Biochemistry

Question 1

A newborn presents with severe acidosis, vomiting, hypotonia, and neurologic deficits. Laboratory analysis reveals elevated levels of lactate and alanine. These observations suggest a deficiency of which of the following enzymes?

Answer 1

Pyruvate dehydrogenase

Pyruvate dehydrogenase (PDH) catalyzes the irreversible conversion of pyruvate to acetyl-CoA. If PDH is absent, pyruvate will be used in other pathways instead. Pyruvate will be converted to alanine via alanine aminotransferase and to lactate via lactate dehydrogenase.

Glutamate dehydrogenase is involved in oxidative deamination, releasing ammonium ion for urea synthesis. Deficiency of this enzyme would not cause the symptoms described.

Phenylethanolamine N-methyltransferase is an enzyme involved in the synthesis of epinephrine.

Question 2

Several members of a family have an autosomal recessive disease characterized by intellectual deterioration, weakness, ataxia, seizures, and death at a young age. Deficiency of cytochrome C oxidase activity. Which of the following subcellular organelles is defective in afected members of this family?

Answer 2

Mitochondria

This is leighs disease, a rare condition also known as subacute necrotizing encephalomyelopathy. The underlying problem is a defective form of cytochomre C oxidase, an electron transport chain component, in the mitochondria of muscle and brain. Clinically, the features described in the quesiton stem are seen. Pathologically, there is a symmetric necrosis that affects central areas of the nervous system rom the thalamus to the spinal cord. No effective treatment exists at this time.

Question 3

17 y/o missed last menstrual period. She has sickle-cell trait and mild asthma but otherwise healthy. bf also has sickle-cell trait. Determined to contiue pregnancy if pregnant. Main cocern is whether her child will have sickle-cell anemia. What is the chance of baby being sickle-celled if both parents have it.

Answer 3

25%

Sickle cell anemia arises due to the mutation of the beta-globin gene. Because both of the beta-globin genes have to be abnormal to produce the disease, it is an example of autosomal recessive. When boht parents are carriers o the abnormal gene the risk of having the disease with two abnormal genes is 25%. The probability of having a child who is a carrier one abnormal gene and one sickle is 50%. Child normal with no abnormal gene is 25%

Question 4

prokaryotic operon codes for two enzymes and one regulatory protein. The operon is expressed only in the presence of a particular sugar. Initial mapping of the operon has differentiated the nontranscribed sequences from the transcribed sequences. Deletion of 10 nucleotides from one of the nontranscribed sequences results in transcription of the operon in both the presence and absence of the sugar. Which of the following sequences was most likely affected?

Answer 4

Operator

The description of the operon expression pattern identifies it as an iducible operon. After the deletion, the operon is expressed constitutively. This pattern can occur only if the deletion removes a negative regulator, i.e. a repressor protein, or the DNA sequence to which it binds, called an operator. Since the questions stipulates a nontranscribed DNA sequence the operator must have been affected.

Question 5

Answer 5

Two codon deletion

The RT-PCR product from the patients reticulocytes is 6 nucleotides shorter than the RT-PCR product from normal reticulocytes. This sex base pair deletion would be consistent with removing two codons from the mRNA .

Question 6

36 y/o greek man with viral pneumonia hs a self-limiting episode of hemolysis. Over the next week, he has an increased rate of reticulocytosis. Which of the following compounds serves as a precursor to heme in the reticulocytes?

Answer 6

Succinyl-CoA

The porphyrin ring of heme is derived rom the citric acid cycle intermediate succinyl-CoA

And the amino acid glycine. The initial synthetic step, which is rate-limiting, is catalyzed by aminolevulinic acid synthase (ALA synthase).

Rest dont play a role in the heme.

Question 7

22 y/o woman presents with a fusiform swelling of the achilles tendon, which, when biopsied, shows cholesterol-laden macrophages (foam cells) dispersed among the collagen fibers. She had been troubled with joint pains for several years. Both her mother and father developed arthritis associated with production of xanthomas, but their first symptoms occured in middle age. She was referred to a nutritionist and treated with an enzyme inhibitor. Which of the following is most likely elevated in the blood of this woman?

Answer 7

Low density lipoproteins (LDL)

Deposition of xanthomas around the achilles tendon is characteristic of familial type IIa hyperlipidemia, and autosomal dominant deficiency of the LDL receptor. The resultant reduced rate of LDL clearance leads to elevated LDL levels and hypercholesterolemia. For some poorly understood reason, arthritic pain in various joints, often prior to any appearance of xanthomas,, is a characteristic of familial type IIa hyperlipidemia manifestation. Although achilles tendonitis with associated xanthomas is common, other tendons may be involed. Symptoms in the heterozygous patient usually do not occur until the third or fouth decase, but homozygotes are affected much earlier, oten in childhood.

The patient was most likely treated with an HMG CoA reductase inhibitor such as simvastatin. Her condition carries a strong risk for cardiovascular disease. Although xanthomas are common in other familial hyperlipemias, such as type III and hepatic lipase deficiency, these are not associated with achilles tendonitis.

Question 8

23 y/o 8th month of pregnancy. First child born at home and breast fed had prolonged diarrhea and died form an intracranial hemorrhage at 1 month of age. Resident gives her a free prescription for a vitamin and advises her to take one 20mg tablet each day to make this not happen. Also informs infant should receive an injection of this vitamin soon after birth. The vitamin prescribed is required as a coenzyme by which of the following enzymes?

Answer 8

y-Glutamyl carboxylase

Late onset vitamin K deficiency bleeding can occur between 1 and 6 months after birth. Risk factors include exclusive breast feeding because human milk is low in vitamin K. In hospital settings infants are given injections of vitamin K after birth, but the irst child of this patient was born at home. Additonal factors contributing to itamin K deficiency in infants include poor transer of vitamin K from maternal to fetal blood and sterile intestines (in this case exacerbated by prolonged diarrhea). In addition, hepatic storage of vitamin K is about 1 month. This is the time when late onset vitamin K deficiency may manifest in infants.

Vitamin K is an important coenzyme for y-glutamyl carboxylase, an enzyme that catalyses a post translational modification of a group of calcium-binding proteins. Important examples include factors II, VII, IX, and X, as well as proteins C and S. Vitamin K deficiencies in infants may manifest as gastrointestinal bleeding, skin hemorrhages, and intracranial hemorrhage.

Question 9

26 y/o pregnant woman complains of persistent dry, ulcerated skin over her knees and elbows. Examination of her eyes reveals small, gray plaques on the conjunctiva. Which of hte following is the most likely diagnosis?

Answer 9

Retinol deficiency

This patient has vitamin A (retinol) deficiency, which typically occurs in people with increased requirements for the nutrient, e.g. pregnant wome and patients with chronic diseases. Night blindness is usually the earliest manifestation of this deficiency and may be followed by retinal degeneration and blindness in utreated patients. Drying of the bulbar conjunctiva (xerosis_) and bitot spots (small, gray plaques on the conjunctiva) may also occur. Keratomalacia (corneal necrosis) is a serious potential complication of this deficiency.

Question 10

pregnant mother of 6 y/o boy son with glucose-6-phosphate dehydrogenase deficiency is very wories that her female fetus will have the disease. The fatehr and mother are clinically normal. Which of the following is true about her baby?

Answer 10

The baby has a 50% chance of being a carrier.

Glucose-6-phosphate dehydrogenase disease is an X-linked condition. the father has a normal X chromosome since he does not have the clinical disease. The mother has one normal X chromosome and one defective X chromosome, since her 6 y/o son has the disease and she does not. Thereore, the baby has 50% chance of having one normal and one abnormal X chromosome, making her a carrier.

Glucose-6-phosphate dehydrogenase deficiency is a defect in the pentose phosphate pathway of glucose metabolism, leading to defective production of reduced NADPH. Reduced NADPH is used in many biochemical pathways and is specifically used to regenerate the reduced from of glutathione that protects the body against oxidant drugs.

Question 11

Genetics researcher is trying to identify a ptoential gene from a gene signature/motif that encodes a seven-helix transmembrane domain. Which of the following is an example of a glycosylated, integral membrane protein with seven transmembrane segments.

Answer 11

Beta-Adrenergic receptor for epinephrine

This question requires that you recognize the family of receptors tha tinteract with G proteins to initiate a signal transduction cascade. These receptors are all glycosylated integral membrane proteins that have seven transmembrane segments. Beta-adrenergic receptors for epinephrine are an example.

Question 12

A previously normal child begins deteriorating developmentally at about 6 months of age. She is seen by an opthalmologist because she no longer responds to visual stimuli. A cherry red spot on the macula is noted on opthalmologic examination. The enzyme that is deficient in this child normally carries otu which of the following functions?

Answer 12

Degradation of glycolipids in the brain

Tay-Sachs disease is a severe autosomal recessive disease characterized clinically by mental retardation, blindness, muscular weakness, and death by 3 years of age.

The chery-red spot is a classic clue for Tay-sachs disease. The disease is prevalent in Ashkenazi (eastern european) Jews. hexosaminidase A normally functions to hydrolyze a bond between N-acetylglucosamine and galactose in the polar head of the ganglioside GM2

an N-acetylneuraminic acid-containing glycolipid found in high concentration in the brain without hexosaminidase A activity, this ganglioside accumulates, leading to degenerative CNS changes.

Question 13

30 y/o vegetarian presents to his physician complaining of diminished sensation in his lower extremities. he has not eaten meat for the past 15 years. A complete blood count reveals hypersegmented neurophils and elevated corpuscular volume. Which of the following findings would be expected on urine analysis?

Answer 13

Methylmalonic aciduria

This patient has vitamin B12 deficiency (a USMLE fav). Absence of vitamin B12 as a cofactor in the conversion of methylmalonyl-CoA to succinyl-CoA results in the accumulation of methylmalonyl-CoA, which is subsequently excreted in the urine. Methylmalonyl-CoA mutase deficiency can also cause this type of aciduria.

methylmalonyl-CoA —–(Vitamin B12)–> Succinyl CoA

Methylmalonyl-CoA mutase as well in place of B12

Question 14

12 y/o boy with genetic disease. Mother is a carrier of the mutated gene, but his father is not clinically affected and is not a carrier. The man has four siblings, a sister and a brother who are not clinically affected and are not carriers, a sister who is a carrier but is not clinically affected, and a brother hwo is clinically affected. This inheritance pattern is consistent with which of the following diseases.

Alpha 1 - antitrypsin deficiency

cystic fibrosis

duchenne muscular dystrophy

phenylketonuria

Tay-Sachs disease

Answer 14

Duchenne-type muscular dystrophy is the only disease listed that is X-linked, the other diseases are autosomal recessive. Remembering the genetics of myriad diseases is very problematic, but here are some imperfect rules of thumb that can help you out if you encouunter an unfamiliar disease on an examination. Deficiencies of most enzymes are recessive (either autosomal or x-linked), since each person has at least two copies (from each of the parents) of each enzyme, and one working gene is usually enough (although careful evaluation of herozygotes often shows mildly altered physiology).

In contrast, alterations in structural proteins are often autosomal dominant, since having any amount of abnormal protein in structures such as basement mebranes or in collagen tends to perturb their function. The number of common x-linked diseases (almost all recessive) is small and can be memorized: Duchenne muscular dystrophy, hemophilia A and B, chronic granulomatous disease, glucose-6-phosphate dehydrogenase deficiency, agamaglobulinemia, wiskott-Aldrich syndrome, diabetes insipidus, Lesch-Nyhan syndrome, fragile X-syndrome, and color blindness.

Question 15

A newborn vomits after each feeding of milk-based formula and does not gain weight. Biochemical testing reveals a severe deficiency of galactose-1-phosphate uridyltransferase, consistent with homozygosity. If this condition goes untreated, which of the following is the likely outcome for this patient?

Answer 15

Death in infancy

Galactosemia occurs in two very different clinical forms. Deficiency of galactokinase produces very mild disease, the only significant complication is cataract formation. In contrast, homozygous deficiency of galactose-1-phosphate uridyltransferase produces severe disease, culminating in death in infancy.

In addition to galactosemia and galactosuria, these patients have impaired renal tubular resorption leading to aminoaciduria, gastrointestinal symptoms, hepatosplenomegaly, cataracts, bleeding diatheses, hypoglycemia, and mental retardation. Pathologically, the CNS shows neuronal loss and gliosis, and the liver shows fatty change and progressing to cirrhosis.

Question 16

Following the ingestion of glyburide a type 2 diabetic patient begins to experience anxiety, diaphoresis, and hunger. The patient subsequently ingests a health food bar containing glucose. The glycolytic degradation of the ingested glucose commences with the action of which of the following enzymes?

Answer 16

Hexokinase

the process of glycolysis is defined as the sequence of reactions that converts glucose into pyruvate with the concomitant production of ATP.

Glycolysis begins when glucose is converted by hexokinase to glucose-6-phosphate. When this compound interacts with the enzyme phosphoglucose isomerase, fructose-6-phosphate is formed. Fructose -6-phosphate is then converted by phosphofructokinase to form fructose 1,6-biphosphate, which is subsequently converted to glyceraldehyde-3-phosphate by aldolase.

After a number of enzymatic reactions, phosphoenolpyruvate is formed. Phosphoenolpyruvate is converted to pyruvate by pyruvate kinase and the glycolytic pathway is then completed.

Question 17

A 24 year old woman with phenylketonuria (PKU) gives birth to first child. Although there is no history of PKU in the father’s family, the couple could not afford genetic testing of the father or consistent prenatal care. At birth, the child is small, microcephalic, and has elevated blood phenylalanine. What is the most likely explanation for this neonate’s symptoms?

• Definitions*
• phenylketonuria - an inherited disease due to faulty metabolism of phenylalanine, characterized by phenylketones in the urine and usually first noted by signs of mental retardation in infancy*
• phenylalanine - a crystalline, water-soluble, essential amino acid*
• microcephalic - having a head with a small brain case*

Answer 17

The child is affected at birth, indicating in the utero exposure to high levels of maternally derived phenylalanine. Women with PKU must strictly adhere to their special diet during pregnancy to avoid adversely affecting the fetus.

utero - in the uterus; unborn

Question 18

Answer 18

The father is a carrier of a reciprocal translocation between 7p and 12q. Although this has no described phenotypic effect on the father, it is likely to increase the rate of spontanious pregnancy loss as described in this question. The most recently aborted fetus has a partial but extensive trisiomy 7 and a small partial monosomy 12. All autosomal trisomies except 21, 18 and 13 are imcompatible with life, as are autosomal monosomies. The cause of the fetal karyotype was adjacent segregation during spermatogenesis in the father.

reciprocal translocation - an exchange of segments between two nonhomologous chromosomes

karyotype - the chromosomes of a cell, usually displayed as a systematized arrangement of chromosome pairs in descending order of size

spermatogenesis - the origin and development of spermatozoa.

Question 19

A 30 year old man has been fasting for religious reasons for several days. His blood glucose level is now about 60% of its normal value, but he does not feel lightheaded because his brain as reduced its need for serum glucose by using which of the following substances as an alternate energy source?

Answer 19

Ketone bodies, which include acetoacetate, beta-hydoxybutyrate, and acetone, are produced by the liver in the fasting state by beta-oxidation of the fatty acids. They are then released in to the blood stream, where they can be used as alternative energy sources for other organs, such as muscle, kidney, and the brain. The brain specifically still requires a small amount of circulating glucasde to function, but the amount required is reduced when ketone bodies are available.

Ketone bodies - any of three compounds, acetoacetic acid, beta-hydroxybutyric acid, or acetone, that are intermediate in the metabolism of fatty acids

Question 20

A lethal mutation occurs in a bacterium, rendering it incapable of replicating its chromosome. Because of this mutation, DNA synthesis produces many short fragments of DNA that have RNA sequences at their 5’ ends. The mutation is most likely in a gene encoding

Answer 20

DNA polymerase I in bacteria digests the RNA primers from the 5’ ends of the short Okazaki fragments and extends the adjacet fragment in to the digested area

• Definitions:*
• DNA polymerase I (or Pol I) is an enzyme that participates in the process of prokaryotic DNA replication*
• prokaryotic - a cellular organism that has no nuclear membrane and no organelles in the cytoplasm except ribosomes*

Question 21

A 2 year old boy has a past medical history significant for mental retardation, hepatosplenomegaly, foam cells in the bone marrow, and neurologic deficits. The boy dies by the age of 3. Which of the following enzymes was most likely deficient in this child?

Hepatosplenomegaly - Enlargement of the liver and the spleen.

Answer 21

The patient has characteristics of Niemann-Pick disease, an autosomal-recessive condition caused by a deficiency of the enzyme sphingomyelinase. This condition is associated with mental retardation, hepatosplenomegaly, foam cells in the bone marrow, cherry red spots on the maculae in 40% of cases and neurolocgic deficits, Death by 3 years of age is common

• Definitions:*
• autosomal - adjective Genetics. occurring on or transmitted by a chromosome other than one of the sex chromosomes:*

Question 22

A 58 year old woman is admitted to the hospital with fever, abdominal cramps, and severe watery diarrhea. The symptoms began one day after attending a banquet. She was treated wih ciprofloxacin, but her condition did not improve. PCR amplification and analysis of a gene region in the bacteria isolated from the patient revealed a missense mutation that conferred resistance to ciprofloxacin. The missense mutation is most likely in the gene encoding an enzyme essential for which of the following functions?

Answer 22

Ciprofloxacin, a quinolone derivative, inhibits DNA gyrase (prokaryotic topoisomerase II) essential for DNA replication. A mutation in the gene for DNA gyrase has made the enzyme insensitive to the effects of ciprofloxacin

• quinolone - any of a group of synthetic antibiotics, including ciprofloxacin, that inactivate an enzyme required for the replication of certain microorganisms*
• DNA gyrase - a bacterial enzyme that causes supercoiling of DNA.*

Question 23

A 62 year old man is prescribed a pharmaceutical agent that inhibits the activity of the enzyme HMG-CoA reductase. This patient most likely has which condition?

Answer 23

Familial hypercholesterolemia. The “statins” are a class of medications that inhibit the activity of the enzyme HMG-CoA reductase. Medications such as pravastatin and simvastatin produce a reversible inhibition of this enzyme, which subesquently leads to a reduction in LDL, total cholesterol, and triglycerides in patients with familial hypercholesterolemia.

Definitions:

Familial hypercholesterolemia - an inherited metabolic disorder caused by a lack or malfunction of receptors for the low-density lipoproteins that activate removal of cholesterol from the blood

hypercholesterolemia - the presence of an excessive amount of cholesterol in the blood.

Question 24

A 5 year old, mentally retarded boy is brought to the city from a rural community for evaluation. A carefull history reveals mental retardation in a number of other family members, especially the males. Physical examination is remarkable for a long face with large ears, a large jaw, an bilateral enlargement of the testes. This presentation is suggestive of?

Answer 24

Enlarged testes are the most specific phenotypic feature to suggest Fragile X syndrome in an individual who appears to have a hereditary mental retardation. The condition has unusual genetics, as it is related to expansion of a CGG repeat sequence lacated on the X chromosome. The larger the number of repeats, the higher the probability of significant retardation: hence, the retardation tends to become more severe in successive generations, as more CGG repeats accummulate. Sisters of affected males tend to show milder retardation than their brothers.

Definitions:

Fragile X syndrome - a widespread form of mental retardation caused by a faulty gene on the X chromosome.

Question 25

Liver cells in culture were kept at 0 C (32 F) and treated with trypsin to digest the receptors on the cell surface. The temperature was then rasised to 37 C (98.6 F), and radioactive LDL was added to the culture media. Several hours later, the labeled LDL was found to be inside the cells. This specific process of LDL uptake is?

• LDL medical abbreviation:*
• Abbreviation for low density lipoprotein. see under lipoprotein. n. A lipoprotein with a relatively high concentration of lipids and low concentration of protein that incorporates cholesterol and transports it to cells. High levels are associated with an increased risk of atherosclerosis and coronary artery disease. Also called LDL cholesterol.*
• Trypsin - a proteolytic enzyme of the pancreatic juice, capable of converting proteins into peptone.*

Answer 25

Even though the surface LDL receptors were digested by the tryspin, the recycling of unoccupied receptors to the cell surface probides a continual supply of new receptors to bind the labeled LDL. The LDL - receptor complex is internalized by receptor- mediated endocytosis

Endocytosis - the transport of solid matter or liquid into a cell by means of a coated vacuole or vesicle

Question 26

On physical examination, a newborn is found to have micrognathia, a prominent occiput, low-set ears, and rocker-bottom feet. There is very little mental development during the first months of life, and the infant dies of cardiac complications after 8 months of life. A complete karyotype of this child would show which of the following?

• micrognathia - Abnormal smallness of the jaws, especially of the mandible.*
• occiput - the back part of the head or skull.*
• karyotype - the chromosomes of a cell, usually displayed as a systematized arrangement of chromosome pairs in descending order of size.*

Answer 26

Answer - Trisomy 18

The chromosomal aberration is known as Edwards syndrome, or trisomy 18. It is characterized by

• Other facts:*
• Trisomy 13 - Patau syndrome - mental retardation, nervous system malfunction, rocker bottom feet, polydactyly, cleft lip and palate*
• Trisomy 18 - Edwards syndrome - mental deficiency, growth retardation, prominent occiput, micrognathia, low set ears, rocker bottom feet, ventricular septal defect​*
• Trisomy 21 - Down syndrome - mental retardation, protruding tongue, simian crease, congenital bridge heart defect, flat nasal bridge*
• XO - Turner syndrome - short stature, webbed neck, hypogonadism (diminished hormonal or reproductive functioning in the testes or the ovaries.​)*
• XXY - Klinefleter syndrome - tall stature, male hypogonadism, mental retardation*

Question 27

A college student goes to a fraternity party and consumes a large quantity of beer. The alcohol in the beer is metabolized by the liver, with almost half the alcohol being oxidized to acetaldehyde. In which site does this reaction occur?

acetaldehyde - a volatile, colorless, water-soluble liquid, C2H4O, having a pungent, fruitlike odor: used chiefly in the silvering of mirrors and in organic synthesis.

Answer 27

Answer - Peroxisomes - these are interesting cell orgaelles that were historically neglected because they are present in only small numbers in most mammalian cells. In the liver, however, these single membrane bound organelles are present in large numbers and are important in detoxification and long chain fatty acid metabolism. The clinically important degradation of ethanol to (potentially toxic) acetaldehyde occurs in humans in both peroxisomes and the smooth endoplasmic reticulum (P450 system). Disulfiram (Antabuse), used to discourage alcoholics from drinking, blocks the next reaction, in which acetaldhyde is oxidized to (nontoxic) acetate. This reaction, catalyzed by aldehyde dehydrogenase, occures in the mitochandria

Cytochromes P450 ( CYPs) are a superfamily of enzymes containing heme as a cofactor that function as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various compounds, as well as for hormone synthesis and breakdown.

Question 28

A patient has an enlarged liver and kidneys, gout, and xanthomas. Studies show that he has a genetic deficiency of glucose 6-phosphatase. Additional studies would most likely show what set of labratory results?

xanthomas - a yellow papule or nodule in the skin, containing lipid deposits.

Answer 28

This is von Gierke disease - one of the qlycogen storage diseases. The defect in glucose 6-phosphatase prevents release of glucose from glycogen across the liver cell membranes. The glucose 6-pholspahtase trapped in the liver cell is degraded to lactate and pyrubate, which are then released into the serum

• glycogen - a white, tasteless polysaccharide, (C6H10O5)n, molecularly similar to starch, constituting the principal carbohydrate storage material in animals and occurring chiefly in the liver, in muscle, and in fungi and yeasts.*
• pyrubate - an ester or salt of pyruvic acid*

Question 29

A neonate with ambiguous genitalia and mircocephaly is supected of having a genetic disease characterized by failure to metabolize 7- dehydrocholesterol to cholesterol. What is the diagnosis?

• neonate - a newborn child, or one in its first 28 days.*
• mircocephaly - A condition that causes a baby’s head to be small and not fully developed.*
• ambiguous genitalia - Ambiguous genitalia is a rare condition in which an infant’s external genitals don’t appear to be clearly either male or female.*

Answer 29

Smith Lemli Opitz syndrome - is a recently described recessive genetic disease. It is the first single-gene (as opposed to trisomy or other gross chromosomal alterations) disease to be associated with mulitple malformations. It presents with combinations of the following: microcephal, mental retardation, hypotonia, incomplete development of the genitalia, high forehead, pyloric stenosis, and syndactyly of the second and third toes. The disease appears to have a carrier rate comparable to sickle cell disease or cystic fibrosis in selected populations (which are not yet well defined). Experiments reveal that nutritional therapies (such as the administraion of vegetable oil supplemented by large amounts of cholesterol) begun within the first few weeks of life may markedly alter the course of this disease (unlike other devastating neurologic diseases that lack effective treatments).

• hypotonia - A condition characterized by low muscle tone​*
• pyloric stenosis - an abnormal narrowing of the valve at the outlet from the stomach, preventing normal passage of food into the small intestine.*
• syndactyly of the second and third toes - A congenital condition characterized by fusion of the bone or skin in the foot digits*

Question 30

A researcher is trying to identify a specific protein within a mixture. He subjects the mixture to gel electrophoresis and then transfers the separation to nitrocellulose filters. The filters are incubated with antibody to the specific protein, and the excess antibody is washed off. The antibody-protein complex is then incubated with a radiolabeled protein that binds to the antibody. Autoradiography is performed to detect the presence of the protein. This technique represents:

Answer 30

Western blotting - Western blots are used to identify a specific protein that may be present in very small concentrations in a complex protein mixture. This technique is used to comfirm the presence of anti-HIV antibodies in the serum of infected patients. Note that ELISA (enzyme-linked immunoabsorbent assay), which measures the amount of HIV antibody present, is used initially to detect the presence of anti-HIV antibody. All patients with positive ELISAs are further evaluated with the Western blot to rule out false-positive results. Because the Western blot is based on an intitial electrophoretic separation, it gives additional information about the molecular weight of the suspected HIV protein that makes the identification more accurate.

Question 31

A rapid way to purify proteins that are targeted to lysosomes would be to use affinity chromatography. An appropriate antibody to use on an affinity chromatography column would be directed against?

• Affinity Chromatography*
• Affinity chromatography is a method of separating a biomolecule from a mixture, based on a highly specific macromolecular binding interaction between the biomolecule and another substance. The specific type of binding interaction depends on the biomolecule of interest; antigen and antibody, enzyme and substrate, receptor and ligand, or protein and nucleic acid binding interactions are frequently exploited for isolation of various biomolecules. Affinity chromatography is useful for its high selectivity and resolution of separation, compared to other chromatographic methods.*

Answer 31

Mannose-6-phosphate

Proteins that are targeted for lyosomes have mannose-6-phosphate on their sugar chains. The monnose-6-phosphate is on an external site on the protein so that it can be recognized by the mannose-6-phosphate receptor on the lysosomal surface, and would be easily accessible to an antibody directed against it.

Lyosomes - a cell organelle containing enzymes that digest particles and that disintegrate the cell after its death.

Question 32

When a cloned DNA fragment is used as a probe, a restriction fragment length polymorphism (RFLP) is revealed in the region adjacent to the centromere of chromosome 21. Four haplotypes exist: A,B,C, and D. An AB woman and a CD man have an ACC child with trisomy 21. Nondisjunction occurred in?

haplotypes - Genetics. a combination of closely linked DNA sequences on one chromosome that are often inherited together: By comparing haplotypes of a mother and father with those of a fetus, scientists can study how new genetic changes arise.

Answer 32

The father during meiosis II

The RFLP detects a region near the centromere of chromosome 21. The region around the centromere exhibits a phenomenon called crossover suppression. Since genetic exchange cannot happen in this area, the probe is a reliable marker for the individual chromosomes. During meiosis II, sister chromatids, which are two identical copies of the same chromosome, should separate. If a nondisjunction event occurs in this division, two copies of the same chromosome are passed to the progeny. In this case, both parents are heterozygous for the RFLP. The child received an A from the mother and two C’s from the father, leading us to conclude that the probelm occurred in the father during meiosis II.

• RFLP - restriction fragment length polymorphism*
• meiosis II - Cell Biology. part of the process of gamete formation, consisting of chromosome conjugation and two cell divisions, in the course of which the diploid chromosome number becomes reduced to the haploid.*
• heterozygous - having dissimilar pairs of genes for any hereditary characteristic.*

Question 33

A 5 year old boy has temporary weakness and cramping of skeletal muscle after exercise. He has normal mental development. This child most likely has deficiency of which of the following enzymes?

Answer 33

Glyogen phosphorylase

The key to excelling in bio-chemistry on the USMLE is to master the most clinically important elements of metabolic pathways, eg. rate-limiting steps, irreversible steps, and steps involving enzymes affected by genetic diseases, in this case, glygogen phosphorylase is the enzyme involved in the rate-limiting step of glycogenolysis

• Glycogen phosphorylase is one of the phosphorylase enzymes (EC 2.4.1.1). Glycogen phosphorylase catalyzes the rate-limiting step in glycogenolysis in animals by releasing glucose-1-phosphate from the terminal alpha-1,4-glycosidic bond. Glycogen phosphorylase is also studied as a model protein regulated by both reversible phosphorylation and allosteric effects*
• Metabolic Pathway (image with question)*
• In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reactions catalyzed by enzymes. In most cases of a metabolic pathway, the product of one enzyme acts as the substrate for the next. However, side products are considered waste and removed from the cell. These enzymes often require dietary minerals, vitamins, and other cofactors to function.*

Question 34

A patient with acute lymphocytic leukemia is treated appropriately with antineoplastic therapy. Inhibition of which of the following enzymes will help prevent side effects of this therapy?

antineoplastic therapy - antineoplastic agent. antineoplastic therapy a regimen that includes chemotherapy, aimed at destruction of malignant cells using a variety of agents that directly affect cellular growth and development. Chemotherapy is but one of a variety of methods available in the treatment of cancer.

Answer 34

Xanthine oxidase

Following antineoplastic therapy for treatment of acute lymphocytic leukemia, patients often have a high level of urate secondary to the breakdown of nucleic acids. Therefore patients are often given allopurinol to decrease plasma urat levels. Allopurinol prevents uric acid formation by inhibiting the enzyme xanthine oxidase. Decreasing uric acid levels will help prevent the formation of kidney stones as well as block the appearance of other deleterious effects of hyperuricemia

• Allopurinol - a substance, C5H4N4O, used primarily in the treatment of chronic gout to decrease the synthesis of uric acid.*
• hyperuricemia - an excess of uric acid in the blood, often producing gout.*
• deleterious - injurious to health, harmful*

Question 35

A 4 year old retarded child hurls himself into walls and bites his fingertips so severely that they must be heavily bandaged. This child most likely has a deficiency of which enzyme?

Answer 35

Hypoxanthine - guanine phosphoribosyltransferase

The combination of retardation and sef-mutilation sould lead you immediately to the diagnosis of Lesch-Nylan disease. The hard part is rememebering which enzyme is deficient in this disease. Hypoxanthinge-guanine phosphoribosyltransferase (HGPRT) is part of the purine salvage pathway. A deficiency of the enzyme results in the inability to recycle purines and an overproduction of uric acid. Affected children are mentally retarded, have hyperuricemia and gout, and engage in complusive self-destructive behaviors

• hyperuricemia - an excess of uric acid in the blood, often producing gout.*
• purine salvage pathway - The term often refers to nucleotide salvage in particular, in which nucleotides ( purine and pyrimidine) are synthesized from intermediates in their degradative pathway. Nucleotide salvage pathways are used to recover bases and nucleosides that are formed during degradation of RNA and DNA.*

Question 36

A gene product thought to be involved in the down-regulation of fetal hemoglobin expression is being investigated. Samples of primary tissue cultures of fetal, neonatal, and adult liver and bone marrow, as well as adequate amounts of a DNA probe believed to contain the studied gene, are provided. The best method for determining which of the tissue culture samples expresses the studied gene is?

Answer 36

The Northern blot

In a Northern blot, all the mRNA from a cell type (isolated through the presence of its polyadenosine tails) is electrophoresed on a gel; since all mRNA is negatively charged, the electrophoresis separated it by size. The gel is then blotted onto nitrocellulose, the probe is applied and hybridized, and the excess is washed off; the resultant blot is autoradiographed. Bands on the autoradiograph film indicate the mRNAs that contain sequences complementary to the probe; this indicates that the gene on the probe has been expressed in the mRNA of that cell. This is the best technique for determining whether a gene is expressed in a particular cell type.

• mRNA - Messenger RNA (mRNA) is a single-stranded RNA molecule that is complementary to one of the DNA strands of a gene. The mRNA is an RNA version of the gene that leaves the cell nucleus and moves to the cytoplasm where proteins are made.*
• Polyadenylation - is the addition of a poly(A) tail to a RNA transcript, typically a messenger RNA (mRNA). The poly(A) tail consists of multiple adenosine monophosphates; in other words, it is a stretch of RNA that has only adenine bases.*

Question 37

A 28 year old woman and a 25 year old man present for genetic counseling. Both are white and have one sibling affected with cystic fibrosis. The most appropriate method to assess the risk of transmitting cystic fibrosis to a potential child would be….

Answer 37

Polymerase chain reaction (PCR)

Cystic fibrosis is an autosomal recessive disease caused by point mutations or small deletions in the gene encoding an integral membrane protein that functions as a chloride transporter. Although many mutations within the gene cause the phenotype, it is not practical or cost-effective to sequence the entire region to screen for a carrier. In the white poulation, however, the most frequent mutant allele causing cystic fibrosis is due to a small deletion at phenylalanine 508 in exon 10 chromosome 7. This mutation accounts for greater than 50% of mutant cystic fibrosis in the white populations. An amplification of this region using the polymerase chain reaction (PCR) can be done and the PCR products are sequenced and compared against the normal sequence for this region. If comparison reveals the deletion, the donor of that template DNA would be classified as a carrier. If the mutation is not present, the probability that the DNA donor is a carrier of cystic fibrosis is greatly reduced, but still exists. Most diagnostic laboratories will use this method to screen for between 4 and 10 of the most common mutations. Note that if a person belongs to a different group, PCR must be used to amplify the exons that contain the majority of cystic fibrosis mutation with inat particular ethic group.

• cystic fibrosis - a hereditary chronic disease of the exocrine glands, characterized by the production of viscid mucus that obstructs the pancreatic ducts and bronchi, leading to infection and fibrosis.*
• allele - any of several forms of a gene, usually arising through mutation, that are responsible for hereditary variation.*
• Polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it to a large enough amount to study in detail.*

Question 38

A 7 year old girl is brought to the emergency department by her parents with a complaint of severe polyuria and polydipsa. Laboratory examination reveals ketones in her urine. What is the most likely source of the ketones?

• polyuria - the passing of an excessive quantity of urine, as in diabetes, in certain nervous diseases, etc.*
• polydipsia - excessive thirst.*
• ketones - any of a class of organic compounds containing a carbonyl group, CO, attached to two alkyl groups, as CH3COCH3 or CH3COC2H5.*

Answer 38

Free fatty acid breakdown

The patient is presenting with signs and syptoms highly suggestive of type 1 diabetes. The primary source of ketones in the urine is free fatty acid breakdown. Ketone body formation occurs as follows; insulin deficiency …. activated lipolysis …..increased plasma free fatty acids ….increased hepatic fatty acids …. accelerate ketogenesis. In summary as fatty acids break down, acetyl-CoA is generated. As acetyl-CoA levels increase, ketone bodies begin to form. When excessive amounts of ketone bodies are formed, the pathologic state known as ketosis can occur.

• lipolysis - the hydrolysis of fats into fatty acids and glycerol, as by lipase.*
• lipase - any of a class of enzymes that break down fats, produced by the liver, pancreas, and other digestive organs*
• hepatic - of or relating to the liver*

Question 39

See photo

Answer 39

Pyruvate kinase (PK) deficiency

Pyruvate kinase is the enzyme involved in the last step of glycolysis

Taken together, severe neonatal jaundice, elevated indirect bilirubin, and exchange blood transfusion strongly suggest hemolysis. The hemoglobin and hernatocrit values are low and reticulocytosis is present, indicating an anemia. Thus, this infant has a hemolytic anemia. Increased levels of 2,3-BPG, 3-PG, 2-PG, and PEP (all distal glycolysis intermediates) are consistent with a block in glycolysis below phosphoenolpyruvate. Only a hemolytic anemia induced by a PK deficiency meets all these criteria. Hemolytic anemias have been reported in association with deficiencies of most of the enzymes of glycolysis. Most are quite rare, but PK deficiency is the most common. There are many variant forms of PK deficiency, some affecting Km, some Vmax, and some allosteric regulation. Most have a relatively mild clinical symptomology limited to the red cell. The tissue specficity is a consequence of the fact that there is a re-cell-specific PK isozyme. The relative lack of severe symptoms is a consequence of the fact that many of the aberrations affect kinetic parameters rather than total activity and because the body compensates with accelerated RBC production and the high 2,3 DPG level. The latter causes the hemoglobin to dump oxygen at a lower partial oxygen pressure.

Question 40

Pseudogenes are homologues of functional genes that lack promoters and are therefore part of the unexpressed DNA. It is thought that at least some of these pseudogenes were produced by reverse transcription of mRNA and insertion of the resultant cDNA into a chromosome by a virus. In addition to lacking a promoter and other regulatory elements, a pseudogene produced in this manner will also differ from the authentic gene in the following way

complementary DNA (cDNA) is DNA synthesized from a single-stranded RNA template in a reaction catalyzed by the enzyme reverse transcriptase. cDNA is often used to clone eukaryotic genes in prokaryotes

Answer 40

It will lack introns

Introns - a noncoding segment in a length of DNA that interrupts a gene-coding sequence or nontranslated sequence, the corresponding segment being removed from the RNA copy before transcription

DNA sequences produced by reversed transcription from mRNA templates would lack introns because the intron sequences are removed from RNA in the nucleus prior to its release as mRNA into the cytoplasm

messenger RNA (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of synthesizing a protein

Question 41

Methotrexate is used as therapy for rheumatoid arthritis, but has many side effects related to competitive inhibition of the enzyme dihydrofolate reductase. These side effects can be decreased without interfering with the efficacy of methotrexate by ingestion of additional folate. If patients treated with methotrexate are given sufficient folate, what would likely occur?

Folate - a salt or ester of folic acid

Answer 41

Dihydrofolate reductase will reach the same Vmax as it would in the absence of methrotrexate

Folate (as 7,8 dihydrofolate) is converted to tetrahydrofolate by dihydrofolate reductase. Methotrexate is a competitive inhibitor of dihydrofolate reductase. Therefore, providing sufficiently high concentrations of folate will allow the enzyme to reach the same Vmax as obtained in the absence of inhibitor.

Dihydrofolate reductase, or DHFR, is an enzyme that reduces dihydrofolic acid to tetrahydrofolic acid, using NADPH as electron donor, which can be converted to the kinds of tetrahydrofolate cofactors used in 1-carbon transfer chemistry. In humans, the DHFR enzyme is encoded by the DHFR gene. It is found in the q11→q22 region of chromosome 5. Bacterial species possess distinct DHFR enzymes (based on their pattern of binding diaminoheterocyclic molecules), but mammalian DHFRs are highly similar

Question 42

A 69 year old edentulous alcoholic man, who lives alone, is admitted to the hospital for evaluation of a shoulder wound that is not healing well. On physical examination, numerous ecchymoses are noted on the posterior aspect of his legs and thighs. Careful examination of the man’s skin reveals minute hemorrhages around hair follicles and splinter hemorrages in the nail beds. Laboratory examination is remarkable for a hemoglobin of 10g/dL; no other hematoligic abnormalities are noted. What is the most appropriate therapy for this disorder?

• edentulous - lacking teeth; toothless*
• ecchymoses - a discoloration due to extravasation of blood, as in a bruise.*
• hemogolobin - anemia is diagnosed if a blood test finds less than 13.5 g/dL in a man or less than 12 g/dL in a woman​ (grams per deciliter) Aged male should be between 13.5 and 15*

Answer 42

Vitamin C

The patient has Scurvy as a result from a deficiency of dietary vitamin C. Absence of vitamin C leads to impaired hydroxylation of the proline residues in the nascent procollagen chains, leading to weaknes of blood vessel walls. Clinically, the deficiency syndrone is characterized by perifollicular hemorrhages, fragmentation of hairs, purpura, ecchymoses, splinter hemorrhages, and hemorrahges into muscle. In patients with normal dentition, gum changes (swelling, bleeding, loosening of teeth) are also noted. Without vitamin C supplementation, death may eventually occur

• nascent - beginning to exist or develop*
• procollagen - The soluble precursor of collagen possibly formed by fibroblasts in the process of collagen synthesis*
• purpura - a disease characterized by purple or brownish-red spots on the skin or mucous membranes, caused by the extravasation of blood*