S1B5 - Nutrition: vitamins, minerals, dietary supplements

Question 1

Why must a woman have a pregnancy test before taking high doses of Vitamin A?

Answer 1

Before oral isotretinoin (cis retinoic acid) is prescribed for severe cystic acne, women must have a pregnancy test because high doses of Vitamin A are highly teratogenic (birth defects include cleft palate, cardiac abnormalities, low IQ scores and extremely high risk for spontaneous abortions)

Question 2

What are three important medical uses for vitamin A?

Answer 2

Vitamin A medical uses:

• Tretinoins (all-trans retinoic acid, acid form of vitamin A), which is a topical medication used for mild acne and psoriasis
• APML (acute promyelocytic leukemia, aka M3 subtype of AML) to induce maturation of leukemic cells.
• Measles (specifically used in malnourished children to prevent exfoliative dermatitis)

Question 3

What are the signs of zinc deficiency?

Answer 3

Zinc deficiency can cause:

• Impaired immune function
• Delayed healing of wounds
• Hair loss
• Hypogonadism
• Anosmia (loss of smell)
• Dysgeusia (taste abnormalities)
• Alcoholic cirrhosis

Question 4

Which protein in raw egg whites prevents biotin from being absorbed?

Answer 4

Raw egg whites contain avidin, which binds and sequesters biotin within the GI tract. In the presence of avidin, biotin is eliminated with the feces rather than absorbed. Heating egg whites denatures avidin, rendering it unable to bind biotin.

Question 5

Which vitamin is a cofactor for carboxylation reactions? What are three important carboxylation reactions that use this vitamin cofactor?

Answer 5

Vitamin B7 (biotin): cofactor in several carboxylation reactions including:

• Acetyl-CoA carboxylase: converts acetyl-CoA (2C) to Žmalonyl-CoA (3C) in fatty acid synthesis
• Pyruvate carboxylase: converts pyruvate (3C) to oxaloacetate (4C) in gluconeogenesis
• Propionyl-CoA carboxylase: converts propionyl-CoA (3C)Ž to methylmalonyl-CoA (4C) in the metabolism of odd-chain fatty acids

Question 6

A 13-year-old girl is prescribed all-trans retinoic acid for acne vulgaris. From which vitamin is this medication derived?

A) Vitamin A

B) Vitamin E

C) Niacin

D) Vitamin D

E) Thiamine

Answer 6

Vitamin A

Answer Explanation

Vitamin A derivatives include retinol, retinal, and retinoic acid (includes all-trans retinoic acid).

Question 7

What are the signs of Vitamin K deficiency?

Answer 7

Vitamin K Deficiency (broad spectrum antibiotics, warfarin therapy, newborns, fat malabsorption):

• Prolonged prothrombin time (PT) and prolonged activated thromboplastin time (aPTT)
• Bleeding from clotting factor deficiencies

Question 8

What are the signs of scurvy?

Answer 8

Signs of scurvy include:

• Swollen, spongy and purplish gums that bleed often
• Bleeding into the skin (bruising)
• Red spots under skin from burst capillaries (petechiae)
• Loose teeth
• Bulging of the eyes (proptosis)
• Anemia
• Dry, brittle hair that curls (“corkscrew” hair)
• Slow wound healing
• Bleeding into the joints (hemarthrosis) and muscles, which causes swelling over the bones of the arms and legs

Question 9

What are 2 etiologies of biotin deficiency?

Answer 9

Although relatively rare, biotin deficiency can be caused by antibiotic use or excessive consumption of raw egg whites, which can be seen in individuals who are bodybuilders or boxers (e.g., Rocky Balboa).

Question 10

What are three conditions that can cause fat-soluble vitamin deficiency?

Answer 10

Vitamins A,D,E,K are fat soluble vitamins that are transported in chylomicrons, and stored in liver or adipose tissue.

• fat-soluble vitamins are more likely to reach toxic levels than water-soluble vitamins
• deficiencies of fat soluble vitamins can occur any time fat malabsorption occurs, for example: malabsorption syndromes (e.g. tropical sprue), pancreatic insufficiency, and resected ileum.

Question 11

Name a vitamin that, when taken in excess, can synergistically act with warfarin.

Answer 11

Excess (taking megadoses of vitamin E): decreased synthesis of vitamin K-dependent coagulation factors in the liver, thereby working synergistically with warfarin.

Question 12

Which vitamin is a precursor for NADH and NADPH? What is a pharmacological use of this vitamin?

Answer 12

Vitamin B3 (niacin) is a precursor for NADH and NADPH. It is used pharmacologically to increase HDL and decrease LDL, typically as alternative or supplemental therapy to other first-line anti-lipid medications.

Question 13

Vitamin B6 is a cofactor for which two types of reactions? It is also a cofactor for which enzyme?

Answer 13

Vitamin B6 is a cofactor for:

• Transamination
• Decarboxylation
• Glycogen phosphorylase

Vitamin B6 is also a cofactor for other reactions as well.

Question 14

Water soluble vitamins:

• What letter vitamins?
• is toxicity common?
• Do deficiencies show up rapidly or slowly?

Answer 14

Water soluble vitamins:

• B and C
• Excreted once concentration surpasses the renal threshold
  
  • Toxicities are rare
• Metabolic stores labile, depletion occurs in weeks or months
  
  • Deficiencies show up quickly on inadequate diet

Question 15

What are some food sources of Vitamin K? What are other sources of Vitamin K?

Answer 15

Source: green leafy vegetables (supply K1, phylloquinone) and bacterial synthesis in colon (supply K2, menaquinone).

Question 16

What are the key clinical findings of chronic vitamin A toxicity?

Answer 16

Chronic vitamin A toxicity causes:

• Hepatoxicity
• Hepatomegaly
• Dry skin
• Alopecia
• Arthalgia
• Pseudotumor cerebri

Question 17

Thiamine pyrophosphate is a coenzyme for which four important enzymes?

Answer 17

A vitamin B1 (thiamine) derivative, thiamine pyrophosphate (TPP), is an important coenzyme for several reactions. some of which can be remembered with the mnemonic, ATP:

• α-ketoglutarate dehydrogenase
• Transketolase
• Pyruvate dehydrogenase

Vitamin B1 is also a cofactor for branched-chain ketoacid dehydrogenase.

Question 18

What is the chief symptom of chromium deficiency? What is the reason for this?

Answer 18

The chief symptom of chromium deficiency is impaired glucose tolerance, a result of decreased insulin effectiveness.

Question 19

Name 3 signs/symptoms that result from biotin deficiency.

Answer 19

Biotin deficiency causes:

• Dermatitis
• Alopecia
• Enteritis

Question 20

Water soluble vitamins:

• Are they stored in the body?
• Are they easily destroyed/washed out in food prep?
• In what foods are they found?
• Is using high doses of multivitamins a good ideas?

Answer 20

Water soluble vitamins- quick facts

• B-complex and vitamin C
  
  • are not stored in the body and must be replaced each day.
• Are easily destroyed / washed out during food storage and preparation.
• Found in a variety of foods
  
  • B in cereal grains, meat, poultry, eggs, fish, milk, legumes and fresh vegetables.
  • C in citrus fruits
• Using high doses of multivitamins / supplements is not a good idea.

Question 21

What two conditions can lead to vitamin E deficiency and what are the signs?

Answer 21

Vitamin E deficiency is rare and primarily occurs in:

• children with cystic fibrosis (due to fat malabsorption secondary to decreased bile salts and pancreatic insufficiency)
• abetalipoproteinemia (fat malabsorption)

​Signs are hemolytic anemia, peripheral neuropathy, posterior column degeneration, retinal degeneration, and myopathy. Vitamin E deficiency signs can mimic those of vitamin B12 deficiency, but vitamin E deficiency is without increased methylmalonic acid levels or megaloblastic anemia.

Question 22

Describe the role of ascorbic acid in collagen synthesis.

Answer 22

Vit C serves as a cofactor for prolyl-4-hydroxylase and lysyl hydroxylase, an enzyme involved in collagen synthesis.

Question 23

Which metal commonly serves as a structural ion in transcription factors?

Answer 23

Found in >100 enzymes, serves as a structural ion in transcription factors (“zinc-finger” motif)

Question 24

What causes acrodermatitis enteropathica? What are the symptoms?

Answer 24

Acrodermatitis enteropathica is a very rare inherited zinc deficiency caused by mutations in a gene that encodes a zinc transporter in the small intestine. Symptoms include periorificial (especially mouth and anus) and acral (hand and feet) dermatitis as well as similar symptoms to acquired zinc deficiency. You can remember this disease because Acrodermatitis = acral dermatitis, and enteropathica = pathology of enterocytes of the small intestine.

Question 25

What are the signs of niacin deficiency?

Answer 25

Niacin deficiency leads to **glossitis** and may lead to **pellagra** in severe deficiency. Pellagra is a syndrome of the "3 D's of B3 deficiency," which are: * **D**iarrhea * **D**ermatitis (classically in a "necklace" distribution around the neck) * **D**ementia

Question 26

What 4 neurotransmitter synthesis reactions does vitamin B6 (pyridoxine) help catalyze?

Answer 26

Pyridoxine (B6) is needed to produce many different compounds, and is a cofactor in steps of many different pathways. Pyridoxine is a cofactor for the following 4 neurotransmitter synthesis reactions: * **Glutamate → GABA** * **Tryptophan → Serotonin** * **DOPA → Dopamine** (dopamine can then go on to form norepinephrine and epinephrine) * **Histidine → Histamine**

Question 27

What is the most common cause of vitamin D deficiency? What are some other causes of vitamin D deficiency?

Answer 27

Causes of vitamin D deficiency: * **Renal failure** (most common cause): deficiency of **1-α-hydroxylase** * **Inadequate exposure to sunlight**: decreased synthesis of vitamin D3 * **Fat malabsorption**: cannot reabsorb fat soluble vitamins * **Chronic liver disease**: unable to carry out first hydroxylation of vitamin D3 * **Enhanced liver cytochrome P450 system** (e.g. alcohol, phenytoin, barbiturates): increased conversion of 25-hydroxycholecalciferol into inactive metabolite * **Primary hypoparathyroidism**: need PTH to enhance activity of 1-α-hydroxylase * **Type 1 vitamin D-dependent rickets**: deficiency of 1-α-hydroxylase * **Type 2 vitamin D-dependent rickets**: deficiency of vitamin D receptors

Question 28

Which amino acid is used to synthesize niacin in the body? What B vitamin is a required cofactor?

Answer 28

**Niacin** may also be endogenously synthesized from **tryptophan** in a process using vitamin **B6**. Prolonged and stark vitamin **B6 deficiency may lead to niacin deficiency** as well.

Question 29

Menkes disease is an X-linked disorder charachterized by global deficiency of what?

Answer 29

**Menkes** disease is an X-linked disorder characterized by global **copper deficiency**. * It is caused by mutations in the ATP7A copper transporter that interfere with the ability of intestinal mucosal cells to transport copper into the bloodstream. * The symptoms of Menkes disease are mental retardation, growth retardation, hypothermia, loose skin and joints, hypopigmentation, and kinky hair caused by the inability to load copper-dependent enzymes with copper.

Question 30

A 36-year-old man with longstanding, poorly controlled schizophrenia, is found to have severe osteomalacia. A review of his records reveals that he very rarely leaves his basement and relies on a grocery delivery service to obtain food. He does state that he drinks a pint of milk per day. Which form of vitamin is most likely deficient? A) Cholecalciferol B) Pantothenic acid C) Retinoic acide D) Niacin E) Ergocalciferol

Answer 30

Cholecalciferol (Vit D) **Osteomalacia** occurs with **vitamin D deficiency**. Several forms of vitamin D are required for normal calcium homeostasis: D2 (ergocalciferol), is the form found in food, especially plants. **D3 (cholecalciferol),** is made by skin from D2 in response to sunlight. Patients with **very dark skin** (e.g. black children) or with **minimal sunlight exposure** are at risk for D3 deficiency.

Question 31

What is the role of ergocalciferol and cholecalciferol in calcium homeostasis?

Answer 31

**Vitamin D2 (ergocalciferol, found in plants)** and **vitamin D3 (cholecalciferol):** *increases intestinal absorption of calcium* *and phosphorous* and *renal absorption of calcium.* It also plays a role in *PTH-induced osteoclast activation and resorption of bone*.

Question 32

What vitamin deficiency may be seen in carcinoid syndrome?

Answer 32

Carcinoid syndrome is a complication of carcinoid tumors (most commonly of the small bowel) in which serotonin is systemically elaborated in great excess. *Tryptophan* is compensatorily shunted to make *serotonin*, decreasing conversion to niacin. Therefore, **niacin deficiency** is a potential complication of carcinoid syndrome.

Question 33

Thiamine deficiency results in which 2 syndromes?

Answer 33

**Thiamine deficiency** may lead to beriberi ("**B**er**1B**er**1**") and Wernicke-Korsakoff syndrome. * **Beriberi** has a dry (muscle wasting and neuropathy) and wet (dilated cardiomyopathy) component. * **Wernicke-Korsakoff syndrome** is composed of Wernicke’s encephalopathy (triad of confusion, ophthalmoplegia, and ataxia) and Korsakoff’s psychosis (memory loss, confabulation and personality change).

Question 34

Which vitamin is used in the synthesis of coenzyme A?

Answer 34

**Vitamin B5 (pantothenic acid)** is needed to form **coenzyme-A (CoA).** Vitamin B5 deficiency is rare. The signs of deficiency include: * Dermatitis * Enteritis * Alopecia * Adrenal insufficiency

Question 35

What is the etiology of pellagra in the setting of carcinoid syndrome? A) Autoimmune destruction B) Inflammatory process C) Thiamine toxicity D) Nutrient deficiency E) Neoplastic process

Answer 35

Nutrient deficiency **Pellagra** occurs in the setting of **niacin deficiency**. Niacin is made from **tryptophan**. In carcinoid syndrome, **serotonin is produced in excess amounts,** which consumes the **precursor of serotonin: tryptophan**. Recall that niacin biosynthesis requires tryptophan, thus carcinoid syndrome can lead to pellagra.

Question 36

What are the main symptoms of Mg2+ deficiency?

Answer 36

The main symptoms of Mg2+ deficiency are **weakness**, **tremors,** and **cardiac arrhythmia**. * Supplemental Mg2+ may help prevent formation of calcium oxalate stones in the kidney. * Mg2+ supplementation has also been shown to lower blood pressure in several clinical studies, and there is an inverse effect between dietary Mg2+ intake and the risk of stroke.

Question 37

What are the signs of riboflavin deficiency?

Answer 37

Riboflavin deficiency causes: * **Cheilitis** (cracked angles of the mouth) * **Glossitis** with a magenta-colored tongue * **Corneal vascularization**

Question 38

Besides its role as an antioxidant and in collagen synthesis, what are two other functions of vitamin C?

Answer 38

Other roles of Vitamin C: * **Cofactor for dopamine hydroxylase** (dopamine → norepinephrine) * **Reduces iron from Fe3+ → Fe2+, facilitating absorption in the duodenum**

Question 39

Which type of kidney stone is associated with excess consumption of vitamin C?

Answer 39

Excess consumption of Vitamin C can lead to **calcium oxalate nephrolithiasis.**

Question 40

What are two molecules that are derived from riboflavin?

Answer 40

**Vitamin B2 (riboflavin):** key component of **FAD** (used in succinate dehydrogenase of the TCA cycle, aka complex II of the electron transport chain) and **FMN** (used in NADH dehydrogenase, aka complex I of the electron transport chain), which are used as cofactors for redox reactions.

Question 41

What rare inherited defect in amino acid transport can cause niacin deficiency?

Answer 41

**Hartnup** disease is an autosomal recessive defect in intestinal and renal transporters for neutral amino acids. This causes **tryptophan excretion** in urine and leads to **pellagra**.

Question 42

Alcoholism can cause what mineral deficiency?

Answer 42

**Magnesium deficiency occurs in alcoholism**, with use of certain diuretics and in metabolic acidosis.

Question 43

Why are neonates vitamin K deficient?

Answer 43

Since **vitamin K is synthesized by intestinal flora and neonatal intestines are not yet colonized by bacteria**, infants are given a vitamin K injection at birth to prevent hemorrhagic disease of the newborn. (Vitamin K is also **not in breast milk**).

Question 44

What are three pathologies of vitamin D deficiency?

Answer 44

Vitamin D deficiency causes: ​ * **Rickets** in children (imperfect calcification, softening, and distortion of the bones classically resulting in bowed legs) * **Osteomalacia** in adults (soft bones and bone pain). * **Hypocalcemic tetany** (decreased extracellular calcium levels cause increased excitability of voltage gated sodium channels, leading to increased muscle contractions)

Question 45

Hemochromatosis is primarily a genetic disease of overload of what mineral?

Answer 45

Hemochromatosis is primarily a **genetic disease of iron overload.** Patients with a hemochromatosis develop iron deposits in the liver, heart, and endocrine tissue even though they have normal intakes of dietary iron. * Eventually, those iron deposits **can lead to cirrhosis**, cardiomyopathy, diabetes, and other endocrine disorders

Question 46

What is the most abundant mineral in the body?

Answer 46

**Calcium** -most abundant mineral in the body. Mostly in bone, small amount of Ca2+ outside of bone functions in a variety of essential processes.

Question 47

What are the signs of niacin excess?

Answer 47

**Niacin** can be used to treat hyperlipidemia; side effects in this setting include **acute skin flushing** and the potential development of **hyperuricemia and hyperglycemia.**

Question 48

What are some symptoms of copper deficiency?

Answer 48

Symptoms of copper deficiency include * anemia, * hypercholesterolemia, * demineralization of bones, * leukopenia, * fragility of large arteries, * demyelination of neural tissue.

Question 49

What are the signs of vitamin A deficiency?

Answer 49

Vitamin A Deficiency: **night blindness**, squamous metaplasia leading to skin abnormalities (follicular hyperkeratosis), eye abnormalities (Bitot's spots → buildup of keratin debris in conjuctiva), lung abnormalities (bronchitis, pneumonia)

Question 50

What is the role of Vitamin E in protecting cell membranes?

Answer 50

**Vitamin E:** antioxidant and scavenger of free radicals that **protects polyunsaturated fats and fatty acids** in cell membranes **from lipid peroxidation** and **protects LDL from oxidation.**

Question 51

Are B vitamins water soluble? What is the consequence of this?

Answer 51

B vitamins are **water soluble**; therefore, body stores of B vitamins are **quickly depleted** (**except** **folate** and **B12 cobalamin**, which are **stored in the liver**). Each B vitamin is used in a unique type of biochemical reaction. By understanding the biochemical roles of each B vitamin, one can deduce B vitamin cofactors that are necessary based on the name of the enzyme.

Question 52

A 34-year-old woman is prescribed an antagonist of vitamin K epoxide reductase. This class of medication functionally makes her vitamin K deficient. Which protein is least affected by this medication? A) Factor VIII B) Protein C C) Protein S D) Factor II E) Factor X

Answer 52

Factor VIII Answer Explanation Vitamin K epoxide reductase reduces vitamin K after it has been oxidized by carboxylation reactions that are required in the synthesis of factors II, VII, IX, X, and proteins C and S. Any form of vitamin K deficiency will reduce levels of those proteins, but **doesn’t affect factor VIII.**

Question 53

What are the signs of vitamin D toxicity?

Answer 53

Vitamin D toxicity leads to: * **Hypercalcemia** * **Renal calcium stones** * **Nausea and vomiting**

Question 54

What are the signs of vitamin B5 deficiency?

Answer 54

Vitamin B5 deficiency is rare. The signs of deficiency include: * Dermatitis * Enteritis * Alopecia * Adrenal insufficiency

Question 55

What two popular beverages destroy thiamine?

Answer 55

**Coffee** and **tea** destroy thiamine.

Question 56

A 25-year-old female is seen in clinic 2 years following successful bariatric surgery. She has lost 140 pounds from a preoperative weight of 250 pounds. Upon referral for body contouring surgery, it is discovered that she has recently had several weeks of hair loss and diarrhea despite no change in her diet. Which deficiency does she most likely have A) Biotin B) Zinc C) Pantothenic acid D) Ascorbate E) Riboflavin

Answer 56

Zinc Answer Explanation Although zinc and biotin deficiency can both lead to alopecia, biotin deficiency is very rare, whereas **zinc deficiency is relatively common in patients with malabsorption** (e.g. s/p bariatric surgery or sprue). Moreover, biotin deficiency is not known to cause diarrhea, whereas diarrhea is a common feature of zinc deficiency.

Question 57

What are the main functions of Vitamin A?

Answer 57

Vitamin A: functions as a **component of visual pigments** and in **cell differentiation**

Question 58

What is the most important source of vitamin D?

Answer 58

Endogenous vitamin D is produced by **photoconversion of 7-dehydrocholesterol to vitamin D3 in sun-exposed skin** (most important source).

Question 59

What are the signs of pyridoxine deficiency?

Answer 59

Pyridoxine deficiency causes: * **Neurological pathology**: Peripheral neuropathy and convulsions (due to defective neurotransmitter synthesis). * **Anemia**: Sideroblastic anemias due to defective heme synthesis.

Question 60

Copper deficiency is rare and is usually seen only because of excess intake of what?

Answer 60

Copper deficiency is relatively rare and is usually seen only because of **excess zinc intake** (zinc and copper compete for absorption)

Question 61

Deficiency of ascorbate leads to what disease?

Answer 61

Deficiency of vitamin C leads to **scurvy** due to defective collagen synthesis.

Question 62

Excess intake of what vitamin can decrease the efficacy of L-DOPA therapy in Parkinson's disease?

Answer 62

A core treatment in Parkinson's disease is L-DOPA therapy. Supplemental **vitamin B6** can convert L-DOPA to dopamine peripherally (as opposed to in the CNS), rendering treatment *less efficacious*.

Question 63

What are the two major side effects of iron deficiency?

Answer 63

Iron deficiency causes **anemia** and **decreased immuno-competence.**

Question 64

What are the key signs of acute vitamin A toxicity?

Answer 64

Acute vitamin A toxicity causes: * **Nausea** * **Vomiting** * **Visual disturbances** * **Vertigo**

Question 65

Symptoms of Ca2+ deficiency resemble what vitamin deficiency?

Answer 65

Symptoms of Ca2+ deficiency resemble those of **vitamin D deficiency** * but other symptoms such as muscle cramps are possible with marginal deficiencies.

Question 66

Vitamin K gamma-carboxylates glutamate residues and thereby activates 6 serum proteins involved with clotting — 4 pro-coagulants, and 2 anti-coagulants. Name these 6 vitamin K-dependent factors.

Answer 66

**Vitamin K** ("**K**oagulations-Vitamin" in German): gamma-carboxylates glutamate residues and thereby activates: * 4 pro-coagulants: **clotting factors IX, X, VII, II (prothrombin)** * 2 anti-coagulants: **protein C, protein S**

Question 67

In which organ is calcidiol produced?

Answer 67

Next, vitamin D is hydroxylated in the **liver** to produce calcidiol (25-hydroxycholecalciferol) by the P450 system.

Question 68

The formation of what enzyme is stimulated with low serum Ca2+ to inhence intestinal Ca2+ absorption?

Answer 68

Low serum Ca2+ stimulates formation of **1,25 dihydroxycholecalciferol**, which enhances intestinal Ca2+ absorption.

Question 69

What are some good sources of Vitamin A?

Answer 69

Sources of vitamin A: * **Liver** * **Egg yolks** * **Leafy greens** * **Carrots**

Question 70

What are some sources of vitamin D?

Answer 70

Sources of vitamin D include: * **Liver** * **Egg yolk** * **Saltwater fish** * **Vitamin D fortified foods (milk)** Vitamin D3 is **made by skin** after sunlight exposure (although some foods are now fortified with synthetic D3).

Question 71

Which B vitamin generally plays a role in the decarboxylation of α-keto acids?

Answer 71

**Vitamin B1 (thiamine)** most often plays a role in decarboxylation of α-keto acids.

Question 72

What disease is characterized by copper overload, particularly in the liver and the brain?

Answer 72

**Wilson disease** is an autosomal recessive disease characterized by **copper overload**, particularly in the liver and the brain. * It is caused by mutations in the ATP7B copper transporter that prevent the liver and neurological tissue from getting rid of excess copper. * The accumulation of copper in the liver leads to cirrhosis, chronic hepatitis, and eventually liver failure. * In the brain leads to parkinsonian symptoms, seizures and psychiatric symptoms. * Copper also accumulates as a characteristic gold-brown ring, called the KayserFleischer ring, around the periphery of the cornea.

Question 73

What people are at a higher risk for thiamine deficiency?

Answer 73

In the United States, thiamine deficiency is **most common in alcoholics** (due to poor nutrition and that excess alcohol limits the body's ability to absorb and store thiamine.)

Question 74

Deficiency of what vitamin is confirmed by an increase in RBC transketolase activity after vitamin supplementation?

Answer 74

Diagnosis of **thiamine deficiency** can be confirmed by an increase in RBC transketolase activity that occurs *after thiamine is supplemented*.

Question 75

Which enzyme produces calcitriol?

Answer 75

Calcidiol is then hydroxylated to the active form, calcitriol (1,25-dihydroxycholecalciferol) in the kidneys by the enzyme **1-α-hydroxylase.**

Question 76

Why should a patient with hemochromatosis avoid excess consumption of Vitamin C?

Answer 76

Excess consumption of Vitamin C can **further increase the risk of iron toxicity** in patients already predisposed to iron toxicity. Those at risk includes patients that receive frequent transfusions and those with *hereditary hemochromatosis*.