Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 61: Which vitamin, when administered in large doses, is known to decrease both triglyceride and cholesterol levels?

A. Vitamin B1
B. Nicotinic acid (Correct Answer)
C. Vitamin B12
D. Riboflavin

Explanation: **Explanation:** **Nicotinic acid (Vitamin B3)**, when administered in pharmacological doses (1–3 grams/day), acts as a potent lipid-lowering agent. Its primary mechanism involves the inhibition of **hormone-sensitive lipase** in adipose tissue. This reduces the lipolysis of triglycerides into free fatty acids (FFAs). Since the liver depends on these FFAs to synthesize **VLDL**, a decrease in FFA flux leads to reduced VLDL production. Consequently, **LDL** (a product of VLDL metabolism) and **triglycerides** decrease. Additionally, it inhibits the hepatic enzyme *diacylglycerol acyltransferase-2*, further lowering TG synthesis, and increases **HDL-C** levels by reducing the fractional catabolic rate of ApoA-1. **Analysis of Incorrect Options:** * **Vitamin B1 (Thiamine):** Primarily functions as a coenzyme (TPP) in carbohydrate metabolism (e.g., Pyruvate dehydrogenase). It has no direct role in lipid lowering. * **Vitamin B12 (Cobalamin):** Essential for DNA synthesis and myelin formation. Deficiency leads to megaloblastic anemia but does not affect systemic lipid profiles. * **Riboflavin (Vitamin B2):** A precursor for FAD and FMN, involved in redox reactions. While it participates in fatty acid oxidation (Beta-oxidation), it does not lower serum cholesterol or triglycerides when given in large doses. **High-Yield Clinical Pearls for NEET-PG:** * **Side Effects:** The most common side effect is **cutaneous flushing**, mediated by Prostaglandin $D_2$ (can be pre-treated with Aspirin). It can also cause **hyperuricemia** (precipitating gout) and **hyperglycemia** (impaired glucose tolerance). * **Drug of Choice:** It is the most effective drug for increasing HDL levels. * **Note:** Nicotinamide (the other form of B3) does *not* have lipid-lowering properties; only Nicotinic acid is effective.

Question 62: Acetyl CoA carboxylase requires which vitamin for its function?

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

Explanation: **Explanation:** **Acetyl CoA Carboxylase (ACC)** is the rate-limiting enzyme in fatty acid synthesis. It converts Acetyl CoA to Malonyl CoA. This reaction is a **carboxylation** process, which fundamentally requires **Biotin (Vitamin B7)** as a coenzyme. Biotin acts as a carrier of activated carbon dioxide ($CO_2$). The mechanism involves the covalent attachment of biotin to a lysine residue of the enzyme (forming biocytin), which then picks up $CO_2$ to transfer it to the substrate. **Analysis of Incorrect Options:** * **Thiamin (B1):** Functions as Thiamine Pyrophosphate (TPP). It is involved in **oxidative decarboxylation** (e.g., Pyruvate Dehydrogenase) and transketolase reactions, not carboxylation. * **Riboflavin (B2):** Functions as FAD/FMN. It is involved in **oxidation-reduction (redox)** reactions (e.g., Succinate Dehydrogenase). * **Niacin (B3):** Functions as NAD/NADP. It is primarily involved in **redox reactions** and electron transport, acting as a hydrogen carrier. **High-Yield Clinical Pearls for NEET-PG:** * **The "ABC" Rule:** Most carboxylases require **A**TP, **B**iotin, and **C**O₂. * **Key Biotin-Dependent Enzymes:** 1. Pyruvate Carboxylase (Gluconeogenesis) 2. Acetyl CoA Carboxylase (Fatty acid synthesis) 3. Propionyl CoA Carboxylase (VLCFA metabolism) * **Clinical Correlation:** Consumption of **raw egg whites** (containing the protein **Avidin**) can lead to biotin deficiency because avidin binds biotin with high affinity, preventing its absorption. * **Localization:** Acetyl CoA Carboxylase is located in the **cytosol**.

Question 63: Which of the following is a pantothenic group?

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

Explanation: **Explanation:** The correct answer is **Pantothenic group (Vitamin B5)**. **1. Why it is correct:** Pantothenic acid (Vitamin B5) is a vital constituent of **Coenzyme A (CoA-SH)** and the **Acyl Carrier Protein (ACP)**. The functional part of these molecules is the "pantothenic group," which consists of pantoic acid joined to β-alanine. Coenzyme A plays a central role in metabolism by acting as a carrier for acyl groups (forming Acetyl-CoA, Succinyl-CoA, etc.) via a high-energy thioester bond. It is essential for the TCA cycle, fatty acid synthesis, and fatty acid oxidation. **2. Why the other options are incorrect:** * **Biotin (Vitamin B7):** Acts as a coenzyme for **carboxylation reactions** (e.g., Pyruvate carboxylase). It is often referred to as the "CO2 carrier." * **Folic Acid (Vitamin B9):** Functions as Tetrahydrofolate (THF), which is involved in **one-carbon metabolism** (transfer of methyl, formyl, or methylene groups), crucial for DNA synthesis. * **Cobalamin (Vitamin B12):** Contains a corrin ring with a central cobalt atom. It is a coenzyme for only two human enzymes: Methionine synthase and Methylmalonyl-CoA mutase. **3. NEET-PG High-Yield Pearls:** * **Active Form:** The active form of Pantothenic acid is Coenzyme A. * **Deficiency:** Though rare, deficiency leads to **"Burning Feet Syndrome"** (Gopalan’s syndrome). * **Key Enzyme Complex:** Pantothenic acid is a component of the **Fatty Acid Synthase (FAS) multienzyme complex** (specifically the ACP arm). * **Link Reaction:** It is a required cofactor for the Pyruvate Dehydrogenase (PDH) complex (along with B1, B2, B3, and Lipoic acid).

Question 64: What is the common name for pyridoxine?

A. Vitamin B1
B. Vitamin B2
C. Vitamin B3
D. Vitamin B6 (Correct Answer)

Explanation: **Explanation:** **Vitamin B6** is the collective term for a group of three related compounds: **pyridoxine**, pyridoxal, and pyridoxamine. These are precursors to the active coenzyme **Pyridoxal Phosphate (PLP)**. PLP is a versatile cofactor essential for over 100 enzymatic reactions, most notably in **amino acid metabolism** (transamination, decarboxylation, and deamination). It is also crucial for heme synthesis (cofactor for ALA synthase) and the conversion of tryptophan to niacin. **Analysis of Incorrect Options:** * **A. Vitamin B1 (Thiamine):** Its active form is Thiamine Pyrophosphate (TPP), which acts as a coenzyme for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase) and the transketolase reaction in the HMP shunt. * **B. Vitamin B2 (Riboflavin):** It is the precursor for FMN and FAD, which act as hydrogen carriers in redox reactions (e.g., Succinate Dehydrogenase in the TCA cycle). * **C. Vitamin B3 (Niacin/Nicotinic acid):** It forms NAD and NADP, essential for electron transfer in glycolysis, the TCA cycle, and fatty acid synthesis. **Clinical Pearls for NEET-PG:** * **Drug Interaction:** **Isoniazid (INH)**, used in TB treatment, inhibits pyridoxine kinase, leading to B6 deficiency. This manifests as **peripheral neuropathy**; hence, B6 is always co-prescribed with INH. * **Sideroblastic Anemia:** Since B6 is a cofactor for ALA synthase, its deficiency impairs heme synthesis, leading to microcytic hypochromic anemia with ringed sideroblasts. * **Homocystinuria:** B6 is a cofactor for **Cystathionine beta-synthase**. Deficiency can lead to elevated homocysteine levels, a risk factor for thrombosis.

Question 65: What is the Recommended Dietary Allowance (RDA) of Retinol during pregnancy?

A. 500 mcg
B. 600 mcg
C. 700 mcg
D. 800 mcg (Correct Answer)

Explanation: **Explanation:** The Recommended Dietary Allowance (RDA) for Vitamin A (Retinol) is a high-yield topic for NEET-PG, specifically following the **ICMR-NIN 2020 guidelines**. **1. Why Option D (800 mcg) is Correct:** During pregnancy, there is an increased physiological demand for Vitamin A to support fetal growth, organogenesis, and the maintenance of maternal epithelial integrity. According to the latest ICMR guidelines, the RDA for a non-pregnant adult woman is **840 mcg/day**. However, for **pregnancy**, the requirement is specifically categorized as **800 mcg/day of Retinol** (or 6400 mcg of Beta-carotene). This ensures adequate placental transfer without reaching teratogenic thresholds. **2. Analysis of Incorrect Options:** * **Option A (500 mcg):** This value is significantly below the required intake for any adult female category and would put the mother at risk of night blindness. * **Option B (600 mcg):** Previously, older guidelines (ICMR 2010) suggested lower values, but these have been revised upward in the 2020 update. * **Option C (700 mcg):** While closer to the baseline, it does not meet the specific 800 mcg threshold established for the gestational period. **3. Clinical Pearls & High-Yield Facts:** * **Lactation:** The RDA increases significantly during lactation to **950 mcg/day** to compensate for Vitamin A lost in breast milk. * **Teratogenicity:** While deficiency is dangerous, excessive intake (>3000 mcg/day) of preformed Vitamin A is **teratogenic**, potentially causing craniofacial and cardiac defects (Retinoic acid embryopathy). * **Conversion:** 1 mcg of Retinol = 8 mcg of Beta-carotene (as per ICMR 2020). * **Storage:** Vitamin A is stored in the liver in **Ito cells** (Stellate cells) as retinyl palmitate.

Question 66: Which condition is caused by thiamine deficiency?

A. Pellagra
B. Beri-beri (Correct Answer)
C. Keshan's disease
D. Rickets

Explanation: ### Explanation **Correct Answer: B. Beri-beri** **Medical Concept:** Thiamine (Vitamin B1) is a crucial water-soluble vitamin that acts as a precursor to **Thiamine Pyrophosphate (TPP)**. TPP serves as a coenzyme for key enzymes in carbohydrate metabolism, including Pyruvate Dehydrogenase, $\alpha$-ketoglutarate dehydrogenase, and Transketolase. Deficiency leads to impaired ATP production and the accumulation of pyruvate/lactate, primarily affecting high-energy demand tissues like the heart and brain. This clinical manifestation is known as **Beri-beri**. **Analysis of Options:** * **Pellagra (Option A):** Caused by a deficiency of **Niacin (Vitamin B3)**. It is characterized by the "4 Ds": Dermatitis, Diarrhea, Dementia, and Death. * **Keshan’s Disease (Option C):** A cardiomyopathy caused by a deficiency of the trace element **Selenium**. It was traditionally endemic in parts of China with selenium-poor soil. * **Rickets (Option D):** Caused by a deficiency of **Vitamin D**, calcium, or phosphate in children, leading to impaired mineralization of the growth plate and bone deformities. **High-Yield Clinical Pearls for NEET-PG:** 1. **Types of Beri-beri:** * **Dry Beri-beri:** Characterized by peripheral neuropathy and muscle wasting. * **Wet Beri-beri:** Characterized by high-output heart failure and edema. 2. **Wernicke-Korsakoff Syndrome:** A severe CNS manifestation of thiamine deficiency common in chronic alcoholics, presenting with the triad of ataxia, ophthalmoplegia, and confusion. 3. **Diagnostic Marker:** The most reliable biochemical test for thiamine status is the measurement of **Erythrocyte Transketolase Activity**. 4. **Clinical Caution:** Always administer thiamine *before* glucose in malnourished or alcoholic patients to prevent precipitating Wernicke encephalopathy.

Question 67: Which vitamin is excreted in the urine?

A. Vitamin A
B. Vitamin C (Correct Answer)
C. Vitamin D
D. Vitamin K

Explanation: **Explanation:** The correct answer is **Vitamin C**. The fundamental concept behind this question is the classification of vitamins based on their solubility. **1. Why Vitamin C is correct:** Vitamins are categorized into **Water-soluble** (B-complex and C) and **Fat-soluble** (A, D, E, and K). Water-soluble vitamins are not stored in the body to any significant extent (except Vitamin B12). When consumed in excess of the body's requirements, they are easily filtered by the kidneys and **excreted in the urine**. Vitamin C (Ascorbic acid) is highly water-soluble; once the renal threshold is exceeded, the surplus is eliminated via the urinary tract. **2. Why the other options are incorrect:** * **Vitamins A, D, and K:** These are **Fat-soluble vitamins**. They are absorbed along with dietary lipids and stored in the liver and adipose tissue. Because they are not soluble in water, they cannot be easily filtered by the glomerulus and excreted in urine. Instead, they are primarily excreted via the **bile/feces**. Due to their storage capacity, excessive intake of these vitamins can lead to toxicity (Hypervitaminosis). **NEET-PG High-Yield Pearls:** * **Vitamin B12 Exception:** Although water-soluble, Vitamin B12 is stored in the liver for 3–5 years. * **Renal Threshold:** Vitamin C acts as a threshold substance; it appears in urine only after plasma levels exceed ~1.4 mg/dL. * **Clinical Correlation:** Large doses of Vitamin C can lead to **Oxalate stones** in the kidney, as oxalate is a metabolic end-product of ascorbic acid. * **Heat Lability:** Vitamin C is the most heat-labile vitamin; it is easily destroyed by cooking.

Question 68: Which of the following vitamins or coenzymes is NOT required for the conversion of pyruvate to acetyl CoA?

A. Thiamine
B. Thiamine pyrophosphate (TPP)
C. Pyridoxine (Correct Answer)
D. Flavin adenine dinucleotide (FAD)

Explanation: The conversion of pyruvate to acetyl CoA is catalyzed by the **Pyruvate Dehydrogenase (PDH) Complex**, a multi-enzyme system located in the mitochondrial matrix. This reaction is a critical "link reaction" between glycolysis and the TCA cycle. ### Why Pyridoxine is the Correct Answer **Pyridoxine (Vitamin B6)**, in its active form Pyridoxal Phosphate (PLP), is primarily involved in **transamination**, decarboxylation of amino acids, and glycogen phosphorylase activity. It plays **no role** in the PDH complex. Therefore, it is the correct answer as the "exception." ### Analysis of Other Options (The 5 Required Cofactors) The PDH complex requires five specific cofactors to function (Mnemonic: **"Tender Loving Care For Nancy"**): 1. **Thiamine pyrophosphate (TPP):** Derived from **Thiamine (Vitamin B1)**. It acts as a coenzyme for the E1 subunit (pyruvate dehydrogenase) to facilitate decarboxylation. (Options A and B are required). 2. **Lipoic Acid (Lipoamide):** Acts as an electron and acyl group carrier. 3. **Coenzyme A (CoA):** Derived from Pantothenic acid (B5); accepts the acetyl group. 4. **FAD:** Derived from **Riboflavin (Vitamin B2)**; acts as a prosthetic group for the E3 subunit. (Option D is required). 5. **NAD+:** Derived from Niacin (B3); serves as the final electron acceptor. ### NEET-PG High-Yield Pearls * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Thiamine Deficiency:** Leads to impaired PDH activity, causing **Wernicke-Korsakoff syndrome** or Beriberi, as the brain cannot effectively oxidize glucose for energy. * **Regulation:** PDH is inhibited by its products (Acetyl CoA, NADH) and activated by ADP and $Ca^{2+}$.

Question 69: Benedict's test will be positive in the urine after administration of which of the following?

A. Folic acid
B. Ascorbic acid (Correct Answer)
C. Pantothetic acid
D. Retinoic acid

Explanation: **Explanation:** **Correct Answer: B. Ascorbic acid** **Mechanism:** Benedict’s test is a semi-quantitative test used to detect **reducing substances** in the urine. It relies on the ability of a reducing agent to convert cupric ions ($Cu^{2+}$) in the Benedict’s reagent to cuprous ions ($Cu^+$), forming a colored precipitate (cuprous oxide). **Ascorbic acid (Vitamin C)** is a potent reducing agent. When consumed in high doses, it is excreted in the urine. Due to its chemical structure, it can reduce the copper sulfate in Benedict’s reagent, leading to a **false-positive** result for glycosuria. This is a classic biochemical pitfall where a non-sugar substance interferes with a test intended for reducing sugars like glucose, fructose, or galactose. **Analysis of Incorrect Options:** * **A. Folic Acid (B9):** A water-soluble vitamin involved in one-carbon metabolism. It does not possess the strong reducing properties required to react with Benedict’s reagent. * **C. Pantothenic Acid (B5):** A precursor to Coenzyme A. It is excreted in urine but does not act as a reducing agent in this chemical environment. * **D. Retinoic Acid (Vitamin A):** A fat-soluble vitamin. It is not excreted in significant quantities in the urine in a form that would react with aqueous Benedict’s reagent. **Clinical Pearls for NEET-PG:** 1. **False Positives in Benedict's Test:** Other non-sugar reducing substances include **Urates, Creatinine, Salicylates, and Homogentisic acid** (seen in Alkaptonuria). 2. **Specific Test for Glucose:** To differentiate between glucose and other reducing substances (like Vitamin C), the **Glucose Oxidase method (Dipstick)** is used, as it is specific only to glucose. 3. **Scurvy Connection:** Ascorbic acid is essential for the hydroxylation of proline and lysine residues during **collagen synthesis**. Its deficiency leads to Scurvy.

Question 70: Vitamin E deficiency is associated with which of the following conditions?

A. Ataxia (Correct Answer)
B. Cholestasis
C. Hyperkeratosis
D. Pseudotumour cerebri

Explanation: **Explanation:** **Vitamin E (Tocopherol)** acts as a potent lipid-soluble antioxidant, protecting cell membranes from oxidative damage caused by free radicals. It is particularly crucial for maintaining the integrity of long axons and red blood cell membranes. **Why Ataxia is correct:** Vitamin E deficiency leads to a clinical syndrome characterized by **spinocerebellar ataxia**, posterior column loss (loss of vibration and position sense), and peripheral neuropathy. The lack of antioxidant protection leads to the degeneration of large myelinated axons in the posterior columns and demyelination of the spinocerebellar tracts. This manifests as gait instability and incoordination, mimicking Friedreich’s ataxia. **Why the other options are incorrect:** * **B. Cholestasis:** This is a *cause* of Vitamin E deficiency, not a result. Since Vitamin E is fat-soluble, it requires bile for absorption; thus, chronic cholestatic liver disease leads to its malabsorption. * **C. Hyperkeratosis:** Specifically "Phrynoderma" (follicular hyperkeratosis), is classically associated with **Vitamin A** deficiency. * **D. Pseudotumour cerebri:** This is a clinical feature of **Vitamin A toxicity** (Hypervitaminosis A), not Vitamin E deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Hemolytic Anemia:** In newborns (especially pre-term), Vitamin E deficiency causes oxidative stress on RBCs, leading to hemolysis. * **Acanthocytosis:** Look for "thorny" RBCs on a peripheral smear in cases of severe deficiency (often seen in Abetalipoproteinemia). * **Differential Diagnosis:** Always differentiate Vitamin E deficiency from Vitamin B12 deficiency; both present with posterior column loss, but Vitamin E deficiency **lacks** the megaloblastic anemia and hypersegmented neutrophils seen in B12 deficiency.

Practice by Chapter

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

Practice Questions

Vitamin A and Vision

Practice Questions

Vitamin D and Calcium Metabolism

Practice Questions

Vitamin E and Antioxidant Functions

Practice Questions

Vitamin K and Blood Coagulation

Practice Questions

Water-Soluble Vitamins: B Complex and C

Practice Questions

Thiamine (B1) and Pyruvate Dehydrogenase

Practice Questions

Riboflavin (B2) and Flavin Coenzymes

Practice Questions

Niacin and NAD/NADP

Practice Questions

Vitamin B6 and Transamination

Practice Questions

Folate and Vitamin B12 in One-Carbon Metabolism

Practice Questions

Vitamin C and Collagen Synthesis

Practice Questions

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