (choice B) is present in cartilage, but does not predispose the cartilage for uncontrollable infection. Collagen is not innervated (choice C), but this does not contribute to the development of uncontrollable infection. The tendency of cartilage to undergo calcification (choice E), even when not acting as the site of ossification, does not predispose for uncontrollable infection.

Answer 74

The correct answer is choice C. Hepatitis C is a disease about which much has been learned recently, but much more is unknown. It seems to be transferred through both bloodborne and sexual routes (not fecal/oral). It is harder to contract than HBV, but easier than HIV. It is distinguished by having the highest percentage of cases going into a chronic destructive stage, of all hepatitis types. It is a single stranded RNA virus. It was formerly known as NANB (Non-A Non-B) or "transfusion hepatitis" because tests for hepatitis A or B did not detect it. Hepatitis B also has a destructive carrier state, but it is less common. Hepatitis A has NO carrier state, and exposure to Hepatitis A gives lifelong immunity.

Answer 75

The correct answer is D. The vagina of prepubertal girls and postmenopausal women is colonized by colonic and skin bacteria, including Staphylococcus epidermidis, which is normally found on the skin. The vagina of women of childbearing age tends to be colonized by Lactobacillus (choice B) species, yeasts such as Candida (choice A), and Streptococcus species (choice E). The presence of Neisseria (choice C), such as N. gonorrhoeae (the cause of gonorrhea), in the vagina of a 5-year-old girl strongly suggests sexual abuse.

Answer 76

The correct answer is A. The answer to this question requires that the student realize that pus consists of bacteria and dead and dying neutrophils. This fact reflects that the patient has typical (rather than atypical) pneumonia. In the United States, the most common agent of this would be Streptococcus pneumoniae, a gram-positive extracellular pathogen rich in teichoic acids and peptidoglycan that elicit the neutrophilic exudate. The capsule of this organism is a polysaccharide and primarily elicits an antibody response rather than attracting neutrophils (choice B). Streptococcus pneumoniae is an extracellular, not an intracellular (choice C) organism. Streptococcus pneumoniae is alpha hemolytic, not beta hemolytic (choice D). This organism can be distinguished from other bacteria by the inhibition of growth in the presence of optochin or bile. Another common alpha hemolytic bacteria is the Viridans streptococci group. Streptococcus pneumoniae does produce an IgA protease (choice E) that enhances the ability of the organism to infect the respiratory mucosa, but this does not contribute to pus formation.

Answer 77

The correct answer is D. Ethylene oxide is used for sterilization of heat sensitive instruments and materials. Its advantage is that it does not melt, damage, or corrode instruments, as heat may. Its disadvantages include long cycle time (8-10 hours) and additional time to air out the materials so that they do not contain ethylene oxide residue. A period of 20-30 minutes is a common steam autoclave time. A period of 1-2 hours is a common dry heat autoclave time. A period of 24 hours was a time recommended for "cold sterilization" by chemicals. Cold sterilization is a misnomer, as it is actually high level disinfecton.

Answer 78

The correct answer is D. Pseudomembranous colitis is caused by a necrolytic toxin produced by Clostridium difficile, which can proliferate in the bowel secondary to the use of antibiotics that disrupt the normal bowel flora. Almost all antibiotics (but most commonly clindamycin and ampicillin) can cause this disorder, which results in profuse watery or bloody diarrhea. The most effective treatment for pseudomembranous colitis is vancomycin or metronidazole. Metronidazole is a bactericidal agent that is indicated for the treatment of serious infections caused by anaerobic bacteria and is active against most protozoa. It is primarily used for the treatment of amebiasis and trichomoniasis in outpatient settings and for prophylactic use in colorectal, gynecologic, and abdominal surgery. The use of antidiarrheal agents, such as attapulgite (choice A) and loperamide (choice C), is contraindicated. Clindamycin (choice B) is a bacteriostatic antibiotic used primarily in the treatment of many different anaerobic infections. This is one of the primary antibiotics implicated in the development of pseudomembranous colitis, and so will not be used to treat it.

Answer 79

The correct answer is A. Amphotericin B is the most appropriate drug listed for the treatment of cryptococcal meningitis. It is a polyene antibiotic that binds to ergosterol in the fungal cell membrane, 41 creating an artificial pore. Flucytosine is often prescribed as an adjunct medication. Fluconazole is used long term to prevent recurrence in AIDS patients. These are all systemic antifungal. Isoniazid (choice B) inhibits the biosynthesis of mycolic acids in the mycobacterial cell wall. It is the primary drug used against tuberculosis. It is used alone for TB prophylaxis and in combination with other antituberculars to treat patients with active disease. Clotrimazole (choice C) is a typical antifungal that, like nystatin, is used primarily in the treatment of oral candidiasis. It is from the imidazole class of antifungals. Metronidazole (choice D) is an antiprotozoal drug useful in treating a variety of parasitic infections. It is the drug of choice for trichomoniasis and giardiasis and provides general anaerobic coverage. Nystatin (choice E) is an antifungal polyene that is usually used topically but can be taken orally for oral and esophageal candidiasis. Candidal infections of the skin, mucous membranes, and vagina usually respond well to this drug. It may also be used to prevent intestinal fungal overgrowth in patients on chemotherapy.

Answer 80

The correct answer is D. The disease caused by HIV virus progresses in stages that vary greatly in length from individual to individual. HIV+ individuals early in the disease are often asymptomatic and otherwise healthy. The HIV test shows only exposure to the virus and formation of antibody (the test tests for antibody, not virus). This asymptomatic stage may continue for several years. Much later, AIDS defining conditions (e.g., Kaposi sarcoma, cryptococcal meningitis, PCP pneumonia) combined with HIV+ blood test, combine to give the diagnosis of AIDS. In between however, an early symptomatic stage is known as ARC (AIDS-related complex), AIDS prodrome or pre-AIDS. Oral candidiasis, night sweats, and weight loss are common at this stage.

Answer 81

The correct answer is B. IgM, produced during the immune response and is the first antibody that an infant makes, is the most efficient immunoglobulin at activating complement. IgG (choice A) is produced during the secondary immune response and is the only maternal immunoglobulin to cross the placenta. IgA (choice C) is the main immunoglobulin in external secretions, such as saliva and tears. It also protects mucosal surfaces. IgE (choice D) mediates immune hypersensitivity reactions and is important in providing immunity against some parasites. IgD (choice E) is present in the membrane of mature B cells and functions in antigen recognition by B cells.

Answer 82

The correct answer is B. Penicillin is a bactericidal agent with broad gram-negative and grampositive coverage. This agent inhibits the biosynthesis of cell wall mucopeptides. Bacterial growth in a closed system is characterized by four phases: (1) In the initial lag phase, no growth occurs as the organisms adapt to the new environment. (2) In the exponential, or log phase, the organisms grow at the fastest rate and antibiotics that interfere with cell growth or division are most likely to be effective. (3) In the stationary phase, when nutrients have been largely exhausted, organisms tend to stop growing but may remain viable for long periods of time. (4) In the phase of decline, cell deaths increase due to cell starvation or exposure to toxins.

Answer 83

The correct answer is E. Patients with sickle cell anemia typically present with a fiery red "slapped cheek" appearance. Parvoviruses are small single-stranded DNA viruses, of which only serotype B19 is pathogenic for humans. This virus causes three distinct syndromes: a childhood febrile rash known as erythema infectiosum ("Fifth disease"); aplastic crisis in individuals with chronic hemolytic diseases (e.g., sickle cell anemia, thalassemia); and congenital infections that can present as stillbirth, hydrops fetalis (analogous to severe Rh incompatibility), or severe anemia. In immunosuppressed patients and those with sickle cell disease, anemia caused by red cell hypoplasia occurs as a consequence of binding to erythrocyte P antigen. Coxsackie viruses (choice A) usually cause cold-like illness, but can cause herpangina, myocarditis, and meningitis. Echoviruses (choice B) can infect a variety of organ systems (gastrointestinal, CNS, eyes, heart, respiratory, skin), but are not a cause of aplastic crises. Hepadnavirus (choice C) is the causative agent of hepatitis B. Herpes viruses (choice D) cause a variety of acute to chronic infections, including herpes simplex types I and II, chicken pox, chronic herpes zoster, cytomegalovirus infection, and Epstein Barr virus infections.

Answer 84

The correct answer is B. The three major causes of neonatal meningitis are group B streptococci (Streptococcus agalactiae; choice D), Escherichia coli (choice A), and Listeria monocytogenes. All can be found in the vaginal tract of normal women and may contaminate the infant during passage through the birth canal. They colonize the upper respiratory tract and can cause pneumonia, septicemia, and/or meningitis in the neonate. They are readily distinguished on morphologic grounds; the streptococci are gram-positive cocci in chains, E. coli is a gram-negative rod, and L. monocytogenes is a gram-positive pleomorphic rod. There are other gram-positive rods that resemble Listeria (e.g., the diphtheroid bacilli found in the upper respiratory tract and on the skin), hence a motility test is done to confirm the identification. L. monocytogenes is also associated with drinking unpasteurized milk. Neisseria meningitidis(choice C) is the most common cause of meningitis in school-age children and young adults. It is a fastidious, non-motile, gram-negative diplococcus that would be a very rare cause of meningeal disease in very young patients, such as this one. Streptococcus pneumoniae(choice E) is a gram-positive coccus that grows in pairs and short chains. It is the number one cause of pneumonia, septicemia, and meningitis in the elderly. There is a vaccine composed of the capsular carbohydrate of 23 serotypes of this organism that is routinely given to individuals over the age of 60, as well as to individuals with splenic abnormalities (e.g., sickle cell disease) who are at increased risk for the development of pneumococcal sepsis.

Answer 85

The correct answer is D.Pseudomonas aeruginosa is a common opportunist in burn patients, in whom it classically causes secondary wound infections and septicemia. Pseudomonal infections are also commonly seen in patients on ventilators. It may also cause cystitis in patients with urinary catheters and pneumonia in patients with cystic fibrosis. The organism is found in water and usually gains access to the body as a contaminant in the water used in respirators or in water baths used to cleanse wounds. This organism is a nonfermenter, that is, it does not metabolize sugars by classic pathways. It produces a blue-green, water-soluble pigment (pyocyanin), and has a fruity odor when grown on laboratory media. Escherichia coli(choice A) is a lactose-fermenting, gram-negative rod commonly seen as normal flora of the intestine. It is the most common cause of urinary bladder infections, pyelonephritis, and sepsis in patients with indwelling urinary catheters. It is also the major cause of traveler's diarrhea and is a very important pathogen in neonates who become infected during passage through the birth canal. Klebsiella pneumoniae(choice B) is a gram-negative, highly encapsulated rod that is a significant pulmonary pathogen in individuals with a respiratory compromise. It is a common cause of aspiration pneumonia and pulmonary abscesses in alcoholics and patients with chronic obstructive pulmonary disease. Legionella pneumophila(choice C) is a fastidious, gram-negative respiratory pathogen that may cause either a fulminating disease or a mild "walking pneumonia-like" condition (i.e., an atypical pneumonia). It usually affects older or debilitated patients. Moratella catarrhalis (choice E) is generally seen in patients with preexisting lung disease, elderly patients, and immunosupressed patients. It is a gram-negative diplococcus that produces a patchy infiltrate with occasional lobar consolidation.

Answer 86

The correct answer is B. The patient's clinical presentation is typical of primary atypical pneumonia. Gram stains will often show PMN's and monocytes. No bacteria is seen. In contrast to bacterial pneumonia, primary atypical pneumonia presents with the following features: - Caused by M. pneumoniae; less frequently by viruses (influenza, respiratory syncytial virus, adenovirus, rhinoviruses, rubeola and varicella virus), Chlamydia, or Coxiella burnetii - Characterized pathologically by interstitial, rather than intra-alveolar, inflammation - Characterized clinically by nonspecific symptomatology and few "localizing" symptoms. Why is M. pneumoniae, and not influenza virus (choice A) or respiratory syncytial virus (choice D), the cause of this patient's pneumonia? First, M. pneumoniae infections are often associated with the appearance of cold agglutinins in the serum, detection of which is diagnostically important. Second, the patient responded quickly to treatment with erythromycin, an antibiotic effective against M. pneumoniae, but obviously not effective in treating viral infections. As a side note: the macrolides, such as erythromycin are the drugs of choice for atypical pneumonia. Pneumocystis carinii(choice C) is a fungal organism causing pneumonia in severely immunocompromised hosts, especially AIDS patients. P. carinii pneumonia (PCP) is characterized by accumulation of a frothy exudate containing numerous organisms within alveolar spaces. Also, P. carinii is not sensitive to erythromycin. The drug of choice for treatment of PCP is trimethoprim-sulfamethoxazole. Streptococcus pneumoniae(choice E) is the usual causative agent of lobar pneumonia, characterized by consolidation of a single lobe due to intra-alveolar acute inflammatory exudation. Lobar pneumonia is more prevalent in young, healthy individuals, whereas primary atypical pneumonia favors old, debilitated patients. S. pneumoniae is highly sensitive to penicillin.

Answer 87

The correct answer is A. The patient described has a urinary tract infection. The presence of white blood cell casts specifically implies renal involvement, because these form when the leukocytes are compressed together in the renal tubules. Similarly, the presence of red blood cell casts in a bloody urine would indicate that at least some of the bleeding was occurring in the kidney. White cell casts are seen in pyelonephritis and interstitial nephritis and are indicative of infection or inflammation. Note also that the presence of bacteria merely indicates infection somewhere in the excretory system. Infection of other sites in the urinary tract, including ureters (choice B), bladder (choice C), and urethra (choice D), does not cause cast formation. Although spread of a urinary tract infection to the uterus (choice E) is always of concern in a pregnant woman, the presence of casts does not specifically suggest that this has occurred.

Answer 88

The correct answer is D. Respiratory syncytial virus is the most common cause of bronchiolitis, tracheobronchitis, and pneumonia in children younger than 1 year of age. The average incubation period is 5 days. Outbreaks occur seasonally in winter and early spring. Infection does not result in lasting immunity, and reinfection can occur. In bronchiolitis, proliferation and necrosis of bronchiolar epithelium develop, producing obstruction from sloughed epithelium and increased mucus secretion. Signs often include fever, tachypnea, and wheezes. Hyperinflated lungs and otitis media are frequent conditions. Coronavirus (choice A) causes approximately 20% of common colds (nasal obstruction and discharge, sneezing, no fever or mild fever, occasional sore throat, and cough) and acute pharyngitis (sore throat, with or without cervical adenopathy, ulceration, and conjunctivitis). Influenza type A (choice B) is the leading cause of influenza. Influenza is a systemic illness characterized by the sudden onset of fever, headache, myalgias, malaise, and prostration, followed by cough, nasal obstruction, and sore throat. The lower respiratory tract may also be involved. Parainfluenza viruses (choice C) are the leading cause of croup, or acute laryngotracheobronchitis, in children. This infection involves the upper and lower respiratory tracts. Inflammation in the subglottal area leads to hoarseness, dyspnea, a barking cough, and inspiratory stridor. Rhinovirus (choice E) is the most common cause of the common cold.

Answer 89

The correct answer is C. This is a microbiology question that requires knowledge about the physiology of bacteria. Lysozyme is present in tears, saliva, mucus, vaginal secretions, and several other body fluids. This material lyses the peptidoglycan layer of the cell wall of bacteria without participation of any of the elements of the oxidative killing pathway. Hydrogen peroxide (choice A) is formed in the oxidative killing pathway from the superoxide ions. This material is also very toxic to bacterial pathogens. Hypochlorous acid (choice B) is formed from hydrogen peroxide and halide ions in the presence of myeloperoxidase. Myeloperoxidase (choice D) is an enzyme present in the oxidative killing pathway that breaks down hydrogen peroxide. Superoxide ions (choice E) are formed by NADPH oxidase in the first reaction of the oxidative killing pathway. These superoxide ions are extremely toxic to bacterial organisms.

Answer 90

The correct answer is B. Tissue grafts in which the same individual acts as both the donor and the recipient are termed autografts. Allografts (choice A) refer to tissue transplants from one person to another. Homograft (choice C) is a synonym for allograft. Isografts (choice D) refer to tissue transplants between genetically identical individuals (e.g., an identical twin donates her kidney to her twin sister). Xenografts (choice E) refer to tissue transplants from another species (e.g., baboon heart transplanted into a human).

Answer 91

The correct answer is A. There are more than 40 antigenic types of adenoviruses that produce a variety of symptoms. The incubation period is 4-9 days. Adenoviruses are nonenveloped (naked), icosahedral DNA viruses causing a variety of clinical syndromes. Adenoviruses cause a pharyngoconjunctivitis that affects children and sometimes adults who are living in the same household. Contaminated swimming pools have been implicated as sources for the virus. The virus is latent in the lymphoepithelial tissue of the nasopharynx and other sites. Adenoviruses also cause watery, nonbloody diarrhea. Fever often lasts 2-12 days and is accompanied by malaise and myalgia. Sore throat is often manifested by a patchy exudate and cervical lymphadenopathy. Conjunctivitis is often present. Chlamydia spp. (choice B) produce a variety of clinical syndromes, including a sexually transmitted urethritis, pelvic inflammatory disease, neonatal pneumonia and inclusion conjunctivitis, lymphogranuloma venereum, adult interstitial pneumonia, and a zoonotic pneumonitis. A gramnegative diplococcus (choice C) would be a Neisseria spp. or Moraxella. Neisseria meningitidis would be the logical choice, because it is associated with outbreaks among children. The clinical syndrome caused by infection with the meningococcus, however, is one of a fulminating, progressive septicemia or meningitis with fever, vascular collapse, and disseminated intravascular coagulation. N. gonorrhoeae does not cause a clinical syndrome as described in the question. Moraxella catarrhalis causes pneumonia in patients with chronic obstructive pulmonary disease. Gram-positive enterococcus (choice D) would refer to Group D streptococcus. These organisms are associated with endocarditis and genitourinary infections. Herpesviruses (choice E) are large, enveloped DNA viruses with an icosahedral shape. Possible diagnoses for this patient include Epstein-Barr virus, producing infectious mononucleosis, or cytomegalovirus, producing a mononucleosis-like syndrome. The patients in the question did not have hepatosplenomegaly (characteristic of mononucleosis), but did have keratoconjunctivitis. The organism causing the outbreak in the question also has a higher level of infectivity and a short incubation time.

Answer 92

The correct answer is E. Pneumonia can be caused by a large variety of agents including bacteria, Mycoplasma, fungi, and viruses. Streptococcus pneumoniae, or Pneumococci are classically described as lancet-shaped gram-positive cocci in pairs, or diplococci. Historically, this organism was specifically associated with lobar pneumonia, but with modern rapid antibiotic therapy, a bronchopneumonia pattern may also be seen. Pneumococcal pneumonia remains common in community and hospital-acquired settings. Complications include bacteremia, meningitis, endocarditis, pericarditis, and empyema. Haemophilus influenzae(choice A) is a gram-negative rod, more specifically a pleomorphic gram-negative coccobacilli. Mycobacterium tuberculosis(choice B) is an acid-fast rod. Mycoplasma pneumoniae(choice C) is an intracellular organism and an atypical bacteria. Gram stains typically show PMNs and monocytes but no bacteria. Staphylococcus aureus(choice D) is a gram-positive coccus that grows in grape-like clusters. Complications include empyema and endocarditis.

Answer 93

The correct answer is A. The patient is presenting with cervicofacial actinomycosis. This type of infection commonly follows a tooth extraction or other trauma. Actinomyces are normal inhabitants of the gastrointestinal tract that grow under anaerobic and microaerophilic conditions. Although they are gram-positive rods, they grow as branching filaments and have been confused with fungi. The yellow colonies ("sulfur granules") are found in low-oxygen niches like the tonsils and in actinomycotic abscesses. Cervicofacial actinomycosis often develops slowly. The affected area becomes markedly indurated and the overlying skin becomes reddish or cyanotic. Abscesses draining to the surface may also present. There is little pain unless a secondary infection sets in. Trismus indicates that the muscles of mastication are involved. Aspergillus fumigatus (choice B) may be present in the respiratory tract as an opportunistic pathogen; however, fungus balls are generally seen only in preexisting cavities (e.g., bronchiectasis, TB), not in the tonsils. Blastomyces dermatitidis (choice C) is a respiratory pathogen that is seen as thick-walled yeasts within granulomas. Candida albicans (choice D), also a normal inhabitant of the oral cavity, would present as whitish plaques and would appear microscopically as budding yeasts. When these white plaques are removed, they reveal a painful bleeding lesion. Diphtheria, caused by Corynebacteria diphtheriae(choice E), is a gram-positive rod. The disease presents with a gray-white membrane in the oropharynx, and large colonies would not be noted.

Answer 94

The correct answer is B. The combination of repeated staphylococcal infections, normal phagocytic function, and normal delayed hypersensitivity responses is suggestive of Bruton hypogammaglobulinemia. The disease is characterized by an IgG level less than 100 mg/dL, other immunoglobulins deficient or absent, B cells deficient or absent, intact cellular (T-cell) immunity, and onset of infections after the sixth month of life, when maternal antibodies are no longer present. Bcell- dependent areas, therefore, such as germinal centers, would be absent or greatly diminished in size and number. Neither postcapillary venules (choice A) nor thymus-dependent areas (choice E) are depleted in this disease. The chemotactic response of neutrophils (choice C) is part of the phagocytic function assay, which was normal in the case presented. Because T-cell functions are intact, the architecture of the portions of lymphoid tissues where T cells reside (e.g., paracortical areas, choice D) should be normal.

Answer 95

The correct answer is E. The findings described are those of tabes dorsalis, a form of tertiary syphilis caused by Treponema pallidum. Tabes dorsalis and other forms of tertiary syphilis are now 47 uncommon in this country, possibly because the common use of antibiotics may "treat" many unsuspected cases of syphilis. Late (tertiary) syphilis is associated with infiltrative tumors of skin, bones, and liver (gummas). Aortic regurgitation and CNS disorders (including meningovascular and degenerative changes), shooting pains, and abnormal reflexes are seen. Haemophilus influenzae (choice A) and Neisseria meningitidis (choice D) can cause meningitis. Neisseria gonorrhoeae (choice C) causes gonorrhea, which usually does not involve the CNS. Herpes simplex I (choice B) can cause an encephalitis that typically involves the frontal and temporal lobes. Cardiac and liver involvement, however, are not seen.

Answer 96

The correct answer is A. The symptoms in the question suggest a deficiency known as common variable hypogammaglobulinemia, characterized by very low serum levels of IgG. In this congenital disease, the number of B cells is normal, but their ability to synthesize IgG and the other immunoglobulins is severely compromised, leading to recurrent streptococcal and Haemophilus infections. The onset of the recurrent infections usually begins between 6 and 12 months of age, concurrent with the decreasing levels of maternal IgG in the newborn. Treatment often involves giving intravenous gamma-globulin to reduce the number of infections. There are no known diseases or infections associated with deficiencies in macrophages (choice B) or natural killer (NK) cells (choice C). Furthermore, one would not expect a deficiency in macrophage or NK cell function to present as recurrent bacterial infections, because these cells are primarily involved in cellular, rather than humoral, immunity. Platelets (choice D) are not directly involved in the immune response. Deficiencies in platelets lead to problems in clotting and produce bleeding disorders. T-cell deficiencies (choice E) result in severe viral, fungal, and protozoal infections, rather than recurrent bacterial infections.

Answer 97

The correct answer is B. Infection with hepatitis B virus (HBV) at birth or a very young age is associated with chronic HBV infection and the development of hepatocellular carcinoma later in life. In fact, infants born to hepatitis B surface antigen (HBsAg)-positive mothers are commonly infected, and approximately 90% become chronic carriers of the virus. In chronic carriers, hepatocellular carcinoma develops at an incidence more than 200 times higher than in noncarriers. The current recommendation for infants born of HBsAg-positive mothers is administration of hepatitis B immunoglobulin (HBIg) in the delivery room, with the first dose of the hepatitis B vaccine given at the same time or within 1 week. The second and third doses of the vaccine are then given at 1 and 6 months. With this protocol, 94% protection is achieved. The Epstein-Barr virus (EBV; choice A) is the agent of heterophile-positive infectious mononucleosis. In children, primary EBV infection is often asymptomatic. The measles virus (choice C) often causes a more severe disease in adults. The incidence of complications, including pneumonia, bacterial superinfection of the respiratory tract, bronchopspasm, and hepatitis, is much higher in adults older than 20 than in children. Poliovirus (choice D) causes asymptomatic or inapparent infections 95% of the time. Frank paralysis occurs in approximately 0.1% of all poliovirus infections. However, the probability of paralysis increases with increasing age. Varicella zoster virus (choice E) is the agent of chickenpox and shingles. In immunocompetent children, it is a benign illness with a mortality rate of less than 2 per 100,000 cases. This risk is increased more than 15-fold in adults. Much of the increase is due to varicella pneumonitis, a complication that occurs more frequently in adults.

Answer 98

The correct answer is E.Because of the prevalence of AIDS, it is essential that the dental student be familiar with these concepts. The Western blot is the most appropriate test for confirmation of HIV infection. It identifies several different antibodies against HIV (anti-gp120, -gp41, -p24, and -p17). The Western blot is generally performed when an ELISA test is positive. The initial HIV test this patient had was most likely an ELISA. The ELISA (choice A) can be used to detect p24 antibody in the patient, but is not as specific as the Western blot. The ELISA test is the standard test used in the screening of HIV infections. Fluorescence activated cell sorting (choice B) is a technique used to separate and count specific numbers and types of cells in a sample. An example of this would be to count the number of B cells and T cells in a specific blood sample. RAST testing (choice C) is used to determine the level of specific IgE present in a patient that reacts with a specific allergen that has been applied to a disk or glass bead. Radial immunodiffusion (choice D) is an excellent test used for quantitation of immunoglobulin levels in patients. This is used to determine the IgG, IgM, and IgA levels in patient's serum.

Answer 99

The correct answer is B. The nuchal rigidity (stiff neck) suggests meningitis. Numerous neutrophils in the cerebrospinal fluid suggest a bacterial pathogen. Furthermore, low CSF glucose is common in bacterial meningitis. The best answer of those listed is Escherichia coli, which is normally a gut organism, but can infect neonates who acquire the organism during passage through the birth canal. The common characteristics of bacterial meningitis include: elevated polymorphonuclear neutrophils, low CSF glucose, elevated CSF protein and markedly elevated opening pressure. Coxsackievirus (choice A) is a cause of acute lymphocytic meningitis. Herpes virus (choice C) is a cause of lymphocytic meningitis. Mycobacterium tuberculosis(choice D) is a cause of chronic meningitis and is the causitive pathogen in tuberculosis. Neisseria meningitidis(choice E) causes bacterial meningitis, usually in the second or third decade of life. It is rare to see this bacteria in infants.

Answer 100

The correct answer is C. NBDE Part 1 questions often involve antibiotic mechanisms. Fluoroquinolone antibiotics, such as ciprofloxacin and lomefloxacin, inhibit DNA-gyrase, which is an enzyme necessary for bacterial DNA replication and repair. The fluoroquinolones are generally not used for dental procedures because they have very poor anaerobic coverage. They have very good 49 gram-negative coverage, however. These agents are commonly used in the treatment of upper and lower respiratory infections and in genitourinary tract infections. Sulfonamide antibiotics exert their antibacterial effect through the competitive inhibition of para-amino-benzoic acid (PABA) (choice A), thereby inhibiting folic acid biosynthesis required for bacterial growth. Cephalosporins (cephalexin, cefuroxime, and cefixime) are believed to exert their antibacterial effect through the inhibition of bacterial cell wall synthesis (choice B). These agents bind to one or more of the penicillin-binding proteins located on the cell walls of susceptible organisms which results in the inhibition of the third and final stage of bacterial cell wall synthesis. These effects account for the bactericidal effect of cephalosporins and their cousins the penicillins. Aminoglycosides, such as gentamicin and streptomycin, irreversibly bind the 30S subunit of bacterial ribosomes (choice D), which inhibits bacterial protein synthesis. Lincosamides, such as clindamycin, irreversibly bind to the 50S subunit of bacterial ribosomes (choice E), which suppresses bacterial protein synthesis. Note: macrolides, such as erythromycin, also bind to the 50S subunit of bacterial ribosomes, which suppresses bacterial protein synthesis.

Answer 101

The correct answer is B. The concomitant administration of bacteriostatic and bactericidal antibiotics would result in antagonism of the antibacterial effects of both agents. The reason for the antagonism between the two is that each agent interferes with the mechanism of action of the other agent. By definition, a bacteriostatic agent is one that prohibits the growth and development of bacteria and a bactericidal agent is one that directly causes the death of a microbe during the growth and development stages. An extension of bacterial coverage between two agents (choice A) is typically seen when two bacteriostatic or two bactericidal agents are given together. The administration of a macrolide and a cephalosporin may cause some discomfort for the patient, such as nausea or stomach cramps; however, profound toxicity for the patients taking both agents (choice C) is very unlikely. Synergism (choice D) is the correlated action of two agents that results in a final effect that is greater than that of each agent acting separately. A classical example of synergism is seen when an aminoglycoside, such as gentamicin, is administered concomitantly with extended spectrum penicillin, such as piperacillin, for the treatment of a pseudomonas infection. The appearance of mild antifungal activity (choice E) would not be seen when bacteriostatic and bactericidal antibiotics are given together, if neither have any antifungal activity.

Answer 102

The correct answer is choice D. The problem of bacteria in dental waterlines is probably going to be the next intensive infection control issue faced by dentistry. The narrow bore of most dental waterlines encourages a thin film colony of bacterial slime (biofilm) to form on the inner circumference of the waterline. Water entering the dental unit, although chlorinated at the source, is clean, but not sterile. The microorganisms it contains can form extensive colonies inside the tubes. While no specific disease cases have been linked to these microorganisms, it is clear that they should be minimized. Several systems of filtration or chemical disinfection are now available. Other alternatives include bottled irrigation water and flushing the lines for several minutes several times each day.

Answer 103

The correct answer is A. Bacillus cereus contaminates grains, such as rice, and produces spores resistant to quick frying and steaming. If you see the words fried rice on the NBDE, the odds are that the correct answer is B. cereus. Campylobacter jejuni (choice B) causes enterocolitis with bloody diarrhea, crampy abdominal pain, malaise, and fever. Clostridium botulinum (choice C) produces a constellation of signs and symptoms, including bulbar palsy, descending weakness or paralysis, progressive respiratory weakness, absence of fever, dry mucous membranes, and autonomic dysfunction. Clostridium difficile (choice D) causes pseudomembranous colitis, classically resulting from clindamycin use. Escherichia coli (choice E) comes in a variety of forms. The enterotoxigenic type is the most common cause of traveler's diarrhea.

Answer 104

The correct answer is E. This presentation (fine, nonblotchy, truncal rash in a not-very-ill child) is characteristic of rubella, or German measles, that is caused by a togavirus. The rash typically spreads from trunk to extremities. IgM specific for rubella can often be detected in serum within 1-2 days of developing the rash. The principal significance of this disease is that it can cause a devastating congenital infection characterized by ocular problems (cataracts, retinopathy, microphthalmos, glaucoma), cardiovascular problems (patent ductus arteriosus, ventricular septal defect, pulmonary stenosis), deafness, thrombocytopenic purpura, leukopenia, hepatosplenomegaly, CNS problems, and bony lesions. Arthralgia is seen, particularly in women. Exposure occurs 14-21 days before onset. Bullous pemphigoid (choice A) produces large, tense blisters in flexural areas that appear with exacerbations and remissions. Dermatitis herpetiformis (choice B) causes recurrent crops of small vesicles or papules that appear mainly on the elbows, knees, buttocks, posterior neck, and scalp. Herpes simplex (choice C) is characterized by crops of vesicles on oral or genital sites, where herpes simplex I causes oral and perioral manifestations and herpes simplex II causes genital lesions. Measles (choice D) causes a blotchy, maculopapular erythematous rash that begins on the face and spreads downward. Patients with measles are usually much sicker than those with German measles, and Koplik spots may be seen on the buccal mucosa.

Answer 105

The correct answer is D. There is a specific association between diabetes (particularly in brittle diabetics who may have episodes of ketoacidosis) and chronic sinusitis caused by saprophytic Zygomycetes, including Mucor and Rhizopus. These fungi can spread rapidly from the sinuses to the nearby skull bones and brain, potentially causing massive tissue destruction and death. The term "rhinocerebral mucormycosis" is used in these cases. Less commonly, other sites may be involved (lung, gastrointestinal tract), depending on the port of entry. The physician should suspect mucormycosis in any patient with chronic sinusitis who appears unusually ill and does not respond to antibiotic therapy. Unfortunately, most cases are diagnosed at autopsy. Actinomyces (choice A) are part of the normal flora of the mouth. Actinomycosis also occurs in humans and may affect the cervicofacial region (typically following dental procedures or maxillofacial injuries), lungs, abdomen (typically following surgery, trauma, or intestinal penetration), or pelvis (related to IUD use). There is no specific association with diabetes mellitus. Aspergillus (choice B) can be present in the sinuses, 51 and does have somewhat increased incidence in patients with diabetes, but it is not the organism about which the physician should be most concerned. Cryptococcus (choice C) is found in pigeon feces and is usually introduced into the body by way of the respiratory tract. It can disseminiate to the meninges and other sites in immunocompromised patients (often AIDS patients). It would not be of particular concern in this patient with sinusitis. Pneumocystis carinii (choice E) causes pneumonia in severely immunosupressed patient ( e.g., patients with AIDS).

Answer 106

The correct answer is E. Although all five pathogens can cause central nervous system (CNS) manifestations, toxoplasmosis (caused by Toxoplasma gondii) presents with seizures in 15-25% of cases. It is the most common cause of secondary CNS infections in AIDS patients. The disease is spread by ingestion of cysts from undercooked meat or from cat feces. Tachyzoites develop from cysts phagocytized by macrophages, then spread to the brain, muscle, and other tissues, where they encyst and multiply. Cryptococcal meningitis (choice A) usually presents as a subacute meningitis with headache, nausea, vomiting, and confusion. Cranial nerve abnormalities are common with cryptococcal infections. Herpes simplex encephalitis (choice B) typically has a subacute onset with headache, meningismus, and personality changes. Mycobacterium tuberculosis(choice C) is a cause of basilar meningitis, which can present insidiously with headache and mental changes over a week or two, or can present acutely as confusion, lethargy, altered sensorium, and a stiff neck. Cranial nerve palsies, focal cerebral ischemia, and hydrocephalus are characteristic. Isospora belli(choice D) is a leading cause of diarrhea in AIDS patients. It typically occurs when the CD4 count is less than 300/uL.

Answer 107

The correct answer is A. All the agents listed can infect the eyeball. The agent specifically associated with contact lens use is Acanthamoeba, which can infect lens solution. This amoeba is dangerous because it causes an intractable ulcerative keratitis that may progress to uveitis. If the lesion is suspected, the clinical laboratory should be notified and specific directions for collecting samples for culture obtained. The parasites may be difficult to see in histologic sections or corneal scrapings. Cytomegalovirus (choice B) and herpes (choice C) infections are most often seen in immunocompromised patients, particularly AIDS patients. Circulating larvae of the helminth Toxocara(choice D) can lodge in the eye (particularly in the vitreous or retina); Toxocara infections are seen more commonly in children. Toxoplasmosis (choice E) of the eye is most often congenital, but it can be acquired.

Answer 108

The correct answer is A. Epiglottitis is the most common disease of the upper respiratory tract produced by Haemophilus influenzae type b, a gram-negative encapsulated rod. Epiglottitis is characterized by an abrupt onset of high fever, drooling, and inability to handle secretions. Stridor and respiratory distress result from laryngeal obstruction. The epiglottis is described as being cherry red and swollen. H. influenzae is also a common cause of otitis media in children and may cause bronchitis, bronchiolitis, and pneumonia in adults. The incidence of serious disease caused by Haemophilus influenzae type b decreased greatly with the introduction of an effective vaccine, which is composed of the H. influenzae type b capsular polysaccharides coupled to a carrier molecule and given to children from 2-15 months of age. The patient had not received the Hib conjugate vaccine and therefore was susceptible to this organism. Klebsiella pneumoniae(choice B) causes pneumonia and pulmonary abscesses, but is not considered to be a pathogen in the upper respiratory tract. Klebsiella is a gram-negative encapsulated rod. Legionella pneumophila(choice C) causes pneumonia in humans. The disease may be mild (an atypical pneumonia) or a fulminating disease with high mortality (30%). On a gram stain, one would see no bacteria because it is an "atypical" organism. Mycoplasma pneumoniae(choice D) causes community-acquired atypical pneumonia. It is the most common cause of pneumonia in young adults. On a gram stain, one would see no bacteria because it is an "atypical" organism. Streptococcus pyogenes(choice E) is the most common cause of pharyngitis; however, this patient's presentation strongly suggests epiglottitis. It is a gram-positive organism.

Answer 109

The correct answer is B. Type II hypersensitivity occurs when antibodies react with antigens present on the surface of cells or other tissue components. It is conveniently subclassified into diseases produced by three distinct mechanisms. One mechanism involves complement fixation by the antibody-antigen complex, which facilitates phagocytosis; this mechanism occurs in transfusion reactions, erythroblastosis fetalis, autoimmune hemolytic anemia, agranulocytosis, or thrombocytopenia. In a second form of type II hypersensitivity, target cells coated with low levels of IgG are lysed (without phagocytosis) by monocytes, neutrophils, or natural killer cells; this mechanism is thought to operate in the destruction of large parasites, possibly some tumor cells, and in graft rejection. The third mechanism involves antibody-mediated cellular dysfunction, such as occurs in Graves disease or myasthenia gravis. Allergic rhinitis (choice A) is an example of type I (allergic or anaphylactic) hypersensitivity. Food allergy (choice C) is an example of type I (anaphylactic) hypersensitivity. Serum sickness (choice D) is an example of type III (immune complex diseases) hypersensitivity. Tuberculosis (choice E) is an example of type IV (cell-mediated or delayed) hypersensitivity

Answer 110

The correct answer is E. This is the classic microscopic description of the pneumococcus Streptococcus pneumoniae, which is a common cause of lobar pneumonia. Most strains are still sensitive to penicillins, although some drug-resistant strains have been isolated. Haemophilus influenzae (choice A) is a gram-negative bacillus. Atypical strains may cause pneumonia in elderly patients with chronic respiratory disease. Neisseria gonorrhoeae (choice B) is a gram-negative diplococcus that is not typically associated with pneumonia. Pneumocystis carinii (choice C) is a small, hat-shaped fungus that is a common cause of pneumonia in HIV-positive patients.Staphylococcus aureus (choice D) occurs as grapelike clusters of large, gram-positive cocci. It may cause pneumonia after surgery or after a viral respiratory infection, such as influenza, and is associated with empyema formation.

Answer 111

The correct answer is A.Borrelia burgdorferi, the tick-transmitted spirochete that causes Lyme disease, can be maintained in nature indefinitely by a tick vector. The tick, Ixodes dammini, can infect white-footed mice and large mammals such as deer during its life cycle, making these animals into reservoirs. The tick itself, however, is a reservoir because it acquires the disease through transovarial passage of the organism. Together these factors make Lyme disease an endemic infection with little hope for eradication. B. burgdorferi is a delicate spirochete that is vulnerable to several chemical and physical agents. It does not have a tough outer coat (unlike choice B). Humans are incidental hosts, not primary reservoirs (choice C), for B. burgdorferi. The primary reservoirs are ticks, mice, and large mammals. B. burgdorferi can be successfully treated with penicillins, tetracycline, and ceftriaxone (unlike choice D). Brill-Zinsser disease (choice E) is actually the reactivation of epidemic typhus infection caused by Rickettsia prowazekii. It can occur many years after an infection that was not treated with antibiotics.

Answer 112

The correct answer is D. Pasteurella multocida is a gram-negative rod that is normal flora of the oral cavity of dogs and cats. It often causes a local abscess following introduction under the skin by an animal bite. Most cases occur in children who are injured while playing with a pet. Note that penicillin G is not highly effective against gram negative rods. Bartonella henselae (choice A)is a very small gram-negative bacterium that is closely related to the rickettsia. It is the cause of cat-scratch disease (a local, chronic lymphadenitis most commonly seen in children) and bacillary angiomatosis (seen particularly in AIDS patients). In this latter patient population, the organism causes proliferation of blood and lymphatic vessels, causing a characteristic "mulberry" lesion in the skin and subcutaneous tissues of the afflicted individual. Brucella canis(choice B) is a gram-negative rod that is a zoonotic agent. Its normal host is the dog, but when it gains access to humans, it causes an undulating febrile disease with malaise, lymphadenopathy, and hepatosplenomegaly. The normal route of exposure is by way of ingestion of the organism. Clostridium tetani (choice C) is a gram-positive spore-forming anaerobic rod. It causes tetanus (a spastic paralysis caused by tetanospasmin, which blocks the release of the inhibitory neurotransmitters glycine and gamma-aminobutyric acid [GABA]). There may be no lesion at the site of innoculation and exudation would be extremely rare. Toxocara canis (choice E), a common intestinal parasite of dogs, is a metazoan parasite that causes visceral larva migrans. Young children are most likely to be affected, as they are most likely to ingest soil contaminated with eggs of the parasite.

Answer 113

The correct answer is B. Viral meningitis is relatively common, accounting for 10,000 cases of meningitis per year in the United States. The vast majority of cases occur in individuals younger than 30 years of age. Usually the symptoms are relatively mild and death is uncommon. Enteroviruses, arboviruses, and type 2 herpes simplex virus are the most common causes of viral meningitis. Also, up to 10% of HIV patients develop an acute meningitis, typically at the time of seroconversion. Adenovirus (choice A) infection is associated with upper respiratory tract infections (URIs), sinusitis, ocular disease, enteric infections, and bladder infections. It does not typically cause aseptic meningitis. Human papillomaviruses (choice C) are associated with warts on the skin and genital areas. Poxviruses (choice D) include the causative agents of smallpox, cowpox, and molluscum contagiosum. Reoviruses include the most common family member, rotavirus, responsible for gastroenteritis in young children. These agents do not typically cause meningitis.

Answer 114

The correct answer is B. Anaerobic bacteria exist as part of the normal flora on the mucosal surfaces (the mouth, gastrointestinal tract, skin, and female genital tract) of humans and animals. Anaerobic infections occur when the normal balance between aerobic and anaerobic bacteria is disrupted.The two medications primarily used for the treatment of anaerobic infections are clindamycin and metronidazole. Clindamycin is typically used in the treatment of serious anaerobic infections when other less toxic medications cannot be used. A potential serious side effect is the onset of pseudomembranous colitis. Metronidazole is a bacterial agent that is indicated for the treatment of serious infections caused by anaerobic bacteria and is active against most protozoa. It is primarily used for the treatment of gynecologic and abdominal infections and can be used for the treatment of antibiotic-induced pseudomembranous colitis. The fluoroquinolones, such as ciprofloxacin (choice A), have very poor anaerobic coverage. Ciprofloxacin therefore would be relatively ineffective for the treatment of an oral infection caused by an anaerobic infection. These agents are commonly used in the treatment of upper and lower respiratory infections and in genitourinary tract infections. Tetracycline (choice C) is primarily used in the treatment of chlamydial and mycoplasmal infections, gonorrhea, and acute exacerbations of chronic bronchitis. Tobramycin (choice D) is an intravenous antibiotic typically reserved for the treatment of serious and lifethreatening gram-negative bacterial infections and some gram-positive infections. Vancomycin (choice E) is also typically reserved for the treatment of serious and life-threatening bacterial infections; however, it is used primarily in the treatment of gram-positive infections.

Answer 115

The correct choice is B. The presence of elevated ALT, HBsAg, anti-HBc, HBeAg, and bilirubin all point to active hepatitis B. An asymptomatic carrier (choice A) would not have elevated ALT and bilirubin. The absence of other symptoms rules out fulminant hepatitis B (choice C). Recovery from acute self-limited HBV (choice D) is associated with the presence of anti-HBs and a decrease in HBsAg and HBeAg. Someone who is vaccinated with HBV (choice E) has only anti-HBs in serum.

Answer 116

The correct answer is D. Toxic Shock Syndrome (TSS), a multisystem syndrome caused by a toxin (TSST-1) formed by certain strains of S. aureus. TSS usually affects several organ systems (gastrointestinal, renal, hepatic, hematopoietic, musculoskeletal, pulmonary) and can result in death. TSS historically has been associated with the use of tampons (as well as cervical cups and diaphragms) in young women, but also can occur in other patient populations. Symptoms often have a very abrupt onset, including fever, hypotension, diarrhea, and diffuse rash with desquamation of the hands and feet. Blood cultures are negative, because sypmtoms are caused by the toxin not the invasive organism. Management of shock, renal failure, and adult respiratory distress syndrome (ARDS) are a priority if these conditions are present, in addition to appropriate antibacterial treatment. C. perfringens (choice A) causes gas gangrene, with necrosis of soft tissues, usually after a traumatic wound. It is also a cause of food poisoning. Although HIV-1 (choice B) can cause many diverse findings and should never be immediately ruled out, the findings in this patient are most specific for TSS. Shigella dysenteriae(choice C) is a cause of dysentery characterized by fever, abdominal cramps, and bloody diarrhea. S. epidermidis (choice E) is part of the normal skin flora, but is notorious for causing infections of intravenous lines and prosthetic heart valves.

Answer 117

The correct answer is E. Candidiasis can appear in any area of the oral mucosa. One of the most common causes of candidiasis in the adult population is the use of oral inhalation corticosteroids, such as fludrocortisone. The pseudomembranous form typically appears with white patches that can be easily wiped off leaving red, bleeding, sore surfaces. There is also an erythematous form that presents with flat red and white lesions that cannot be rubbed off. Based on the description of the signs and symptoms in this question, the patient has the pseudomembranous form of oral candidiasis or oral thrush. Oral candidiasis responds very well to antifungal therapy. Clotrimazole and nystatin are antifungal agents used locally for treatment of infections caused by many different Candida species. Clotrimazole Troches are the generic name for the brand name product Mycelex Troches. A troche is essentially an orally disintegrating tablet that dissolves in the mouth and is generally used in adults with candidiasis. Acyclovir (choice A) is an antiviral agent used in the treatment of infections caused by herpes simplex virus types 1 and 2 and varicella-zoster virus. This agent would be indicated in individuals with herpes zoster infections that typically appear as vesicular eruptions and/or ulcers on the cheek, tongue, gingival or palate. Both ampicillin (choice B) and cephalexin (choice C) are betalactam antibiotics used to treat a number of bacterial infections. Both agents would be ineffective in fungi. Clarithromycin (choice D) is a macrolide antibiotic that may be safely and effectively used for the treatment of bacterial pharyngitis, as well as various other infections caused by susceptible organisms. It is also used in SBE prophylaxis for dental procedures.

Answer 118

The correct answer is B. One of the most efficient mechanisms for eliminating extracellular pathogenic bacteria is by opsonization and phagocytosis by macrophages. The IgG and IgM antibody produced in response to the organism reacts with the capsular structure, stimulating the activation of the classic pathway of the complement system. This pathway produces large amounts of C3b that 56 coat the organism, preparing it for phagocytosis. ADCC cells (choice A) are actually natural killer (NK) cells that find virally infected cells and tumor cells that have been coated with antibody and react with and destroy them. These cells do not destroy antibody-coated bacteria, only body cells that are coated with antibody. Cytotoxic T lymphocytes (choice C) react only with cells that have antigen epitopes presented in association with class I MHC molecules. An example would be a virus epitope from a virally infected cell presented by class I molecules on the surface of the cell. LAK cells (choice D) are NK cells that have been activated by IL-2. They are considered to be superactivated NK cells. NK cells (choice E) are cells of the innate immune system that destroy virally infected cells or tumor cells. This does not involve antibody and it does not involve extracellular pathogens.

Answer 119

The correct answer is D. The specific clue to the cause of the severe encephalitis in this patient is the history of shingles, caused by reactivation of the herpes zoster-varicella virus. In otherwise healthy adults, the virus (which is usually introduced to the body as a childhood case of chickenpox) remains dormant in a dorsal root ganglion, only to reactivate in later life as a painful dermatomal vesicular eruption. In patients with AIDS, the virus can cause a severe, multifocal encephalitis that may be resistant to acyclovir therapy. Cytomegalovirus (choice A), Herpes simplex type I (choice B) and herpes simplex type II (choice C), can cause disseminated disease (including brain infection) in AIDS patients, but is less likely in this patient, given the past history of shingles and the lack of response to acyclovir. Measles virus (choice E) appears to be related to subacute sclerosing panencephalitis, but this condition is not specifically increased in AIDS patients.

Answer 120

The correct answer is E. M. tuberculosis is a non-spore-forming thin aerobic bacteria. Once stained the bacilli are not discolorized by acid alcohol; hence the term acid-fast. Mycobacteria are also considered "acid fast" because they have an envelope that contains large amounts of lipids and even true waxes (unlike envelopes of other types of bacteria) that prevents the acid-fast stain (carbolfuchsin) from leaking out. The other characteristics listed in the answers are commonly tested features of specific bacteria: Nutritional requirement for factors V and X (choice A) is a feature of Haemophilus influenzae. Streptokinase (choice B) is produced by Streptococci. Toxic shock syndrome toxin (choice C) is a feature of Staphylococcus aureus. Visibility under dark field illumination (choice D) is a feature of the syphilis organism Treponema pallidum. Furthermore, T. pallidum does not grow on artificial media and therefore cannot be cultured in a laboratory.

Answer 121

The correct answer is B. Any time you see the development of diarrhea in the same question stem with "treated with antibiotics," you should immediately think of pseudomembranous colitis. This 57 condition is caused by Clostridium difficile and typically occurs as a result of treatment with clindamycin or ampicillin. You would confirm your suspicion by sending a stool culture to be tested for the presence of the C. difficile toxin. The most test-worthy side effect of chloramphenicol (choice A) is aplastic anemia, not diarrhea. In addition, you might have been able to eliminate this choice simply because of the extremely low probability that this patient would receive this antibiotic in the United States. The key side effects of gentamicin (choice C) include ototoxicity and nephrotoxicity. Metronidazole (choice D) and vancomycin (choice E) do not cause pseudomembranous colitis; they are used to treat it.

Answer 122

The correct answer is C. The clinical and radiologic characteristics of this patient's condition are consistent with lobar pneumonia, a respiratory infection that, in its classic presentation, involves a single pulmonary lobe. In 90-95% of cases, the etiologic agent is Streptococcus pneumoniae (AKA pneumococcus), an alpha-hemolytic streptococcus present in the throat of 40-70% of healthy individuals. This organism is best described as a gram-positive diplococcus. Penicillin is the drug of choice for pneumococcal pneumonia, but sputum cultures are necessary to identify the infectious agent and determine its antibiotic sensitivity.S. pneumoniae is a major cause of purulent meningitis in the elderly. Other alpha-hemolytic streptococci (viridans streptococci) cause subacute endocarditis in patients with previously altered cardiac valves. Bacteria belonging to the genera Haemophilus(choice A) and Klebsiella (choice B) cause respiratory infections; however, in immunocompetent individuals, these bacterial infections usually result in bronchopneumonia, which leads to multilobar, and often bilateral, pulmonary involvement. Beta-hemolytic streptococci (choice D) cause countless infections in humans. Recall that classification of beta-hemolytic streptococci is based on their surface antigens known as Lancefield antigens. Human diseases caused by this group of bacteria include: - Streptococcus pyogenes (group A): pharyngitis, scarlet fever, erysipelas, impetigo, rheumatic fever, and glomerulonephritis. - Streptococcus agalactiae (group B): neonatal sepsis and urinary infections. - Enterococcus faecalis (group D): endocarditis and urinary infections. Respiratory involvement is rarely seen. Gamma-hemolytic streptococci (choice E) are streptococci that do not produce hemolysins (nonhemolytic streptococci) and are not a significant cause of human disease.

Answer 123

The correct answer is D. Staphylococcal meningitis is fairly rare, occurring mostly in patients with indwelling ventricular-peritoneal shunts. Staphylococcus epidermidis, a coagulase-negative organism that normally colonizes the skin, is the most common organism causing this disorder; Staphylococcus aureus meningitis occasionally occurs. Bacteroides fragilis(choice A) is a common cause of anaerobic infections, including sepsis and peritonitis, but it does not commonly cause meningitis. Diphtheroids (including Corynebacterium diphtheriae, choice B) are sometimes isolated from indwelling shunts, but in many cases they are simply contaminants, because some diphtheroids are normal skin flora. Escherichia coli(choice C) is isolated from 30-50% of neonates with bacterial meningitis, but is not typically associated with shunt infections. Streptococcus pneumoniae(choice E) is the most common cause of bacterial meningitis in people older than 30 years of age; it is much less common in children.

Answer 124

The correct answer is D. Rubella, or German measles, is a disease caused by a Togavirus, which is a small, enveloped, single-stranded, (+) linear RNA virus. Approximately 40% of patients are asymptomatic or have mild symptoms. In symptomatic patients, the clinical presentation typically consists of an erythematous rash beginning on the head that spreads downward to involve the trunk, lasting for approximately 3 days. In addition to the rash, symptoms include fever, malaise, and arthralgias coinciding with the eruption. Leukopenia also may be seen. Enteroviral rashes may mimic rubella and rubeola. Infectious mononucleosis (choice A) is caused by the Epstein-Barr virus, a herpesvirus. Classic findings include fever, exudative pharyngitis, generalized lymphadenopathy, severe malaise (most common complaint), and hepatosplenomegaly. A rash is not a characteristic feature unless the patient has been treated with ampicillin. This is a self-limited disease that is rarely fatal unless splenic rupture occurs. Lyme disease (choice B) is caused by the spirochete Borrelia burgdorferi. The disease is transmitted by the bite of the tick, Ixodes dammini. Reservoirs in nature include deer and mice. The initial lesion is an annular rash with central clearing and a raised red border (erythema chronicum migrans) at the bite site. The rash is warm, but not painful or itchy. Patients also have fever, malaise, myalgias, arthralgias, headache, generalized lymphadenopathy, and, occasionally, neurologic findings. Roseola (choice C) is caused by human herpesvirus 6. Other names include exanthem subitum or sixth disease. Children have a febrile period of 3-5 days with rapid defervescence followed by an erythematous maculopapular rash lasting 1-3 days. Rubeola (choice E), or measles, is caused by a paramyxovirus. Patients present with an upper respiratory prodrome and characteristic oral lesions (Koplik spots) that precede the rash. The nonpruritic maculopapular rash begins on the face and spreads to the trunk and extremities, including palms and soles. The incubation period is 10-14 days. Patients also have a posterior cervical lymphadenopathy. The virus is not associated with risk to a fetus.

Answer 125

The correct answer is A. The patient has a severe, potentially fatal, pneumonia with prominent systemic symptoms. Culture revealing a coccobacillus is the specific clue that the organism is Legionella pneumophila. The disease is respiratory Legionellosis, also known as Legionnaire disease. Patients tend to be older (40-70 years of age) and may have risk factors including cigarette use, alcoholism, diabetes, chronic illness, or immunosuppressive therapy. The bacteria can also be seen in the sputum with direct fluorescent antibody staining. Listeria monocytogenes(choice B) causes listeriosis and is not a notable cause of pneumonia. Pseudomonas aeruginosa (choice C) is a gram-negative rod that is a cause of pneumonia in patients on ventilators and those with cystic fibrosis. Staphylococcus aureus(choice D) can cause pneumonia, but is a plump gram-positive cocci in clumps. Streptococcus pneumoniae(choice E) can cause pneumonia, but is a gram-positive diplococci.

Answer 126

The correct answer is A. This is a good microbiology question that uses the principles of bacterial physiology. Unlike eukaryotic cells in which the electron transport system is located on mitochondria, the electron transport system of bacteria is located on the cytoplasmic (plasma) membrane. Mesosomes (choice B) and polyribosome DNA aggregates (choice E) are also found in bacterial cells and function in cell division and protein synthesis, respectively. Bacteria do not have mitochondria (choice C) or membrane-bounded nuclei (choice D).

Answer 127

The correct answer is C. The clinically suspected diagnosis is meningitis, which is confirmed by the abnormal cerebrospinal fluid. The markedly increased lymphocytes suggests acute lymphocytic meningitis, which is distinguished from acute pyogenic meningitis (increased neutrophils as well as lymphocytes). Acute lymphocytic meningitis is usually viral in origin. Among the many viruses that have been implicated, mumps, herpes, Epstein-Barr, echovirus, and Coxsackie virus are the most common. One of the easiest ways to differentiate between a viral and bacterial meningitis is the appearance of the cerebral spinal fluid. A viral meningitis will not generally change the color of the CSF. However, a bacterial meningitis often produces a cloudy CFS. Escherichia coli(choice A) and Haemophilus influenzae(choice B) cause acute pyogenic meningitis and would be expected to produce a cloudy cerebral spinal fluid. Mycobacterium tuberculosis(choice D) and Treponema pallidum(choice E) cause chronic meningitis and would be expected to produce a cloudy fluid.

Answer 128

The correct answer is choice B. A Dane particle is the name for the entire Hepatitis B virion (viral particle). It is one of three entities that will test positive as Hepatitis B surface antigens. The other two are pieces of viral coats known as spheres or filaments, which, although they are not complete viral particles, they do contain the surface antigen (HBsAg). Other hepatitis B antigens include the core antigen (HBcAg) and an "e-antigen" (HBeAg). The "e-antigen" is associated with increased viral replication and infectivity of the patient.

Kaplan Micro Flashcards

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