Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 91: What is the major function of Vitamin E in the body?

A. Regulation of energy metabolism
B. Carboxylation reaction
C. Blood clotting
D. Protection of biological membranes from free radical damage (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is correct:** Vitamin E (specifically **α-tocopherol**) is the body’s most potent **lipid-soluble antioxidant**. Its primary role is to protect polyunsaturated fatty acids (PUFAs) in biological membranes and low-density lipoproteins (LDL) from **lipid peroxidation**. It acts as a "chain-breaker" by scavenging free radicals (like superoxide and hydroxyl radicals), donating a hydrogen atom to neutralize them before they can damage the cell membrane. This is particularly vital in red blood cell (RBC) membranes and neurological tissues. **2. Why other options are incorrect:** * **Option A (Energy metabolism):** This is the primary role of **Vitamin B complex** (e.g., Thiamine/B1, Riboflavin/B2, Niacin/B3), which act as coenzymes in the TCA cycle and Electron Transport Chain. * **Option B (Carboxylation):** This is the specific function of **Biotin (B7)** (e.g., Pyruvate carboxylase) and **Vitamin K** (gamma-carboxylation of glutamic acid). * **Option C (Blood clotting):** This is the classic function of **Vitamin K**, which is essential for the post-translational modification of clotting factors II, VII, IX, and X. **3. NEET-PG High-Yield Pearls:** * **Deficiency Manifestations:** Hemolytic anemia (due to RBC membrane fragility), posterior column signs (loss of position and vibration sense), and spinocerebellar ataxia. * **Regeneration:** Vitamin E is regenerated (reduced) back to its active form by **Vitamin C (Ascorbic acid)**. * **Toxicity:** High doses of Vitamin E can interfere with Vitamin K metabolism, leading to an increased risk of bleeding (prolonged PT/INR). * **Storage:** Unlike other fat-soluble vitamins stored in the liver, Vitamin E is primarily stored in **adipose tissue**.

Question 92: Which of the following demonstrates the functional assessment of Vitamin B1 deficiency?

A. RBC Transketolase (Correct Answer)
B. RBC Glutathione Reductase
C. Serum Thiamine Levels
D. RBC Glutathione Peroxidase

Explanation: ### Explanation **Correct Option: A. RBC Transketolase** Vitamin B1 (Thiamine) acts as a vital coenzyme in the form of **Thiamine Pyrophosphate (TPP)**. To assess thiamine status functionally, we measure the activity of the TPP-dependent enzyme **Transketolase** within Red Blood Cells (RBCs). * **The Concept:** In a laboratory setting, RBC transketolase activity is measured before and after the addition of exogenous TPP. An increase in enzyme activity (known as the "TPP effect") of >15–25% indicates a functional deficiency of Vitamin B1. This is considered more reliable than static serum levels because it reflects intracellular metabolic status. **Analysis of Incorrect Options:** * **B. RBC Glutathione Reductase:** This is the functional marker for **Vitamin B2 (Riboflavin)** deficiency. This enzyme requires **FAD** (Flavin Adenine Dinucleotide) as a cofactor. * **C. Serum Thiamine Levels:** While these can be measured, they are "static" markers that reflect recent dietary intake rather than "functional" intracellular stores. They are less sensitive and specific than the transketolase assay. * **D. RBC Glutathione Peroxidase:** This is a functional marker used to assess **Selenium** status. It requires Selenium as a cofactor to protect cells from oxidative damage. **High-Yield Clinical Pearls for NEET-PG:** * **Key TPP-dependent enzymes:** Pyruvate Dehydrogenase (PDH), $\alpha$-Ketoglutarate Dehydrogenase, and Transketolase (HMP Shunt). * **Clinical Triad of Wernicke’s Encephalopathy:** Ophthalmoplegia, Ataxia, and Confusion. * **Korsakoff Psychosis:** Characterized by irreversible confabulation and amnesia (damage to mammillary bodies). * **Management Rule:** Always administer Thiamine **before** Glucose in a malnourished/alcoholic patient to prevent precipitating acute Wernicke’s (as glucose oxidation consumes remaining thiamine).

Question 93: Vitamin required for the activity of transaminases is:

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

Explanation: **Explanation:** The correct answer is **Pyridoxine (Vitamin B6)**. **1. Why Pyridoxine is Correct:** Pyridoxine, in its active coenzyme form **Pyridoxal Phosphate (PLP)**, is essential for the metabolism of amino acids. Transamination is the process where an amino group is transferred from an amino acid to a keto-acid, catalyzed by enzymes called **transaminases** (e.g., ALT and AST). PLP acts as a temporary carrier of the amino group, shifting between its aldehyde form (PLP) and its aminated form (Pyridoxamine phosphate) during the reaction. **2. Why Other Options are Incorrect:** * **Folic Acid (B9):** Primarily involved in **one-carbon metabolism** (transfer of methyl, formyl, or methylene groups), essential for DNA synthesis and erythropoiesis. * **Nicotinic Acid (B3):** Functions as NAD/NADP, which are coenzymes for **redox (oxidation-reduction) reactions**, such as those in glycolysis and the TCA cycle. * **Thiamine (B1):** Its active form, TPP, is a cofactor for **oxidative decarboxylation** (e.g., Pyruvate Dehydrogenase) and the **transketolase** reaction in the HMP shunt. **3. High-Yield Clinical Pearls for NEET-PG:** * **PLP-Dependent Reactions:** Besides transamination, PLP is required for **decarboxylation** (e.g., synthesis of GABA, Histamine, Serotonin, Heme) and **cystathionine synthesis**. * **Isoniazid (INH) Link:** INH (anti-TB drug) inhibits pyridoxine kinase, leading to B6 deficiency and **peripheral neuropathy**. Always co-administer B6 with INH. * **Xanthurenic Acid:** Deficiency of B6 leads to increased urinary excretion of xanthurenic acid due to impaired tryptophan metabolism.

Question 94: Deficiency of which of the following vitamins does not affect tooth development?

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

Explanation: **Explanation:** The correct answer is **Vitamin K**. Tooth development (odontogenesis) relies on the coordinated action of vitamins that regulate epithelial differentiation, mineral homeostasis, and collagen synthesis. Vitamin K is primarily involved in the gamma-carboxylation of clotting factors (II, VII, IX, X) and bone proteins (osteocalcin). While it plays a role in bone mineralization, its deficiency does not have a recognized direct impact on the structural development of teeth. **Why the other options are incorrect:** * **Vitamin A:** Essential for the differentiation of epithelial cells. In the context of teeth, it is required for **ameloblasts** (enamel-forming cells). Deficiency leads to enamel hypoplasia and defective dentin formation. * **Vitamin D:** Crucial for calcium and phosphorus homeostasis. Deficiency during tooth development leads to **delayed eruption**, enamel hypoplasia, and a widened predentin layer (similar to Rickets in bone). * **Vitamin C:** Necessary for the hydroxylation of proline and lysine during **collagen synthesis**. Since dentin is approximately 90% collagen, deficiency (Scurvy) leads to defective dentin formation and atrophy of **odontoblasts**. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin A deficiency:** Causes keratinizing metaplasia of specialized epithelium. * **Vitamin C deficiency:** Leads to "scorbutic teeth," characterized by internal hemorrhages in the pulp and gingival bleeding. * **Vitamin D-resistant Rickets:** Often presents with spontaneous periapical abscesses due to large pulp chambers and thin dentin. * **Vitamin K:** Remember the mnemonic **"1972"** for the clotting factors it activates (Factors II, VII, IX, and X).

Question 95: Harup disease causes deficiency of?

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

Explanation: **Explanation:** **Hartnup disease** is an autosomal recessive metabolic disorder characterized by a defect in the **SLC6A19 transporter**. This transporter is responsible for the absorption of neutral amino acids (especially **Tryptophan**) in the small intestine and their reabsorption in the renal proximal tubules. **Why Nicotinamide is the correct answer:** Tryptophan is a vital precursor for the endogenous synthesis of **Niacin (Vitamin B3)**; approximately 60 mg of Tryptophan yields 1 mg of Niacin. In Hartnup disease, the profound malabsorption and renal loss of Tryptophan lead to a secondary deficiency of Niacin/Nicotinamide. This results in **Pellagra-like symptoms**, as Niacin is essential for the formation of NAD+ and NADP+, which are critical coenzymes in redox reactions. **Why other options are incorrect:** * **Thiamine (B1):** Deficiency causes Beriberi or Wernicke-Korsakoff syndrome; its transport is unrelated to the SLC6A19 transporter. * **Riboflavin (B2):** Deficiency causes cheilosis and corneal vascularization; it is not derived from Tryptophan. * **Biotin (B7):** Deficiency is usually associated with raw egg white consumption (avidin) or carboxylase defects, not neutral amino acid transport. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Dermatitis (photosensitive rash/Casal’s necklace), Cerebellar Ataxia, and Aminoaciduria. * **Diagnostic Hallmark:** **Neutral Aminoaciduria** (detected via chromatography). Note that proline, hydroxyproline, and arginine are excreted normally (distinguishing it from Fanconi syndrome). * **Treatment:** High-protein diet and **Nicotinamide** supplementation. * **Blue Diaper Syndrome:** A related condition where bacterial degradation of unabsorbed tryptophan in the gut leads to indicanuria, turning diapers blue.

Question 96: Which vitamin also acts as a hormone?

A. Vitamin D (Correct Answer)
B. Vitamin A
C. Vitamin B1
D. Vitamin C

Explanation: **Explanation:** **Vitamin D** is considered a hormone rather than just a vitamin because it is synthesized endogenously in the skin (via UV light), travels through the bloodstream to distant target organs, and exerts its effects via specific intracellular receptors (**VDR**). The active form, **1,25-dihydroxycholicalciferol (Calcitriol)**, functions like a steroid hormone by binding to nuclear receptors and regulating gene expression to maintain calcium and phosphorus homeostasis. **Analysis of Options:** * **Vitamin A (Retinol):** While its derivative, Retinoic Acid, acts via nuclear receptors to regulate cell differentiation, Vitamin D is the classic answer for this question as it strictly follows the endocrine definition (produced in one site, acting on distant sites like the gut and bone). * **Vitamin B1 (Thiamine):** This is a water-soluble vitamin that acts as a coenzyme (Thiamine Pyrophosphate - TPP) for decarboxylation reactions (e.g., Pyruvate Dehydrogenase). It has no hormonal activity. * **Vitamin C (Ascorbic Acid):** This acts as a water-soluble antioxidant and a co-factor for hydroxylation reactions (e.g., collagen synthesis). It does not function through hormonal signaling pathways. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** 1-alpha-hydroxylase (located in the proximal convoluted tubule of the kidney). * **Storage form:** 25-hydroxyvitamin D [25(OH)D] is the major circulating form used to clinically assess Vitamin D status. * **Mechanism:** Calcitriol increases the synthesis of **Calbindin** in intestinal cells to facilitate calcium absorption. * **Clinical Correlation:** Deficiency leads to Rickets (children) and Osteomalacia (adults).

Question 97: In folic acid deficiency, which of the following is excreted in urine?

A. Kynurenine
B. Xantheurenic acid
C. Formiminoglutamic acid (FIGLU) (Correct Answer)
D. Methylmalonate

Explanation: **Explanation:** The correct answer is **Formiminoglutamic acid (FIGLU)**. **1. Why FIGLU is the correct answer:** The metabolism of the amino acid **Histidine** involves its conversion to Glutamate. A key intermediate in this pathway is FIGLU. Under normal conditions, the enzyme *formiminotransferase* transfers the formimino group from FIGLU to **Tetrahydrofolate (THF)**, producing Glutamate and N5-formimino-THF. In **Folic acid deficiency**, THF is unavailable to accept the formimino group. Consequently, FIGLU accumulates in the body and is excreted in the urine. The **FIGLU Excretion Test** (often performed after a histidine load) is a sensitive functional indicator of folate status. **2. Why the other options are incorrect:** * **Kynurenine & Xanthurenic acid (Options A & B):** These are intermediates in the metabolism of **Tryptophan**. Their excretion in urine is a hallmark of **Vitamin B6 (Pyridoxine) deficiency**, as the enzyme kynureninase requires B6. * **Methylmalonate (Option D):** Methylmalonic acid (MMA) is excreted in the urine in **Vitamin B12 (Cobalamin) deficiency**. B12 is a cofactor for *methylmalonyl-CoA mutase*; its absence leads to MMA accumulation. This is a crucial lab finding to differentiate B12 deficiency from Folate deficiency. **3. Clinical Pearls for NEET-PG:** * **FIGLU Test:** Used for Folate deficiency. * **MMA Test:** Used for B12 deficiency (More specific than homocysteine). * **Homocysteine:** Elevated in **both** B12 and Folate deficiency. * **Schilling Test:** Historically used to determine the cause of B12 malabsorption (e.g., Pernicious anemia). * **Histidinemia:** A rare metabolic disorder where histidine levels are high due to histidase deficiency, but FIGLU is not elevated.

Question 98: Acyl carrier protein (ACP) occurs in a bound state to a derivative of which vitamin?

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

Explanation: **Explanation:** **Acyl Carrier Protein (ACP)** is a key component of the **Fatty Acid Synthase (FAS) multienzyme complex** involved in de novo lipogenesis. The correct answer is **Pantothenate (Vitamin B5)** because ACP contains a prosthetic group called **4'-phosphopantetheine**. 1. **Why Pantothenate is correct:** Pantothenic acid is the precursor for 4'-phosphopantetheine. In the FAS complex, this derivative is covalently linked to a serine residue of the ACP. It acts as a "swinging arm," carrying the growing fatty acyl chain between the different catalytic sites of the enzyme complex. This is analogous to how Coenzyme A (which also contains pantothenate) carries acyl groups in other metabolic pathways. 2. **Why other options are incorrect:** * **Biotin (B7):** Acts as a coenzyme for **carboxylation** reactions (e.g., Acetyl-CoA carboxylase, the rate-limiting step of fatty acid synthesis). It carries $CO_2$, not acyl groups. * **Folic acid (B9):** Functions in **one-carbon metabolism** (transfer of methyl, formyl groups), essential for DNA synthesis. * **Pyridoxine (B6):** Its active form, Pyridoxal Phosphate (PLP), is the coenzyme for **transamination**, decarboxylation, and heme synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **The "Swinging Arm" trio:** Three enzymes use flexible arms to move substrates: **ACP** (4'-phosphopantetheine), **Pyruvate Dehydrogenase** (Lipoamide), and **Acetyl-CoA Carboxylase** (Biotin). * **FAS Complex:** In humans, it is a homodimer where each monomer has 7 enzyme activities and an ACP domain. * **Vitamin B5 Deficiency:** Extremely rare, but classically associated with **"Burning Feet Syndrome"** (Gopalan’s syndrome).

Question 99: Why is nicotinamide generally preferred over niacin?

A. Niacin causes flushing. (Correct Answer)
B. Niacin causes abdominal pain.
C. Niacin causes breathlessness.
D. Niacin has lower bioavailability.

Explanation: **Explanation:** **1. Why Option A is Correct:** Nicotinic acid (Niacin) and Nicotinamide are both forms of Vitamin B3. However, in clinical practice, nicotinamide is preferred for treating deficiency (Pellagra) because **Niacin causes intense cutaneous flushing**, itching, and burning sensations. This occurs because nicotinic acid triggers the release of **Prostaglandins (PGD2 and PGE2)** in the skin, leading to vasodilation. Nicotinamide does not have this vasodilatory effect and is therefore better tolerated by patients. **2. Why Other Options are Incorrect:** * **Option B & C:** While high doses of niacin can occasionally cause GI distress or exacerbate asthma, these are not the primary or most common reasons for preferring nicotinamide. Flushing is the classic, dose-limiting side effect unique to the nicotinic acid form. * **Option D:** Both forms have excellent bioavailability and are readily absorbed from the gastrointestinal tract. The preference is based on the side-effect profile, not pharmacokinetics. **3. NEET-PG High-Yield Pearls:** * **The "Niacin Flush":** Can be minimized by pre-treating with **Aspirin** (which inhibits prostaglandin synthesis) or by taking the drug with meals. * **Lipid Profile:** Only **Niacin** (not nicotinamide) is used to treat hyperlipidemia as it inhibits lipolysis in adipose tissue and reduces VLDL synthesis. * **Pellagra Symptoms:** Remember the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Biochemical Precursor:** Niacin can be synthesized in the body from the amino acid **Tryptophan** (60 mg Tryptophan = 1 mg Niacin). This process requires Vitamin B6 (Pyridoxine) as a cofactor.

Question 100: Which vitamin is required for glucose-6-phosphate dehydrogenase activity?

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

Explanation: **Explanation:** The correct answer is **Niacin (Vitamin B3)**. Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of the **Pentose Phosphate Pathway (Hexose Monophosphate Shunt)**. This enzyme catalyzes the conversion of glucose-6-phosphate to 6-phosphogluconolactone. During this process, it reduces **NADP+ to NADPH**. Niacin is the essential precursor for the synthesis of nicotinamide adenine dinucleotide phosphate (NADP+). Therefore, without niacin, the coenzyme required for G6PD activity would be unavailable. **Analysis of Incorrect Options:** * **Riboflavin (B2):** Serves as a precursor for FAD and FMN. While it is involved in redox reactions (e.g., Glutathione reductase), it is not the coenzyme for G6PD. * **Thiamine (B1):** Its active form, TPP, is a cofactor for Transketolase in the HMP shunt, but not for G6PD. It is also vital for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. * **Biotin (B7):** Acts as a coenzyme for carboxylation reactions (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). **High-Yield Clinical Pearls for NEET-PG:** * **NADPH Utility:** The NADPH generated by G6PD is crucial for maintaining the pool of **reduced glutathione**, which protects red blood cells against oxidative stress. * **G6PD Deficiency:** This is the most common enzymopathy worldwide, leading to neonatal jaundice and drug-induced (e.g., Primaquine) or fava bean-induced hemolytic anemia. * **Diagnostic Hallmark:** Look for **Heinz bodies** (denatured hemoglobin) and **Bite cells** on a peripheral smear in G6PD deficiency cases. * **Niacin Deficiency:** Leads to **Pellagra**, characterized by the 3 Ds: Dermatitis, Diarrhea, and Dementia.

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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