Vitamins (Fat-Soluble and Water-Soluble, Deficiencies) MCQs

Biochemistry · 73 free questions with answers & explanations.

1. A 45-year-old alcoholic presents with bilateral foot drop, ophthalmoplegia, nystagmus, and ataxia, followed by confusion and memory impairment. The vitamin deficiency most responsible for this clinical picture is:
2. A breastfed infant develops rickets despite adequate sunlight exposure. His mother has been exclusively taking cholestyramine for years. The most likely mechanism is:
3. A patient on isoniazid (INH) therapy for tuberculosis develops peripheral neuropathy. INH causes relative deficiency of which vitamin by competitively inhibiting its phosphorylation?
4. Vitamin K is essential for the post-translational modification of clotting factors II, VII, IX, and X. Which specific modification does vitamin K facilitate?
5. A 28-year-old woman in her first trimester presents with megaloblastic anemia and a previous child with a neural tube defect (NTD). Which biochemical function of folic acid is most directly related to NTD prevention?
6. A 32-year-old woman on long-term isoniazid therapy develops peripheral neuropathy with glove-and-stocking sensory loss. The biochemical mechanism by which isoniazid causes this deficiency involves:
7. Biotin is covalently attached to carboxylase enzymes via a specific lysine residue in a reaction catalyzed by holocarboxylase synthetase. Multiple carboxylase deficiency in infants can result from either holocarboxylase synthetase deficiency or biotinidase deficiency. Which carboxylase is the mitochondrial rate-regulating enzyme for gluconeogenesis whose activity would be impaired?
8. A patient receiving total parenteral nutrition without trace element supplementation develops a dermatitis around the eyes, nose, and mouth, along with impaired wound healing and dysgeusia. Serum alkaline phosphatase and lactate dehydrogenase activities are decreased. The deficient micronutrient is a cofactor for which category of enzymes?
9. Vitamin K-dependent carboxylation of glutamate residues to gamma-carboxyglutamate (Gla) is required for functional coagulation factors II, VII, IX, and X, as well as proteins C and S. Which enzyme is specifically inhibited by warfarin to block this carboxylation cycle?
10. A 55-year-old man with chronic alcoholism and poor diet develops bilateral symmetrical dermatitis on sun-exposed skin, diarrhea, and dementia. The condition is caused by deficiency of niacin (vitamin B3). Pellagra can also result from dietary tryptophan deficiency because tryptophan can be converted to niacin. Approximately how many milligrams of tryptophan are required to synthesize 1 mg of niacin, and which vitamin is an essential cofactor for this conversion?
11. A patient on long-term isoniazid therapy develops peripheral neuropathy with sensory loss, burning paresthesias, and elevated serum homocysteine. Which biochemical mechanism explains the neuropathy?
12. Biotin-dependent carboxylases include pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase, and 3-methylcrotonyl-CoA carboxylase. Biotinidase deficiency causes acquired multiple carboxylase deficiency. Which metabolite accumulation is most useful in confirming this diagnosis?
13. Vitamin K acts as a cofactor for gamma-glutamyl carboxylase, which carboxylates glutamate residues in target proteins. In addition to clotting factors II, VII, IX, X, which other biologically important process requires this vitamin K-dependent carboxylation?
14. Thiamine (B1) deficiency causes Wernicke-Korsakoff syndrome in alcoholics. Beyond its known role in pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, which other thiamine-dependent enzyme, when deficient, explains the elevated pentose phosphates and ribose-5-phosphate found in Wernicke encephalopathy?
15. Pellagra (niacin/tryptophan deficiency) can occur despite adequate dietary niacin in patients with Hartnup disease. What is the precise mechanism?
16. Vitamin K-dependent carboxylation converts glutamic acid (Glu) residues to gamma-carboxyglutamic acid (Gla) residues in clotting factors. This reaction requires reduced vitamin K (KH2). After carboxylation, KH2 is oxidized to vitamin K 2,3-epoxide. Warfarin inhibits vitamin K epoxide reductase (VKOR). In a patient on stable warfarin with INR 2.5, which newly added drug would MOST predictably raise INR by pharmacokinetic interaction?
17. Biotin is covalently attached to carboxylase enzymes via an amide bond with a specific lysine residue, a process catalyzed by holocarboxylase synthetase. Biotinidase then recycles biotin from degraded carboxylases. A child with biotinidase deficiency has defects in which FOUR carboxylases?
18. A 28-year-old woman exclusively breastfeeding her infant presents with the infant having seizures at 4 months of age. Serum pyridoxine-5'-phosphate levels are elevated in the infant. The mother takes isoniazid for latent TB. Isoniazid forms a hydrazone with pyridoxal-5'-phosphate (PLP), inactivating it. PLP is required as cofactor for aromatic L-amino acid decarboxylase. Deficiency of this enzyme would MOST directly reduce synthesis of which two neurotransmitters?
19. Vitamin D3 (cholecalciferol) undergoes two sequential hydroxylations to form the active hormone 1,25-dihydroxycholecalciferol (calcitriol). The first hydroxylation at C-25 occurs in the liver by CYP2R1/CYP27A1. The second at C-1 occurs in the kidney by CYP27B1 (1-alpha hydroxylase). In a patient with hereditary 1,25-dihydroxyvitamin D resistant rickets (HVDRR, vitamin D dependent rickets type 2), the defect is in the vitamin D receptor (VDR). Which laboratory finding would BEST distinguish HVDRR from CYP27B1 deficiency (VDDR type 1)?
20. Biotin (vitamin B7) is a covalently attached cofactor for carboxylase enzymes. A patient with biotinidase deficiency presents with skin rash, alopecia, lactic acidosis, and organic aciduria. The accumulated metabolites (methylcitrate, 3-hydroxypropionate, 3-methylcrotonylglycine) implicate deficient activity of all EXCEPT which enzyme?
21. A chronic alcoholic presents with Wernicke's encephalopathy (confusion, ophthalmoplegia, ataxia). The biochemical basis is deficiency of thiamine (B1) required as TPP for three key enzyme complexes. Which of the following enzymes does NOT require TPP?
22. Vitamin K-dependent carboxylation converts glutamate (Glu) residues in clotting factor precursors to gamma-carboxyglutamate (Gla) residues. Warfarin inhibits which enzyme in the vitamin K cycle, and which clotting factors are affected?
23. A vegetarian patient presents with megaloblastic anemia and neurological symptoms (subacute combined degeneration). Serum B12 is low; serum folate is normal. The key biochemical link between B12 and folate metabolism that explains why B12 deficiency causes a functional folate trap is:
24. Vitamin D3 (cholecalciferol) undergoes two hydroxylation steps before becoming the active hormone. Which is the physiologically regulated, rate-limiting hydroxylation step, and what stimulates it?
25. A 35-year-old woman on long-term isoniazid therapy develops peripheral neuropathy, cheilosis, and confusion. Urinary xanthurenic acid is elevated after a tryptophan load test. What is the biochemical basis of isoniazid-induced vitamin B6 deficiency?
26. Biotin serves as a cofactor for carboxylase enzymes. A patient with raw egg consumption-induced biotin deficiency will primarily show impaired activity of which set of enzymes?
27. Vitamin K is required for gamma-carboxylation of glutamate residues in clotting factors II, VII, IX, and X. Warfarin inhibits vitamin K epoxide reductase (VKORC1). In addition to clotting factors, which other proteins require vitamin K-dependent gamma-carboxylation?
28. A 52-year-old alcoholic man presents with bilateral pretibial edema, weakness, and cardiomegaly. ECG shows sinus tachycardia. He has no proteinuria. Which thiamine-dependent enzyme deficiency underlies the cardiovascular manifestations of wet beriberi?
29. Vitamin D3 (cholecalciferol) undergoes sequential hydroxylation to become active 1,25-dihydroxycholecalciferol (calcitriol). Which hydroxylase step is the tightly regulated, physiologically critical step, and what is its principal inducer?
30. A 40-year-old woman on long-term isoniazid and hydralazine therapy develops a peripheral neuropathy with sensory loss and burning pain. Her symptoms partially respond to pyridoxine. What is the precise biochemical mechanism by which these drugs cause pyridoxine (B6) deficiency?
31. Vitamin K activates clotting factors II, VII, IX, and X, as well as proteins C and S, by facilitating γ-carboxylation of glutamate residues. Which enzyme performs this carboxylation, and what is the immediate consequence of warfarin administration on vitamin K metabolism?
32. A strict vegan presents with macrocytic anemia, subacute combined degeneration of the spinal cord, and elevated plasma methylmalonic acid. Serum B12 is 85 pg/mL. B12 serves as a cofactor for only two human enzymes. Which of the following correctly pairs these two enzymes with their cofactor form?
33. Biotin deficiency may result from avidin in raw egg white, prolonged TPN without biotin supplementation, or biotinidase deficiency. Biotinidase deficiency impairs recycling of biotin, causing multiple carboxylase deficiency. Which set of carboxylases is affected, and what is the diagnostic metabolite pattern?
34. A 55-year-old alcoholic with chronic liver disease presents with ophthalmoplegia, ataxia, and confusion. He is treated with intravenous thiamine. Thiamine pyrophosphate (TPP) is a cofactor for four enzyme complexes. Which is NOT one of them?
35. Biotin (vitamin B7) functions as a carboxyl carrier. A patient on prolonged raw egg-white diet develops alopecia, dermatitis, and neuropsychiatric symptoms. The enzyme most critically dependent on biotin whose deficiency produces lactic acidosis in this context is:
36. Vitamin K-dependent gamma-carboxylation of glutamate residues is essential for coagulation factors II, VII, IX, X and proteins C and S. Which subcellular compartment performs this carboxylation reaction?
37. A patient with exclusive parenteral nutrition (PN) for 3 months develops peripheral neuropathy. Plasma pyruvate is elevated. The vitamin deficiency most likely responsible is:
38. Vitamin D3 (cholecalciferol) undergoes sequential hydroxylations. The hydroxylation that produces the most physiologically active form (1,25-(OH)2D3 / calcitriol) and is tightly regulated by PTH, FGF-23, and calcium is performed by which enzyme in which location?
39. The clinical syndrome of pellagra (niacin deficiency) can occur even with adequate dietary niacin in a patient with which metabolic disorder?
40. Biotin (vitamin B7) serves as a covalently-attached cofactor for carboxylase enzymes. The enzyme that covalently attaches biotin to the apocarboxylase (biotinylation) and the clinical consequence of its deficiency are:
41. Vitamin K-dependent carboxylation of glutamate residues is required for the activation of clotting factors II, VII, IX, and X, as well as proteins C and S. The specific amino acid modification performed and the vitamin K cycle enzyme inhibited by warfarin are:
42. Pellagra from niacin deficiency causes dermatitis, diarrhoea, and dementia (3 Ds). Niacin can be synthesised endogenously from tryptophan. A patient on isoniazid therapy develops pellagra despite normal dietary tryptophan intake. The mechanism is:
43. A patient receiving long-term total parenteral nutrition (TPN) without supplementation develops confusion, ophthalmoplegia, and ataxia. Which vitamin deficiency is most likely, and what is the specific biochemical test to confirm subclinical deficiency before overt symptoms appear?
44. Biotin (vitamin B7) is a covalently bound cofactor for carboxylase enzymes. Which enzyme's biotin dependence explains the organic aciduria in biotinidase deficiency?
45. Vitamin K is required for gamma-carboxylation of glutamate residues in coagulation factors II, VII, IX, and X. Warfarin inhibits vitamin K epoxide reductase (VKOR). Which additional vitamin K-dependent proteins are clinically relevant and have anticoagulant/anti-calcification roles?
46. A patient on long-term isoniazid therapy develops peripheral neuropathy. The BIOCHEMICAL basis of pyridoxine (B6) deficiency in this context is:
47. Biotin-dependent carboxylases are inhibited by avidin (raw egg white). Which metabolic pathway is MOST critically impaired, leading to the characteristic finding of lactic acidosis and organic aciduria?
48. Vitamin K-dependent gamma-carboxylation of glutamate residues is required for the biological activity of coagulation factors. Which reaction does vitamin K (as KH2) directly participate in?
49. A patient on long-term isoniazid (INH) therapy develops peripheral neuropathy. The mechanism involves which vitamin?
50. Vitamin K is required for carboxylation of glutamate residues in clotting factors. Which enzyme catalyses this reaction, and what happens to vitamin K in the process?
51. Pellagra presents with dermatitis, diarrhea, and dementia (3Ds). In carcinoid syndrome, pellagra occurs despite adequate dietary niacin. What is the biochemical explanation?
52. Biotin deficiency causes alopecia, dermatitis, and neurological symptoms. Biotin serves as a prosthetic group for carboxylases. Which enzyme is NOT biotin-dependent?
53. A 35-year-old with Crohn's disease has ileal resection and develops neurological symptoms: subacute combined degeneration of the cord with megaloblastic anaemia. Vitamin B12 absorption requires intrinsic factor binding in the stomach and absorption at the:
54. Wernicke's encephalopathy in a chronic alcoholic is due to thiamine deficiency. Which biochemical reactions are impaired, explaining the selective vulnerability of high-metabolic-demand brain regions?
55. A patient on isoniazid and hydralazine develops pellagra-like dermatitis, despite adequate dietary niacin. The biochemical mechanism involves:
56. Biotin serves as a CO2 carrier in carboxylase reactions. Which of the following enzymes does NOT require biotin?
57. Vitamin K-dependent carboxylation activates coagulation factors II, VII, IX and X. The specific amino acid residue modified is:
58. Thiamine pyrophosphate (TPP) is the active form of thiamine. In severe thiamine deficiency (Wernicke's encephalopathy), which enzymes are primarily affected, explaining the clinical presentation?
59. Vitamin K is essential for post-translational gamma-carboxylation of glutamate residues in coagulation factors II, VII, IX, X, and proteins C and S. The reaction requires which form of vitamin K as the active carboxylase cofactor?
60. Niacin (vitamin B3) deficiency causes pellagra (3 Ds: dermatitis, diarrhea, dementia). Tryptophan can serve as a niacin precursor. Pellagra occurs secondary to carcinoid syndrome because:
61. Biotin deficiency presents with alopecia, dermatitis, and neurological symptoms. The biochemical reason is impairment of which enzymatic reactions?
62. A patient taking isoniazid, pyrazinamide, and rifampicin for TB develops pellagra-like dermatitis, diarrhoea, and dementia after 3 months. The BIOCHEMICAL mechanism linking anti-TB therapy to niacin deficiency is:
63. Vitamin K undergoes a specific enzymatic regeneration cycle after each round of carboxylation. Which enzyme catalyses the reduction of vitamin K epoxide back to active vitamin K (hydroquinone form)?
64. A patient with chronic alcohol use develops Wernicke's encephalopathy. He is given IV dextrose before thiamine. This worsens his neurological status. The biochemical explanation is:
65. Biotin-dependent carboxylase deficiencies (multiple carboxylase deficiency) present with organic acidaemia, skin rash, and alopecia. All four biotin-dependent carboxylases in humans share the common mechanism of:
66. Biotin is a cofactor for carboxylase enzymes. A patient on long-term raw egg white diet develops biotin deficiency because:
67. Niacin (vitamin B3) is unique among water-soluble vitamins in having an alternative biosynthetic route. The amino acid precursor and the limiting enzyme for endogenous niacin synthesis in the liver is:
68. Biotin (vitamin B7) serves as a CO2 carrier in carboxylation reactions. Biotin deficiency (or biotinidase deficiency) impairs which set of clinically important enzymes?
69. Vitamin K-dependent carboxylation activates clotting factors II, VII, IX, X, and proteins C and S by gamma-carboxylating glutamate residues. The reaction requires which co-substrate that is recycled by vitamin K epoxide reductase (VKORC1)?
70. A patient on long-term isoniazid therapy develops peripheral neuropathy. The mechanism relates to isoniazid's interference with which vitamin pathway?
71. Biotin (vitamin B7) is covalently bound to carboxylase enzymes. Biotinylation of which enzyme is essential for fatty acid synthesis?
72. Vitamin K-dependent clotting factors (II, VII, IX, X) require gamma-carboxylation of glutamate residues. The specific reaction catalysed requires vitamin K in which form?
73. A patient receiving isoniazid (INH) for tuberculosis develops peripheral neuropathy. The mechanism is: