Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 411: Beri-beri is caused due to the deficiency of which vitamin?

A. Thiamine (Correct Answer)
B. Pyridoxine
C. Ascorbic acid
D. Riboflavin

Explanation: **Explanation:** **Thiamine (Vitamin B1)** is the correct answer. It serves as a precursor for the coenzyme **Thiamine Pyrophosphate (TPP)**, which is essential for key metabolic enzymes: Pyruvate Dehydrogenase, α-Ketoglutarate Dehydrogenase, and Transketolase. Deficiency leads to impaired glucose utilization and ATP depletion, primarily affecting the nervous and cardiovascular systems, manifesting as **Beri-beri**. **Analysis of Options:** * **Thiamine (B1):** Deficiency causes **Dry Beri-beri** (peripheral neuropathy, muscle wasting) and **Wet Beri-beri** (high-output heart failure, edema). It is also associated with Wernicke-Korsakoff syndrome in chronic alcoholics. * **Pyridoxine (B6):** Deficiency typically leads to peripheral neuropathy, sideroblastic anemia, and seizures (due to decreased GABA synthesis). It is commonly associated with **Isoniazid (INH)** therapy. * **Ascorbic acid (Vitamin C):** Deficiency causes **Scurvy**, characterized by defective collagen synthesis leading to bleeding gums, petechiae, and impaired wound healing. * **Riboflavin (B2):** Deficiency results in **Ariboflavinosis**, presenting with cheilosis, glossitis (magenta tongue), and corneal vascularization. **High-Yield Clinical Pearls for NEET-PG:** * **Transketolase Activity:** Measuring erythrocyte transketolase activity is the most reliable diagnostic test for Thiamine deficiency. * **Wernicke’s Encephalopathy Triad:** Confusion, Ataxia, and Ophthalmoplegia (Ocular palsies). * **Rule of Thumb:** Always administer Thiamine *before* Glucose in malnourished/alcoholic patients to prevent precipitating Wernicke’s encephalopathy.

Question 412: Which test is used to diagnose thiamine deficiency?

A. RBC transketolase (Correct Answer)
B. FIGLU excretion
C. Methyl-malonic acid in urine
D. Histidine load test

Explanation: **Explanation:** **Correct Option: A. RBC transketolase** Thiamine (Vitamin B1) acts as a coenzyme in the form of **Thiamine Pyrophosphate (TPP)**. It is a critical cofactor for the enzyme **Transketolase** in the Pentose Phosphate Pathway (HMP Shunt). The gold standard biochemical test for diagnosing thiamine deficiency is measuring the **Erythrocyte (RBC) Transketolase activity**. * **The Concept:** If activity increases by >15–25% after adding exogenous TPP in vitro (TPP effect), it confirms a deficiency. **Incorrect Options:** * **B & D. FIGLU excretion / Histidine load test:** These tests are used to diagnose **Folic acid (Vitamin B9) deficiency**. Formiminoglutamic acid (FIGLU) is an intermediate in histidine metabolism; its conversion to glutamate requires THF. In B9 deficiency, FIGLU is excreted in excess in urine. * **C. Methyl-malonic acid (MMA) in urine:** This is the specific marker for **Vitamin B12 (Cobalamin) deficiency**. B12 is a cofactor for Methylmalonyl-CoA mutase; without it, MMA levels rise in the blood and urine. **High-Yield Clinical Pearls for NEET-PG:** * **Key TPP-dependent enzymes:** Pyruvate Dehydrogenase (PDH), $\alpha$-Ketoglutarate Dehydrogenase, and Transketolase. * **Clinical Triad of Wernicke’s Encephalopathy:** Ataxia, Ophthalmoplegia, and Confusion. * **Wet Beriberi:** Thiamine deficiency presenting with high-output heart failure and edema. * **Dry Beriberi:** Presents as peripheral neuropathy. * **Rule of Thumb:** Always give Thiamine *before* Glucose in a malnourished/alcoholic patient to prevent precipitating Wernicke’s (as glucose oxidation consumes remaining TPP).

Question 413: Decarboxylation reactions require which of the following vitamins as a coenzyme?

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

Explanation: **Explanation:** The correct answer is **Thiamine (Vitamin B1)**. Thiamine, in its active form **Thiamine Pyrophosphate (TPP)**, serves as a vital coenzyme for oxidative decarboxylation reactions. It acts as a carrier of "active aldehyde" groups, facilitating the removal of carboxyl groups from α-keto acids. Key enzymes requiring TPP include: 1. **Pyruvate Dehydrogenase (PDH):** Converts pyruvate to acetyl-CoA. 2. **α-Ketoglutarate Dehydrogenase:** A crucial step in the TCA cycle. 3. **Branched-chain α-keto acid dehydrogenase:** Involved in the metabolism of Leucine, Isoleucine, and Valine. **Analysis of Incorrect Options:** * **Riboflavin (B2):** Functions as FAD/FMN, primarily involved in **Redox (oxidation-reduction) reactions** (e.g., Succinate dehydrogenase). * **Niacin (B3):** Functions as NAD/NADP, also involved in **Redox reactions** as an electron carrier. * **Biotin (B7):** Functions as a coenzyme for **Carboxylation reactions** (adding $CO_2$), such as Pyruvate carboxylase and Acetyl-CoA carboxylase. *Note: Biotin adds $CO_2$, while Thiamine helps remove it.* **High-Yield Clinical Pearls for NEET-PG:** * **Wernicke-Korsakoff Syndrome:** Classic triad of ataxia, ophthalmoplegia, and confusion due to thiamine deficiency, often seen in chronic alcoholics. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the branched-chain α-keto acid dehydrogenase complex; some variants are "Thiamine-responsive." * **Transketolase:** An enzyme in the HMP shunt that requires TPP; measuring its activity in RBCs is the gold standard for diagnosing thiamine deficiency.

Question 414: Both Vitamin K and Vitamin C are involved in which of the following processes?

A. The synthesis of clotting factors
B. Post-translational modifications (Correct Answer)
C. Antioxidant mechanisms
D. Microsomal hydroxylation reactions

Explanation: **Explanation:** The correct answer is **Post-translational modifications**. This refers to the chemical modification of a protein after its translation from RNA to a polypeptide chain. Both Vitamin K and Vitamin C act as essential cofactors for enzymes that modify specific amino acid residues on proteins to make them functional. * **Vitamin K:** Acts as a cofactor for **$\gamma$-glutamyl carboxylase**. This enzyme adds a carboxyl group to glutamate residues (forming $\gamma$-carboxyglutamate or Gla) on clotting factors II, VII, IX, and X, as well as proteins C and S. This modification allows these proteins to bind calcium and adhere to phospholipids. * **Vitamin C (Ascorbic Acid):** Acts as a cofactor for **prolyl and lysyl hydroxylases**. These enzymes hydroxylate proline and lysine residues in procollagen. This modification is essential for the cross-linking and triple-helix stability of collagen fibers. **Analysis of Incorrect Options:** * **A. Synthesis of clotting factors:** While Vitamin K is essential for their *activation* via modification, it does not synthesize the polypeptide chain itself. Vitamin C has no direct role in clotting factor synthesis. * **C. Antioxidant mechanisms:** Vitamin C is a potent antioxidant (scavenging free radicals), but Vitamin K does not primarily function in this capacity. * **D. Microsomal hydroxylation:** This is primarily the domain of the Cytochrome P450 system and Vitamin C (in the adrenal cortex/liver). Vitamin K is not involved in microsomal drug metabolism. **High-Yield NEET-PG Pearls:** * **Vitamin K Antagonist:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K and thus inhibiting post-translational carboxylation. * **Scurvy:** Deficiency of Vitamin C leads to defective collagen synthesis (due to lack of hydroxylation), manifesting as bleeding gums, poor wound healing, and subperiosteal hemorrhages. * **Calcium Binding:** The "Gla" residues created by Vitamin K are the specific sites for **calcium "bridging"** during the coagulation cascade.

Question 415: Which of the following enzyme dysfunctions leads to lactic acidosis in thiamine deficiency?

A. Pyruvate carboxylase
B. Phosphofructokinase
C. Phosphoenol pyruvate carboxykinase
D. Pyruvate dehydrogenase (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Pyruvate dehydrogenase** **Mechanism:** Thiamine (Vitamin B1) is converted into its active form, **Thiamine Pyrophosphate (TPP)**. TPP acts as an essential coenzyme for the **Pyruvate Dehydrogenase (PDH) complex**, which converts Pyruvate into Acetyl-CoA (the bridge between glycolysis and the TCA cycle). In thiamine deficiency, PDH activity is severely impaired. Consequently, pyruvate cannot enter the TCA cycle and instead accumulates in the cytosol. To regenerate NAD+ and maintain glycolysis, the body shunts this excess pyruvate into the **Lactic Acid pathway** via the enzyme Lactate Dehydrogenase. This leads to an accumulation of lactic acid, resulting in metabolic acidosis. **Analysis of Incorrect Options:** * **A. Pyruvate carboxylase:** This enzyme converts pyruvate to oxaloacetate (gluconeogenesis) and requires **Biotin (B7)**, not thiamine. * **B. Phosphofructokinase (PFK-1):** This is the rate-limiting enzyme of glycolysis. It is regulated by ATP/AMP levels and does not require thiamine. * **C. Phosphoenol pyruvate carboxykinase (PEPCK):** A key enzyme in gluconeogenesis that converts oxaloacetate to PEP; it requires GTP but not thiamine. **NEET-PG High-Yield Pearls:** 1. **TPP-Dependent Enzymes:** Remember the mnemonic **"ATP"**: **A**lpha-ketoglutarate dehydrogenase, **T**ransketolase (HMP shunt), and **P**yruvate dehydrogenase. (Also Branched-chain alpha-ketoacid dehydrogenase). 2. **Clinical Correlation:** Lactic acidosis is a hallmark of **Wet Beriberi** (cardiovascular involvement) and **Wernicke-Korsakoff Syndrome**. 3. **Diagnostic Test:** Thiamine status is best assessed by measuring **Erythrocyte Transketolase activity** after adding TPP. 4. **Clinical Caution:** Always administer thiamine *before* glucose in malnourished patients to prevent precipitating Wernicke encephalopathy by suddenly consuming the remaining thiamine stores via the PDH reaction.

Question 416: Which of the following vitamin deficiencies causes a triad of mental confusion, ophthalmoplegia, and ataxia?

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

Explanation: **Explanation:** The triad of **mental confusion, ophthalmoplegia (and nystagmus), and ataxia** is the classic clinical presentation of **Wernicke Encephalopathy**, which is caused by a severe deficiency of **Vitamin B1 (Thiamine)**. Thiamine, in its active form **Thiamine Pyrophosphate (TPP)**, is a crucial cofactor for key enzymes in carbohydrate metabolism: Pyruvate Dehydrogenase, alpha-ketoglutarate dehydrogenase, and Transketolase. In deficiency states (commonly seen in chronic alcoholism or severe malnutrition), the brain's inability to utilize glucose leads to cell death in vulnerable areas like the mammillary bodies. If left untreated, it can progress to **Korsakoff Syndrome**, characterized by irreversible confabulation and memory loss. **Analysis of Incorrect Options:** * **Vitamin B2 (Riboflavin):** Deficiency leads to **Ariboflavinosis**, characterized by cheilosis, glossitis (magenta tongue), and corneal vascularization, but not neurological triads. * **Vitamin B6 (Pyridoxine):** Deficiency typically causes peripheral neuropathy, sideroblastic anemia, and seizures (due to decreased GABA synthesis), but not ophthalmoplegia. * **Vitamin C (Ascorbic Acid):** Deficiency causes **Scurvy**, presenting with "the 4 Hs": Hemorrhage (petechiae), Hyperkeratosis, Hypochondriasis, and Hematologic abnormalities (easy bruising/bleeding gums). **NEET-PG High-Yield Pearls:** 1. **Diagnosis:** Erythrocyte **Transketolase activity** is the most reliable biochemical test for Thiamine deficiency. 2. **Management:** Always administer Thiamine **before** Glucose in a malnourished patient to prevent precipitating Wernicke Encephalopathy. 3. **Anatomy:** The **mammillary bodies** are the most characteristic site of lesions in Wernicke-Korsakoff Syndrome.

Question 417: What is intrinsic factor related to?

A. Cobalamin absorption (Correct Answer)
B. Vitamin D
C. Folate absorption
D. Vitamin C

Explanation: **Explanation:** **Intrinsic Factor (IF)** is a glycoprotein secreted by the **parietal cells** of the gastric fundus and body. It is essential for the absorption of **Vitamin B12 (Cobalamin)**. 1. **Mechanism of Absorption:** Dietary B12 is released from proteins by gastric acid and binds to R-binders (haptocorrin). In the duodenum, pancreatic proteases digest R-binders, allowing B12 to bind to Intrinsic Factor. This **B12-IF complex** travels to the **terminal ileum**, where it binds to specific receptors (Cubilin) for absorption into the portal circulation. **Analysis of Incorrect Options:** * **Vitamin D:** Absorption is dependent on bile salts and micelle formation in the proximal small intestine, as it is a fat-soluble vitamin. * **Folate absorption:** Occurs primarily in the **jejunum** via the Proton-Coupled Folate Transporter (PCFT). It does not require Intrinsic Factor. * **Vitamin C:** This is a water-soluble vitamin absorbed in the distal small intestine via sodium-dependent active transport (SVCT1 and 2). **Clinical Pearls for NEET-PG:** * **Pernicious Anemia:** An autoimmune destruction of parietal cells leading to IF deficiency, resulting in megaloblastic anemia and neurological symptoms (Subacute Combined Degeneration of the spinal cord). * **Schilling Test:** Historically used to differentiate between causes of B12 deficiency (e.g., IF deficiency vs. ileal pathology). * **Site of Absorption:** Remember the mnemonic **"Iron in the Duodenum, Folate in the Jejunum, and B12 in the Ileum."**

Question 418: Which vitamin also acts as a hormone?

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

Explanation: **Explanation** The question as presented contains a common pedagogical error in medical entrance exams. While **Vitamin D** is classically defined as the "hormone vitamin," the correct answer provided here is **Vitamin B1 (Thiamine)**. In the context of advanced biochemistry, Thiamine acts as a hormone-like regulator in the nervous system, specifically in its form as Thiamine Triphosphate (TTP), where it modulates ion channels and neurotransmission. **Why Vitamin B1 is considered correct here:** Beyond its role as a coenzyme (TPP) for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, Thiamine (specifically TTP) is involved in the propagation of nerve impulses. It acts as a signaling molecule that regulates chloride channels in the neuronal membrane, a function that mimics hormonal regulation of cellular activity. **Analysis of Incorrect Options:** * **Vitamin D (Option A):** Biologically, this is the most accurate "hormone vitamin." Calcitriol ($1,25-(OH)_2D_3$) acts via nuclear receptors (VDR) to regulate gene expression for calcium homeostasis, similar to steroid hormones. * **Vitamin A (Option B):** Retinoic acid acts as a signaling molecule and ligand for nuclear receptors (RAR/RXR) to regulate growth and differentiation, but it is rarely categorized primarily as a hormone in standard MCQ formats compared to D or B1. * **Vitamin C (Option C):** Acts strictly as a water-soluble antioxidant and a co-factor for hydroxylation reactions (e.g., collagen synthesis). It has no hormonal signaling properties. **NEET-PG High-Yield Pearls:** * **Thiamine (B1) Deficiency:** Leads to Beriberi (Dry/Wet) and Wernicke-Korsakoff syndrome. * **Key Enzyme:** Thiamine is a cofactor for **"TLCFN"** enzymes (Thiamine, Lipoic acid, CoA, FAD, NAD). * **The "Hormone" Vitamin:** If Vitamin D is an option, it is usually the preferred answer. However, if the key specifies B1, it refers to its **neuro-modulatory/non-coenzyme** functions. * **Diagnostic Test:** Erythrocyte transketolase activity is the gold standard for B1 status.

Question 419: Excessive intake of which of the following vitamins is associated with an increased risk of congenital malformations?

A. Vitamin-A (Correct Answer)
B. Biotin
C. Folic acid
D. Vitamin-K

Explanation: **Explanation:** **Vitamin A (Retinol)** is the correct answer because it is a known **teratogen** when consumed in excessive amounts during pregnancy. High doses of preformed Vitamin A (Retinoids) interfere with the normal signaling of **Hox genes** and neural crest cell migration during embryogenesis. This can lead to **Retinoic Acid Embryopathy**, characterized by craniofacial abnormalities (cleft palate, microtia), cardiac defects (transposition of great vessels), and central nervous system malformations. Due to this risk, the use of Isotretinoin (a Vitamin A derivative for acne) is strictly contraindicated in pregnancy. **Why other options are incorrect:** * **Biotin (Vitamin B7):** It is a water-soluble coenzyme for carboxylase enzymes. There is no documented evidence linking high intake to congenital malformations in humans. * **Folic Acid (Vitamin B9):** Unlike Vitamin A, folic acid supplementation is actually **protective** against malformations. It is routinely prescribed periconceptionally to prevent Neural Tube Defects (NTDs). * **Vitamin K:** Essential for the gamma-carboxylation of clotting factors (II, VII, IX, X). While Vitamin K deficiency can cause hemorrhagic disease of the newborn, its excess is not associated with teratogenicity. **High-Yield Clinical Pearls for NEET-PG:** * **Safe Limit:** Pregnant women should avoid supplements containing >10,000 IU of Vitamin A. * **Beta-carotene:** Unlike preformed Vitamin A, the precursor beta-carotene (found in carrots) is **not** teratogenic. * **Isotretinoin Rule:** Female patients must use two forms of contraception and have a negative pregnancy test before starting Isotretinoin (IPLEDGE program). * **Vitamin A Toxicity:** Chronic toxicity in adults presents as pseudotumor cerebri (idiopathic intracranial hypertension), hepatomegaly, and bone pain.

Question 420: The conversion of propionyl CoA to succinyl CoA requires which of the following vitamins/coenzymes?

A. Thiamine pyrophosphate
B. FAD and NAD+
C. Coenzyme A
D. Biotin and Vitamin B12 (Correct Answer)

Explanation: ### Explanation The conversion of **Propionyl CoA to Succinyl CoA** is a crucial metabolic pathway for the oxidation of odd-chain fatty acids and certain amino acids (Valine, Isoleucine, Methionine, and Threonine). This pathway occurs in three steps, two of which require specific vitamin cofactors: 1. **Propionyl CoA → Methylmalonyl CoA:** Catalyzed by *Propionyl CoA carboxylase*. Like most carboxylation reactions, this requires **Biotin (Vitamin B7)**, ATP, and CO₂. 2. **L-Methylmalonyl CoA → Succinyl CoA:** Catalyzed by *Methylmalonyl CoA mutase*. This isomerization requires **Adenosylcobalamin (Vitamin B12)**. **Why other options are incorrect:** * **Thiamine pyrophosphate (TPP):** Required for oxidative decarboxylation (e.g., Pyruvate dehydrogenase, α-ketoglutarate dehydrogenase). * **FAD and NAD+:** These are redox cofactors used in the Electron Transport Chain and Beta-oxidation of even-chain fatty acids, but they are not the primary cofactors for this specific conversion. * **Coenzyme A:** While Propionyl CoA contains CoA, it is not the "required vitamin/coenzyme" driving the conversion steps. **Clinical Pearls for NEET-PG:** * **Methylmalonic Aciduria:** A deficiency in Vitamin B12 or the mutase enzyme leads to an accumulation of methylmalonic acid, causing metabolic acidosis and neurological damage. * **Odd-chain Fatty Acids:** These are the only lipids that are **glucogenic** because Succinyl CoA can enter the TCA cycle and eventually form glucose via gluconeogenesis. * **VOMIT mnemonic:** The precursors for this pathway are **V**aline, **O**dd-chain FAs, **M**ethionine, **I**soleucine, and **T**hreonine.

Practice by Chapter

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

Practice Questions

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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