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Water-Soluble Vitamins: Clinical MCQs with Explanations & Illustrations
Case 1: A Toddler with Lactic Acidosis
A 2-year-old child presents with irritability, vomiting, and convulsions. Physical examination reveals hypotonia and developmental delay. Laboratory tests show elevated lactate and pyruvate levels. The pediatrician suspects a thiamine-responsive metabolic disorder.
Which of the following enzymes is most likely deficient in this patient?
A) Pyruvate dehydrogenase
B) Succinate dehydrogenase
C) Glucose-6-phosphate dehydrogenase
D) Lactate dehydrogenase
E) Glycogen phosphorylase
Correct Answer: A) Pyruvate dehydrogenase
- Thiamine (B1) is a cofactor for pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, and transketolase.
- Deficiency leads to the accumulation of pyruvate, which is converted to lactate, causing lactic acidosis.
- Pyruvate dehydrogenase deficiency presents with neurological impairment, metabolic acidosis, and developmental delay.
Incorrect Options:
B) Succinate dehydrogenase → Part of the TCA cycle and electron transport chain but does not require thiamine.
C) Glucose-6-phosphate dehydrogenase (G6PD) → Involved in the pentose phosphate pathway, requires NADP⁺ but not thiamine.
D) Lactate dehydrogenase → Converts pyruvate to lactate, but its dysfunction would cause pyruvate buildup without necessarily involving thiamine.
E) Glycogen phosphorylase → Involved in glycogen breakdown, does not require thiamine.
Case 2: The Mystery of the Magenta Tongue
A 23-year-old woman presents with a sore throat, cheilitis (cracks at the corners of the mouth), glossitis, and a magenta-colored tongue. She also has corneal vascularization.
Which coenzyme function is most impaired in this patient?
A) Pyruvate dehydrogenase
B) Succinate dehydrogenase
C) Glucose-6-phosphate dehydrogenase
D) Lactate dehydrogenase
E) Glycogen phosphorylase
Correct Answer: C) Hydrogen transfer in redox reactions
Riboflavin (B2) is a precursor for FAD and FMN, which are essential for redox reactions in oxidative metabolism (e.g., succinate dehydrogenase in the TCA cycle).
Incorrect Options:
A) Carboxylation of pyruvate → Requires biotin (B7), not riboflavin.
B) Transfer of one-carbon units → Requires folate (B9) for DNA synthesis.
D) Conversion of propionyl-CoA to succinyl-CoA → Requires vitamin B12 (cobalamin), not B2.
E) Transamination of amino acids → Requires pyridoxal phosphate (B6), not B2.
Case 3: The Alcoholic with a Sun-Sensitive Rash
A 35-year-old man with alcohol dependence presents with diarrhea, dermatitis, and dementia. His physical exam reveals a hyperpigmented rash in sun-exposed areas.
Which amino acid deficiency could contribute to his condition?
A) Lysine
B) Tyrosine
C) Tryptophan
D) Methionine
E) Cysteine
Correct Answer: C) Tryptophan
- Niacin (B3) is synthesized from tryptophan.
- Deficiency leads to Pellagra (“3 D’s” – Diarrhea, Dermatitis, Dementia), often seen in alcoholics and patients with malabsorptive disorders or Hartnup disease (tryptophan transport defect in kidneys and intestines).
Incorrect Options:
A) Lysine → Essential amino acid but not involved in niacin synthesis.
B) Tyrosine → Precursor for dopamine and melanin but not related to niacin.
D) Methionine → Required for methylation reactions, but not niacin synthesis.
E) Cysteine → Precursor for glutathione, no direct role in niacin production.
Case 4: The Athlete with Burning Feet Syndrome
A 28-year-old woman presents with burning feet syndrome, fatigue, and nausea. She reports a poor diet consisting mainly of processed foods.
Which of the following biochemical pathways would be most affected in this patient?
A) Conversion of oxaloacetate to phosphoenolpyruvate
B) β-oxidation of fatty acids
C) De novo pyrimidine synthesis
D) Synthesis of catecholamines
E) Conversion of homocysteine to methionine
Correct Answer: B) β-oxidation of fatty acids
- Pantothenic acid (B5) is required for Coenzyme A (CoA) synthesis, which is essential for fatty acid oxidation (β-oxidation), TCA cycle, and acetyl-CoA formation.
Incorrect Options:
A) Conversion of oxaloacetate to phosphoenolpyruvate → Requires pyruvate carboxylase (biotin-dependent).
C) De novo pyrimidine synthesis → Requires folate (B9).
D) Synthesis of catecholamines → Requires tyrosine hydroxylase (BH4-dependent).
E) Conversion of homocysteine to methionine → Requires B12 and folate (B9).
Case 5: Neuropathy After Tuberculosis Treatment
A 26-year-old pregnant woman presents with irritability, depression, and peripheral neuropathy. She was recently started on isoniazid therapy for latent tuberculosis.
Which biochemical process is most affected?
A) Neurotransmitter synthesis
B) Collagen cross-linking
C) Electron transport chain function
D) DNA synthesis
E) Fatty acid oxidation
Correct Answer: A) Neurotransmitter synthesis
- Vitamin B6 (pyridoxine) is a cofactor for decarboxylases and transaminases, essential for neurotransmitter synthesis (GABA, dopamine, serotonin, histamine).
- Isoniazid inhibits pyridoxal phosphate (PLP), leading to deficiency-related symptoms like depression, neuropathy, and irritability.
Incorrect Options:
B) Collagen cross-linking → Requires vitamin C, not B6.
C) Electron transport chain function → Depends on CoQ10, iron, and riboflavin (B2), not B6.
D) DNA synthesis → Impaired in B12 or folate (B9) deficiency, not B6.
E) Fatty acid oxidation → Requires Coenzyme A (B5), not B6.
Case 6: Hair Loss and Raw Egg Consumption
A 34-year-old woman presents with hair loss, depression, and scaly dermatitis after consuming large amounts of raw egg whites for several months.
Which enzyme function is most impaired in this patient?
A) Methylmalonyl-CoA mutase
B) Propionyl-CoA carboxylase
C) Thymidylate synthase
D) Ornithine transcarbamylase
E) Tyrosinase
Correct Answer: B) Propionyl-CoA carboxylase
- Biotin (B7) is a cofactor for carboxylase enzymes, including propionyl-CoA carboxylase, pyruvate carboxylase, and acetyl-CoA carboxylase.
- Avidin in raw egg whites binds biotin and leads to deficiency.
Incorrect Options:
A) Methylmalonyl-CoA mutase → Requires B12, not biotin.
C) Thymidylate synthase → Requires folate (B9) for DNA synthesis.
D) Ornithine transcarbamylase → Involved in the urea cycle, not biotin-dependent.
E) Tyrosinase → Converts tyrosine to melanin, requires copper, not biotin.
Case 7: A Mystery of Macrocytic Anemia
A 24-year-old woman presents with fatigue and pallor. Laboratory tests reveal macrocytic anemia with hypersegmented neutrophils.
Which enzyme function is most impaired?
A) Methylmalonyl-CoA mutase
B) Thymidylate synthase
C) Phenylalanine hydroxylase
D) Cystathionine synthase
E) Glucose-6-phosphate dehydrogenase
Correct Answer: B) Thymidylate synthase
- Folate (B9) is required for thymidylate synthase to convert dUMP to dTMP for DNA synthesis.
- Deficiency causes megaloblastic anemia and affects rapidly dividing cells.
Incorrect Options:
A) Methylmalonyl-CoA mutase → Requires B12, not B9.
C) Phenylalanine hydroxylase → Converts phenylalanine to tyrosine, requires BH4, not folate.
D) Cystathionine synthase → Requires B6, not B9.
E) Glucose-6-phosphate dehydrogenase → Requires NADP⁺, not folate.
Case 8: Macrocytic Anemia with Elevated Methylmalonic acid levels.
A 65-year-old vegan presents with paresthesias, weakness, and ataxia. Laboratory studies show macrocytic anemia with elevated methylmalonic acid levels.
Which of the following is most likely impaired?
A) Purine synthesis
B) Conversion of homocysteine to methionine
C) Transamination of amino acids
D) Urea cycle function
E) Collagen cross-linking
Correct Answer: B) Conversion of homocysteine to methionine
- Vitamin B12 is required for methionine synthase, which converts homocysteine to methionine.
- Deficiency causes megaloblastic anemia and neurological symptoms.
Incorrect Options:
A) Purine synthesis → Requires B9, not B12.
C) Transamination of amino acids → Requires B6, not B12.
D) Urea cycle function → Requires arginine, not B12.
E) Collagen cross-linking → Requires vitamin C, not B12.
Case 9: A Mystery of Bleeding Gums
A 40-year-old man presents with fatigue, bleeding gums, and corkscrew hairs. He has a history of poor diet and lack of fresh fruits and vegetables.
Which process is impaired in this patient?
A) Hydroxylation of proline and lysine
B) Carboxylation of glutamate
C) Methylation of DNA
D) Oxidation of succinate to fumarate
E) Conversion of norepinephrine to epinephrine
Correct Answer: A) Hydroxylation of proline and lysine
- Vitamin C is required for proline and lysine hydroxylation in collagen synthesis.
- Deficiency causes scurvy (bleeding gums, poor wound healing, bruising).
Incorrect Options:
B) Carboxylation of glutamate → Requires vitamin K.
C) Methylation of DNA → Requires folate (B9).
D) Oxidation of succinate to fumarate → Requires FAD (B2).
E) Conversion of norepinephrine to epinephrine → Requires SAM (B12, folate-dependent).
Case 10. An Infant with A Burnt Sugar Odor in the urine
A 3-month-old infant presents with poor feeding, lethargy, and a burnt sugar odor in the urine. Laboratory analysis reveals elevated levels of leucine, isoleucine, and valine in the plasma.
Deficiency of which vitamin is most likely contributing to this metabolic disorder?
A) Vitamin B1
B) Vitamin B2
C) Vitamin B3
D) Vitamin B6
E) Vitamin B12
Correct Answer: A) Vitamin B1 (Thiamine)
- Maple syrup urine disease (MSUD) results from a deficiency of branched-chain α-ketoacid dehydrogenase, an enzyme requiring thiamine (B1) as a cofactor.
- B1 supplementation may benefit some milder forms of MSUD.
Incorrect Options:
B2 (Riboflavin) → Required for FAD-dependent redox reactions, not BCAA metabolism.
B3 (Niacin) → Precursor for NAD+/NADP+, not involved in MSUD.
B6 (Pyridoxine) → Cofactor for transaminases, not BCAA degradation.
B12 (Cobalamin) → Involved in odd-chain fatty acid metabolism and homocysteine conversion, not MSUD.
Case 11: Decreased ATP production and Increased Succinate levels
A 28-year-old woman presents with muscle weakness and exertional fatigue. Laboratory tests show decreased ATP production and increased succinate levels in mitochondria.
Which enzyme is most likely impaired in this patient?
A) Complex I (NADH dehydrogenase)
B) Complex II (Succinate dehydrogenase)
C) Complex III (CoQ-cytochrome c reductase)
D) Complex IV (Cytochrome c oxidase)
E) ATP synthase (Complex V)
Correct Answer: B) Complex II (Succinate dehydrogenase)
- Succinate dehydrogenase (Complex II) is the only enzyme in the TCA cycle that is also part of the electron transport chain and requires FAD, which is derived from riboflavin (B2).
- Riboflavin deficiency leads to reduced ATP synthesis and accumulation of succinate.
Incorrect Options:
Complex I (NADH dehydrogenase) → Requires NADH, not FAD.
Complex III, IV, and ATP synthase → Not riboflavin-dependent.
Case 12: Enzyme Function Impairment in Pellagra
A 55-year-old man with chronic alcohol use disorder presents with glossitis, photosensitive rash, and diarrhea. He is diagnosed with Pellagra.
Which enzyme function is most impaired in this condition?
A) Isocitrate dehydrogenase
B) Pyruvate carboxylase
C) Carnitine palmitoyltransferase I
D) Methylmalonyl-CoA mutase
E) HMG-CoA reductase
Correct Answer: A) Isocitrate dehydrogenase
- Niacin (B3) is a precursor for NAD+/NADP+, which is required for dehydrogenase enzymes, including isocitrate dehydrogenase in the TCA cycle.
- Deficiency leads to Pellagra (4 D’s: Diarrhea, Dermatitis, Dementia, and Death).
Incorrect Options:
B) Pyruvate Carboxylase→ Requires B1.
C) Carnitine palmitoyltransferase I → Involved in fatty acid oxidation, not niacin-related.
D) Methylmalonyl-CoA mutase → Requires B12.
E) HMG-CoA reductase → Involved in cholesterol synthesis, not niacin-dependent.
Case 13: An Athlete with low acetyl-CoA levels
A 22-year-old athlete complains of fatigue and muscle cramps. Laboratory results show low acetyl-CoA levels.
Deficiency of which vitamin is most likely responsible?
A) Vitamin B1 (Thiamine)
B) Vitamin B2 (Riboflavin)
C) Vitamin B5 (Pantothenic Acid)
D) Vitamin B6 (Pyridoxine)
E) Vitamin B12 (Cobalamin)
Correct Answer: C) Vitamin B5 (Pantothenic Acid)
- B5 is required for the synthesis of Coenzyme A (CoA), which is essential for the TCA cycle and fatty acid metabolism.
- Deficiency leads to fatigue, muscle cramps, and “burning feet syndrome.”
Incorrect Options:
- B1 → Impairs pyruvate metabolism but does not directly affect CoA synthesis.
- B2, B6, B12 → Not involved in CoA synthesis.
Case 14: Peripheral neuropathy during Isoniazid therapy
A 30-year-old woman on isoniazid therapy develops peripheral neuropathy and irritability.
Which biochemical reaction is most affected?
A) Decarboxylation of glutamate to GABA
B) Conversion of homocysteine to methionine
C) Oxidation of succinate to fumarate
D) Dehydrogenation of pyruvate to acetyl-CoA
E) Methylation of norepinephrine to epinephrine
Correct Answer: A) Decarboxylation of glutamate to GABA
- Vitamin B6 is required for the synthesis of neurotransmitters such as GABA, dopamine, and serotonin.
- Deficiency leads to irritability, depression, and neuropathy.
Incorrect Options:
B) Homocysteine to methionine → Requires B12.
C) Succinate to fumarate → Requires FAD (B2).
D) Pyruvate to acetyl-CoA → Requires B1.
E) Norepinephrine to epinephrine → Requires SAM (B12, folate).
Case 15: Raw Egg white consumption and Hypoglycemia
A 35-year-old woman on a ketogenic diet presents with fatigue, nausea, and muscle pain. She has been consuming large amounts of raw egg whites daily. Laboratory tests show hypoglycemia and metabolic acidosis.
Which enzyme function is most likely impaired?
A) Pyruvate carboxylase
B) Glucose-6-phosphatase
C) Hexokinase
D) Phosphofructokinase-1 (PFK-1)
E) Pyruvate dehydrogenase
Correct Answer: A) Pyruvate carboxylase
- Biotin (B7) is a cofactor for pyruvate carboxylase, which converts pyruvate to oxaloacetate in gluconeogenesis.
- Avidin in raw egg whites binds biotin, leading to deficiency and impaired gluconeogenesis.
Incorrect Options:
B) Glucose-6-phosphatase → Required for gluconeogenesis but does not require biotin.
C) Hexokinase → Glycolytic enzyme, not involved in gluconeogenesis.
D) Phosphofructokinase-1 (PFK-1) → Regulated by ATP but does not require biotin.
E) Pyruvate dehydrogenase → Requires thiamine (B1), not biotin.
Case 16. Pregnancy and neural tube defect
A 28-year-old pregnant woman presents with fatigue and pallor. The blood smear shows macrocytic anemia with hypersegmented neutrophils. Her fetus is found to have neural tube defects on ultrasound.
Which biochemical reaction is most impaired in this patient?
A) Methylation of homocysteine to methionine
B) Synthesis of thymidylate from dUMP
C) Decarboxylation of histidine to histamine
D) Synthesis of glutamate from α-ketoglutarate
E) Transamination of aspartate to oxaloacetate
Correct Answer: B) Synthesis of thymidylate from dUMP
- Folate (B9) is essential for the conversion of dUMP to dTMP via thymidylate synthase.
- Deficiency leads to megaloblastic anemia and neural tube defects.
Incorrect Options:
A) Methylation of homocysteine to methionine → Requires B12, not B9.
C) Decarboxylation of histidine to histamine → Requires B6.
D) Synthesis of glutamate from α-ketoglutarate → Requires B6, not B9.
E) Transamination of aspartate to oxaloacetate → Requires B6.
Case 17: Frequent Bruising and Enzyme Function Impairment
A 32-year-old woman presents with fatigue, mood changes, and frequent bruising. She is diagnosed with vitamin C deficiency.
Which enzyme function is most affected?
A) Dopamine β-hydroxylase
B) Phenylalanine hydroxylase
C) Tyrosine hydroxylase
D) Glutamate decarboxylase
E) Histidine decarboxylase
Correct Answer: A) Dopamine β-hydroxylase
- Vitamin C is required for dopamine β-hydroxylase, which converts dopamine to norepinephrine.
- Deficiency can cause mood disorders and impaired catecholamine synthesis.
Incorrect Options:
B) Phenylalanine hydroxylase → Requires BH4, not vitamin C.
C) Tyrosine hydroxylase → Requires BH4, not vitamin C.
D) Glutamate decarboxylase → Requires B6, not vitamin C.
E) Histidine decarboxylase → Requires B6, not vitamin C.
