Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 351: What vitamin deficiency causes pellagra?

A. Thiamine
B. Riboflavin
C. Niacin (Correct Answer)
D. Biotin

Explanation: **Explanation:** **Niacin (Vitamin B3)** is the precursor for the coenzymes **NAD and NADP**, which are essential for oxidation-reduction reactions in cellular metabolism. Pellagra occurs due to a deficiency of Niacin or its precursor amino acid, **Tryptophan**. It is classically characterized by the **"4 Ds"**: Dermatitis (photosensitive "Casal’s necklace"), Diarrhea, Dementia, and, if untreated, Death. **Analysis of Incorrect Options:** * **Thiamine (B1):** Deficiency leads to **Beriberi** (Dry or Wet) and **Wernicke-Korsakoff syndrome**, primarily affecting the cardiovascular and nervous systems. * **Riboflavin (B2):** Deficiency causes **Ariboflavinosis**, characterized by cheilosis, glossitis (magenta tongue), and corneal neovascularization. * **Biotin (B7):** Deficiency is rare but can be induced by excessive consumption of **raw egg whites** (due to avidin binding). It presents with dermatitis, alopecia, and enteritis. **High-Yield Clinical Pearls for NEET-PG:** * **The 60:1 Rule:** 60 mg of Tryptophan is required to synthesize 1 mg of Niacin. * **Secondary Pellagra:** Can occur in **Hartnup disease** (impaired tryptophan absorption) and **Carcinoid syndrome** (increased tryptophan metabolism into serotonin, diverting it away from niacin synthesis). * **Corn-based diets:** Populations relying on maize are at risk because the niacin in corn is bound (niacytin) and unavailable for absorption unless treated with alkali. * **Isoniazid (INH) Therapy:** This anti-tubercular drug can deplete Vitamin B6, which is a necessary cofactor for the conversion of tryptophan to niacin, potentially leading to pellagra.

Question 352: Which of the following vitamins is a major electron acceptor in the electron transport chain?

A. Vitamin B1 (Thiamine)
B. Vitamin B2 (Riboflavin)
C. Vitamin B3 (Niacin) (Correct Answer)
D. Vitamin B6 (Pyridoxine)

Explanation: **Explanation:** The correct answer is **Vitamin B3 (Niacin)**. **Why Vitamin B3 is correct:** Niacin is the precursor for the coenzymes **NAD+ (Nicotinamide Adenine Dinucleotide)** and **NADP+**. In the Electron Transport Chain (ETC), NAD+ acts as a major electron acceptor during the oxidation of substrates in the TCA cycle (e.g., Isocitrate, α-ketoglutarate, and Malate). It accepts two electrons and one proton to become **NADH**, which then delivers these electrons to **Complex I (NADH Dehydrogenase)** to initiate ATP production via oxidative phosphorylation. **Why other options are incorrect:** * **Vitamin B1 (Thiamine):** Its active form, Thiamine Pyrophosphate (TPP), is involved in oxidative decarboxylation (e.g., Pyruvate Dehydrogenase complex) but does not act as a primary electron carrier in the ETC. * **Vitamin B2 (Riboflavin):** While its derivative **FAD** is an electron acceptor, it is considered a "prosthetic group" tightly bound to enzymes (like Succinate Dehydrogenase). In the context of "major mobile electron acceptors" that shuttle between the TCA cycle and the ETC, Niacin (NAD+) is the more definitive answer for this specific metabolic role. * **Vitamin B6 (Pyridoxine):** Its active form, Pyridoxal Phosphate (PLP), is the essential coenzyme for transamination, decarboxylation, and heme synthesis, but it has no role in the ETC. **High-Yield NEET-PG Pearls:** * **Pellagra:** Deficiency of Niacin leads to the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Hartnup Disease:** A defect in the transport of **Tryptophan** (a precursor for Niacin synthesis) can lead to Niacin deficiency. * **Corn-based diets:** Low in Tryptophan and Niacin, often leading to Pellagra. * **Therapeutic use:** Niacin is used to treat hyperlipidemia as it inhibits lipolysis in adipose tissue.

Question 353: Which of the following is used for the treatment of hypercholesterolemia?

A. Thiamine
B. Biotin
C. Pyridoxine
D. Nicotinic acid (Correct Answer)

Explanation: **Explanation:** **Nicotinic Acid (Vitamin B3)** is the correct answer because, in pharmacological doses (1.5–3 g/day), it acts as a potent lipid-lowering agent. It inhibits the enzyme **hormone-sensitive lipase** in adipose tissue, reducing the lipolysis of triglycerides into free fatty acids (FFAs). Since FFAs are the primary substrate for hepatic VLDL synthesis, nicotinic acid effectively decreases levels of VLDL, IDL, and LDL. Additionally, it is the most effective agent for **increasing HDL-C levels** by reducing the fractional clearance of apoA-I. **Analysis of Incorrect Options:** * **Thiamine (B1):** Functions as a coenzyme (TPP) for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase). Deficiency leads to Beriberi or Wernicke-Korsakoff syndrome, but it has no role in lipid management. * **Biotin (B7):** Acts as a coenzyme for carboxylation reactions (e.g., Acetyl-CoA carboxylase). While involved in fatty acid synthesis, it is not used therapeutically to treat hypercholesterolemia. * **Pyridoxine (B6):** Essential for transamination, deamination, and heme synthesis. It is often co-administered with Isoniazid to prevent peripheral neuropathy but does not lower cholesterol. **High-Yield Clinical Pearls for NEET-PG:** * **Side Effect:** The most common side effect of Nicotinic acid is **cutaneous flushing**, mediated by Prostaglandin $D_2$. This can be mitigated by taking **Aspirin** 30 minutes prior. * **Metabolic Effects:** It can cause hyperuricemia (precipitating gout) and hyperglycemia (impaired glucose tolerance). * **Pellagra:** Deficiency of Vitamin B3 leads to the "3 Ds": Dermatitis, Diarrhea, and Dementia.

Question 354: Active transport of calcium is regulated by which of the following, which is synthesized in the kidneys?

A. Cholecalciferol
B. Ergosterol
C. 25-hydroxycholecalciferol
D. 1,25-dihydroxycholecalciferol (Correct Answer)

Explanation: **Explanation:** The correct answer is **1,25-dihydroxycholecalciferol (Calcitriol)**. This is the physiologically active form of Vitamin D. **Underlying Medical Concept:** Vitamin D undergoes a two-step activation process. First, it is hydroxylated in the liver to form 25-hydroxycholecalciferol. The second, and rate-limiting step, occurs in the **proximal convoluted tubules of the kidneys**, where the enzyme **1α-hydroxylase** converts it into 1,25-dihydroxycholecalciferol. This active hormone then travels to the intestine, where it binds to nuclear receptors to stimulate the synthesis of **Calbindin**, a calcium-binding protein that facilitates the active transport of calcium across the intestinal mucosa. **Analysis of Incorrect Options:** * **A. Cholecalciferol (Vitamin D3):** This is the inactive form synthesized in the skin via UV light or ingested through diet. It has no biological activity until hydroxylated. * **B. Ergosterol:** This is a precursor found in plants and fungi. When irradiated, it forms Ergocalciferol (Vitamin D2), not the active hormone synthesized in human kidneys. * **C. 25-hydroxycholecalciferol (Calcidiol):** This is the major circulating form of Vitamin D and is synthesized in the **liver**. While it is used to clinically measure a patient’s Vitamin D status, it is not the final active form. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** 1α-hydroxylase (stimulated by PTH and low serum phosphate; inhibited by FGF-23). * **Storage form:** 25-hydroxycholecalciferol (longest half-life). * **Clinical Correlation:** In Chronic Kidney Disease (CKD), the loss of 1α-hydroxylase activity leads to secondary hyperparathyroidism and renal osteodystrophy. * **Function:** Calcitriol increases both Calcium and Phosphate absorption in the gut.

Question 355: Which of the following medications can cause pyridoxine deficiency?

A. Isoniazid (Correct Answer)
B. Chronic renal failure
C. Congestive heart failure
D. All of the above

Explanation: **Explanation:** **1. Why Isoniazid (INH) is the correct answer:** Isoniazid, a primary anti-tubercular drug, is the classic cause of drug-induced **Pyridoxine (Vitamin B6) deficiency**. The mechanism is two-fold: * **Chemical Antagonism:** Isoniazid reacts with pyridoxal phosphate (PLP) to form a hydrazone complex, which is then excreted in the urine. * **Enzyme Inhibition:** It inhibits the enzyme **pyridoxine phosphokinase**, which is essential for converting pyridoxine into its active coenzyme form, PLP. This deficiency leads to decreased synthesis of neurotransmitters (like GABA), manifesting clinically as **peripheral neuropathy**. **2. Why the other options are incorrect:** * **Chronic Renal Failure (CRF):** While patients on dialysis may lose water-soluble vitamins, CRF itself is not a primary "medication" or cause specifically linked to B6 antagonism in the same way INH is. * **Congestive Heart Failure (CHF):** CHF is associated with **Thiamine (B1) deficiency**, especially when treated with loop diuretics (furosemide), which increase the urinary excretion of B1. It does not typically cause B6 deficiency. * **All of the above:** Since CHF and CRF are not primary pharmacological causes of B6 deficiency, this option is incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis:** Always co-administer **10–40 mg/day of Pyridoxine** with Isoniazid to prevent peripheral neuropathy. * **Other B6 Antagonists:** Penicillamine (used in Wilson’s disease), Hydralazine (antihypertensive), and Cycloserine. * **Sideroblastic Anemia:** B6 deficiency can cause microcytic anemia because PLP is a cofactor for **ALA synthase**, the rate-limiting enzyme in heme synthesis. * **Homocysteinemia:** B6 is required for the conversion of homocysteine to cystathionine; deficiency leads to elevated homocysteine levels.

Question 356: Which trace element acts as an antioxidant?

A. Chromium
B. Selenium (Correct Answer)
C. Magnesium
D. Iron

Explanation: **Explanation:** **Selenium** is the correct answer because it is an essential component of the enzyme **Glutathione Peroxidase (GPx)**. This enzyme plays a critical role in the cellular antioxidant defense system by reducing hydrogen peroxide and lipid hydroperoxides into harmless water and alcohols, thereby protecting cell membranes from oxidative damage. Selenium exists in the active site of these enzymes as the "21st amino acid," **Selenocysteine**. **Analysis of Incorrect Options:** * **Chromium (A):** Primarily functions as a component of the **Glucose Tolerance Factor (GTF)**. It enhances the action of insulin but does not possess direct antioxidant properties. * **Magnesium (C):** Acts as a cofactor for over 300 enzymes, particularly those involving **ATP utilization** and transfer (kinases). It is a macro-mineral, not a primary antioxidant trace element. * **Iron (D):** While essential for oxygen transport (Hemoglobin) and the Electron Transport Chain (Cytochromes), free iron is actually a **pro-oxidant**. Through the **Fenton reaction**, it generates highly reactive hydroxyl radicals that cause oxidative stress. **High-Yield Clinical Pearls for NEET-PG:** * **Keshan Disease:** A cardiomyopathy resulting from Selenium deficiency (originally described in China). * **Kashin-Beck Disease:** An osteoarthropathy associated with Selenium deficiency. * **Antioxidant Trio:** Remember that Selenium works synergistically with **Vitamin E**. * **Other Antioxidant Enzymes:** Superoxide Dismutase (requires Copper, Zinc, or Manganese) and Catalase (requires Heme/Iron).

Question 357: Which of the following is NOT a coenzyme required for the pyruvate dehydrogenase complex?

A. FAD
B. NAD
C. Folic acid (Correct Answer)
D. Lipoate

Explanation: **Explanation:** The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme cluster that catalyzes the oxidative decarboxylation of pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. This complex requires **five specific coenzymes** to function, often remembered by the mnemonic **"Tender Loving Care For Nancy."** 1. **Thiamine pyrophosphate (TPP):** Derived from Vitamin B1. 2. **Lipoate (Lipoic acid):** Acts as an acyl carrier. 3. **Coenzyme A (CoA):** Derived from Vitamin B5 (Pantothenic acid). 4. **FAD:** Derived from Vitamin B2 (Riboflavin). 5. **NAD+:** Derived from Vitamin B3 (Niacin). **Why Folic Acid is the correct answer:** **Folic acid (Vitamin B9)** is primarily involved in **one-carbon metabolism** (e.g., DNA synthesis and amino acid metabolism). It plays no role in the PDH complex. **Analysis of Incorrect Options:** * **FAD (Option A):** Serves as the prosthetic group for E3 (Dihydrolipoyl dehydrogenase), accepting electrons from reduced lipoamide. * **NAD (Option B):** Acts as the final electron acceptor in the complex, forming NADH. * **Lipoate (Option D):** Attached to E2 (Dihydrolipoyl transacetylase), it undergoes oxidation/reduction to swing the hydroxyethyl group from TPP to CoA. **High-Yield Clinical Pearls for NEET-PG:** * **Arsenic Poisoning:** Arsenic inhibits the PDH complex by binding to the **SH (sulfhydryl) groups of Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Thiamine Deficiency:** Leads to Beriberi and Wernicke-Korsakoff syndrome because PDH and Alpha-ketoglutarate dehydrogenase cannot function without TPP. * The same five cofactors are also required by the **Alpha-ketoglutarate dehydrogenase** and **Branched-chain alpha-keto acid dehydrogenase** complexes.

Question 358: Which vitamin is a component of Pyruvate Dehydrogenase?

A. Biotin
B. Pyridoxine
C. Folic acid
D. Pantothenic acid (Correct Answer)

Explanation: **Explanation:** The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme system that catalyzes the oxidative decarboxylation of pyruvate to Acetyl-CoA, linking glycolysis to the TCA cycle. **Why Pantothenic Acid is Correct:** The PDH complex requires five essential cofactors, often remembered by the mnemonic **"Tender Loving Care For Nancy"**: 1. **T**hiamine (B1) as TPP 2. **L**ipoic acid 3. **C**oenzyme A (derived from **Pantothenic acid/B5**) 4. **F**AD (derived from Riboflavin/B2) 5. **N**AD+ (derived from Niacin/B3) Pantothenic acid is the precursor for **Coenzyme A (CoA-SH)**, which serves as the carrier for the acetyl group produced during the reaction, forming Acetyl-CoA. **Analysis of Incorrect Options:** * **Biotin (B7):** Acts as a cofactor for **carboxylation** reactions (e.g., Pyruvate Carboxylase). It adds CO2, whereas PDH removes it. * **Pyridoxine (B6):** Primarily involved in **transamination** (e.g., ALT/AST) and decarboxylation of amino acids, not keto-acids like pyruvate. * **Folic acid (B9):** Functions in **one-carbon metabolism** (e.g., DNA synthesis), not oxidative decarboxylation. **High-Yield Clinical Pearls for NEET-PG:** * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and garlic breath. * **Thiamine Deficiency:** Leads to impaired PDH activity, causing **Wernicke-Korsakoff syndrome** or Beriberi, as the brain cannot oxidize glucose efficiently. * **Location:** The PDH complex is located in the **mitochondrial matrix**.

Question 359: Which coenzyme is required for dopa decarboxylase activity?

A. Biotin
B. Vitamin B6 (Correct Answer)
C. Vitamin C
D. Vitamin B12

Explanation: **Explanation:** The correct answer is **Vitamin B6 (Pyridoxine)**. **Why Vitamin B6 is correct:** Dopa decarboxylase (also known as aromatic L-amino acid decarboxylase) is the enzyme responsible for converting L-DOPA into Dopamine. This enzyme requires **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6, as a coenzyme. PLP is the universal coenzyme for almost all **decarboxylation** and **transamination** reactions in the body. In the catecholamine synthesis pathway, PLP-dependent decarboxylation is a critical step in producing neurotransmitters like dopamine, norepinephrine, and epinephrine. **Why the other options are incorrect:** * **A. Biotin (B7):** Acts as a coenzyme for **carboxylation** reactions (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). It adds $CO_2$, whereas decarboxylases remove it. * **C. Vitamin C:** Required for the next step in the pathway—the conversion of Dopamine to Norepinephrine by the enzyme **Dopamine $\beta$-hydroxylase**. * **D. Vitamin B12:** Primarily involved in DNA synthesis (via folate metabolism) and the conversion of methylmalonyl-CoA to succinyl-CoA. It is not involved in decarboxylation. **High-Yield Clinical Pearls for NEET-PG:** * **Parkinson’s Disease:** Peripheral Dopa decarboxylase can prematurely convert Levodopa to Dopamine before it crosses the blood-brain barrier. This is why Levodopa is co-administered with **Carbidopa**, a peripheral Dopa decarboxylase inhibitor. * **PLP Dependency:** Other high-yield PLP-dependent enzymes include **AST/ALT**, **Histidine decarboxylase** (Histamine), and **$\delta$-ALA synthase** (Heme synthesis). * **Isoniazid (INH):** This anti-tubercular drug can cause Vitamin B6 deficiency, leading to peripheral neuropathy.

Question 360: A 35-year-old alcoholic presents with diarrhea and scaly rash on his neck, hands, and feet. With which of the following treatments would he likely improve?

A. Vitamin A
B. Thiamine
C. Folic acid
D. Nicotinic acid (Correct Answer)

Explanation: ### Explanation The patient presents with the classic clinical triad of **Pellagra**: **D**ermatitis, **D**iarrhea, and **D**ementia (the "3 Ds"). Pellagra is caused by a deficiency of **Niacin (Vitamin B3)** or its precursor, the amino acid **Tryptophan**. **Why Nicotinic Acid is correct:** Nicotinic acid (Niacin) is the treatment of choice. In alcoholics, poor dietary intake and impaired absorption lead to niacin deficiency. The characteristic "scaly rash" is a photosensitive dermatitis; when it occurs around the neck, it is specifically referred to as **Casal’s necklace**. Niacin is essential for the synthesis of coenzymes **NAD and NADP**, which are critical for redox reactions in energy metabolism. **Why the other options are incorrect:** * **Vitamin A:** Deficiency typically presents with ocular symptoms like night blindness (nyctalopia) and Xerophthalmia, not the 3 Ds of pellagra. * **Thiamine (B1):** While common in alcoholics, B1 deficiency leads to **Beriberi** (cardiac or neurological involvement) or **Wernicke-Korsakoff syndrome**, characterized by ataxia, ophthalmoplegia, and confusion. * **Folic acid:** Deficiency primarily causes **megaloblastic anemia** and glossitis, but not the specific photosensitive dermatitis seen here. **High-Yield Clinical Pearls for NEET-PG:** * **The 4th D:** If untreated, Pellagra leads to **Death**. * **Hartnup Disease:** A genetic disorder involving defective tryptophan transport that can result in "Pellagra-like" symptoms. * **Carcinoid Syndrome:** Can cause niacin deficiency because tryptophan is diverted to overproduce Serotonin. * **Corn-based diets:** Maize is low in tryptophan and contains niacin in a bound, unabsorbable form (niacytin), predisposed to Pellagra.

Practice by Chapter

Fat-Soluble Vitamins: A, D, E, K

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Vitamin A and Vision

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Vitamin D and Calcium Metabolism

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Vitamin E and Antioxidant Functions

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Vitamin K and Blood Coagulation

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Water-Soluble Vitamins: B Complex and C

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Thiamine (B1) and Pyruvate Dehydrogenase

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Riboflavin (B2) and Flavin Coenzymes

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Niacin and NAD/NADP

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Vitamin B6 and Transamination

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Folate and Vitamin B12 in One-Carbon Metabolism

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Vitamin C and Collagen Synthesis

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