Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 541: Which of the following vitamins is NOT obtained from a plant source?

A. Cobalamine (Correct Answer)
B. Riboflavin
C. Thiamine
D. Vitamin A

Explanation: **Explanation:** The correct answer is **Cobalamine (Vitamin B12)**. **1. Why Cobalamine is the correct answer:** Vitamin B12 is unique among vitamins because it is synthesized exclusively by **microorganisms** (bacteria and archaea). It is not produced by plants or animals. Animals obtain it by ingesting contaminated food or through ruminal bacterial synthesis. Consequently, B12 is found naturally only in **animal-sourced foods** (meat, eggs, dairy, and shellfish). Strict vegetarians (vegans) are at a high risk of deficiency because plants do not contain B12 unless they are fortified or contaminated with nitrogen-fixing bacteria. **2. Why the other options are incorrect:** * **Riboflavin (B2):** Widely distributed in both plants and animals. Rich plant sources include green leafy vegetables and enriched cereals. * **Thiamine (B1):** Found abundantly in whole grains, legumes, and nuts. The outer layer of rice (bran) is a major source. * **Vitamin A:** While preformed Vitamin A (retinol) is found in animal products, plants provide **Provitamin A (Beta-carotene)**, which is converted to Vitamin A in the human body. Sources include carrots, spinach, and sweet potatoes. **Clinical Pearls for NEET-PG:** * **Absorption:** B12 requires **Intrinsic Factor (IF)** from gastric parietal cells for absorption in the **terminal ileum**. * **Storage:** Unlike other water-soluble vitamins, B12 is stored in the **liver** for 3–5 years. * **Deficiency:** Leads to **Megaloblastic Anemia** and **Subacute Combined Degeneration (SCD)** of the spinal cord due to defective myelin synthesis (accumulation of methylmalonic acid). * **Key Enzyme:** B12 is a coenzyme for **Methionine Synthase** and **Methylmalonyl-CoA Mutase**.

Question 542: Megaloblastic anemia occurs due to deficiency of which vitamin?

A. Niacin
B. Vitamin C
C. Vitamin B1
D. Vitamin B12 (Correct Answer)

Explanation: **Explanation:** **Megaloblastic anemia** is characterized by the presence of large, nucleated red blood cell precursors (megaloblasts) in the bone marrow. This occurs primarily due to impaired **DNA synthesis**, while RNA and protein synthesis remain unaffected, leading to "nuclear-cytoplasmic asynchrony." **Why Vitamin B12 is correct:** Vitamin B12 (Cobalamin) is a crucial cofactor for the enzyme **Methionine Synthase**. This enzyme converts homocysteine to methionine and simultaneously converts 5-methyltetrahydrofolate (5-methyl THF) back into **Tetrahydrofolate (THF)**. In B12 deficiency, folate remains trapped in the 5-methyl THF form (the **"Folate Trap"**). Since THF is essential for purine and pyrimidine (dTMP) synthesis, its unavailability halts DNA replication, resulting in megaloblastic changes. **Why other options are incorrect:** * **Niacin (B3):** Deficiency leads to **Pellagra**, characterized by the 3 Ds: Dermatitis, Diarrhea, and Dementia. * **Vitamin C:** Deficiency causes **Scurvy**, characterized by defective collagen synthesis, leading to bleeding gums and poor wound healing. * **Vitamin B1 (Thiamine):** Deficiency leads to **Beriberi** (Dry/Wet) or **Wernicke-Korsakoff syndrome**, affecting the cardiovascular and nervous systems. **High-Yield Clinical Pearls for NEET-PG:** * **Folate (B9) Deficiency** also causes megaloblastic anemia, but unlike B12 deficiency, it **does not** cause neurological symptoms (Subacute Combined Degeneration of the Spinal Cord). * **Schilling Test:** Historically used to determine the cause of B12 malabsorption (e.g., Pernicious Anemia). * **Biochemical Markers:** In B12 deficiency, both **Methylmalonic Acid (MMA)** and **Homocysteine** levels are elevated. In Folate deficiency, only Homocysteine is elevated.

Question 543: Which of the following vitamins is involved in the post-translational modification of glutamate residues and plays an important role in the coagulation pathway?

A. Vitamin C
B. Vitamin K (Correct Answer)
C. Vitamin D
D. Vitamin B12

Explanation: **Explanation:** **Correct Option: B (Vitamin K)** Vitamin K acts as a crucial coenzyme for the enzyme **$\gamma$-glutamyl carboxylase**. This enzyme adds a carboxyl group to specific glutamate (Glu) residues on certain proteins, converting them into **$\gamma$-carboxyglutamate (Gla)** residues. This post-translational modification is essential because the two adjacent carboxylic acid groups in Gla residues create a high-affinity binding site for **Calcium ($Ca^{2+}$)**. This "calcium bridge" allows clotting factors to bind to the phospholipid membranes of platelets, a vital step in the coagulation cascade. **Why other options are incorrect:** * **Vitamin C:** Involved in the post-translational **hydroxylation** of proline and lysine residues during collagen synthesis, not carboxylation. * **Vitamin D:** Primarily functions as a hormone regulating calcium and phosphate homeostasis; it does not act as a coenzyme for glutamate modification. * **Vitamin B12:** Acts as a coenzyme for methionine synthase and methylmalonyl-CoA mutase; its deficiency leads to megaloblastic anemia and neurological issues. **High-Yield NEET-PG Pearls:** 1. **Vitamin K-dependent factors:** Factors **II, VII, IX, X**, and Proteins **C and S**. 2. **Warfarin Mechanism:** Inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the regeneration of active (reduced) Vitamin K, thereby inhibiting the $\gamma$-carboxylation process. 3. **Clinical Marker:** Prothrombin Time (PT) is the most sensitive lab test to monitor Vitamin K status/Warfarin therapy. 4. **Newborns:** They are born with a sterile gut and low Vitamin K stores, necessitating a prophylactic Vitamin K injection at birth to prevent **Hemorrhagic Disease of the Newborn**.

Question 544: In vitamin A deficiency, which of the following is NOT a typical complaint?

A. Night blindness
B. Xerophthalmia
C. Keratosis
D. Phosphoric calciuria (Correct Answer)

Explanation: **Explanation:** Vitamin A (Retinol) is essential for vision, epithelial integrity, and immune function. The correct answer is **Phosphoric calciuria** because it is not associated with Vitamin A deficiency; rather, it is a clinical feature related to **Vitamin D** metabolism or parathyroid disorders. **Why Phosphoric calciuria is the correct choice:** Phosphoric calciuria (excessive excretion of calcium and phosphate in urine) is typically seen in conditions like Vitamin D toxicity or hyperparathyroidism. Vitamin A does not play a direct role in the renal handling of phosphate or calcium. **Analysis of Incorrect Options:** * **Night Blindness (Nyctalopia):** This is the earliest symptom of Vitamin A deficiency. It occurs due to the failure to regenerate **Rhodopsin** (visual purple) in the rod cells of the retina, which is necessary for low-light vision. * **Xerophthalmia:** This refers to a spectrum of ocular manifestations. Deficiency leads to the keratinization of the conjunctiva and cornea, causing dryness (xerosis), Bitot’s spots, and eventually keratomalacia (corneal melting). * **Keratosis:** Vitamin A is vital for maintaining healthy epithelial tissues. Deficiency leads to **Follicular Hyperkeratosis** (Phrynoderma or "Toad skin"), where skin becomes rough and scaly due to excessive keratin plugs in hair follicles. **High-Yield Clinical Pearls for NEET-PG:** * **WHO Classification:** The first clinical sign of Vitamin A deficiency is Night Blindness (XN), while the first objective sign is Conjunctival Xerosis (X1A). * **Storage:** Vitamin A is stored in the **Ito cells** (Stellate cells) of the liver. * **Transport:** It is transported in the blood bound to **Retinol Binding Protein (RBP)** and Pre-albumin. * **Teratogenicity:** Isotretinoin (a Vitamin A derivative) is highly teratogenic; a negative pregnancy test is mandatory before prescription.

Question 545: Which enzyme is thiamine dependent?

A. Transketolase (Correct Answer)
B. Transaldolase
C. Amino transferase
D. Eno lase

Explanation: **Explanation:** Thiamine (Vitamin B1) functions in its active form, **Thiamine Pyrophosphate (TPP)**, as a vital coenzyme for oxidative decarboxylation and carbon-transfer reactions. **Why Transketolase is Correct:** Transketolase is a key enzyme in the **Pentose Phosphate Pathway (HMP Shunt)**. It requires TPP to transfer two-carbon units between sugar phosphates. In clinical practice, measuring erythrocyte transketolase activity is the gold standard for diagnosing thiamine deficiency. **Analysis of Incorrect Options:** * **B. Transaldolase:** Also part of the HMP Shunt, but it does **not** require a cofactor. It uses a lysine residue at its active site to form a Schiff base with the substrate. * **C. Amino transferase (Transaminase):** These enzymes (e.g., ALT, AST) require **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6, for the transfer of amino groups. * **D. Enolase:** An enzyme in glycolysis that converts 2-phosphoglycerate to phosphoenolpyruvate. It is dependent on **Magnesium (Mg²⁺)** and is inhibited by fluoride. **High-Yield NEET-PG Pearls:** * **The "T-A-P" Mnemonic:** Thiamine is a cofactor for four major enzymes: 1. **T**ransketolase (HMP Shunt) 2. **A**lpha-ketoglutarate dehydrogenase (TCA Cycle) 3. **P**yruvate dehydrogenase (Link reaction) 4. **B**ranched-chain alpha-ketoacid dehydrogenase (Metabolism of Leucine, Isoleucine, Valine). * **Clinical Correlation:** Deficiency leads to **Beriberi** (Dry/Wet) and **Wernicke-Korsakoff Syndrome**. Always administer thiamine *before* glucose in malnourished patients to prevent precipitating Wernicke’s encephalopathy.

Question 546: Which of the following is a lipid-soluble plasma membrane-associated antioxidant?

A. Ubiquitin
B. Vitamin E (Correct Answer)
C. Vitamin C
D. Glutathione

Explanation: **Explanation:** The correct answer is **Vitamin E (α-tocopherol)**. **1. Why Vitamin E is correct:** Vitamin E is a potent **lipid-soluble** antioxidant. Its primary physiological role is to act as a "chain-breaker" in the process of **lipid peroxidation**. Because of its hydrophobic nature, it integrates into the phospholipid bilayer of plasma membranes and lipoproteins. It protects the membrane from oxidative damage by scavenging free radicals (like superoxide and hydroxyl radicals), specifically preventing the oxidation of polyunsaturated fatty acids (PUFAs). **2. Why the other options are incorrect:** * **Ubiquitin (A):** This is a small regulatory protein, not an antioxidant. It marks misfolded or damaged proteins for degradation via the **Proteasome pathway** (Ubiquitin-Proteasome System). * **Vitamin C (C):** While a powerful antioxidant, Vitamin C (Ascorbic acid) is **water-soluble**. It acts in the aqueous compartments of the cell (cytosol) and is responsible for regenerating the reduced form of Vitamin E. * **Glutathione (D):** This is a tripeptide (Glu-Cys-Gly) that serves as the major **intracellular, water-soluble** antioxidant. It acts as a substrate for Glutathione Peroxidase to neutralize hydrogen peroxide. **3. High-Yield Clinical Pearls for NEET-PG:** * **Synergy:** Vitamin C and Vitamin E work together; Vitamin C reduces the tocopheroxyl radical back to active Vitamin E. * **Deficiency:** Vitamin E deficiency leads to **hemolytic anemia** (due to fragile RBC membranes) and posterior column neurological defects (mimicking Friedreich’s ataxia). * **Toxicity:** Excess Vitamin E can interfere with Vitamin K metabolism, leading to an increased risk of bleeding (prolonged PT/INR). * **Selenium:** This trace element is a cofactor for Glutathione Peroxidase and works synergistically with Vitamin E.

Question 547: Which of the following foods contains the highest quantity of vitamin C?

A. Orange
B. Lemon
C. Indian gooseberry (Correct Answer)
D. Grapes

Explanation: **Explanation:** Vitamin C (Ascorbic acid) is a water-soluble vitamin essential for collagen synthesis, antioxidant defense, and iron absorption. While all citrus fruits are excellent sources, the **Indian Gooseberry (Amla)** is renowned in biochemistry and nutrition as one of the richest natural sources of Vitamin C. * **Why Indian Gooseberry is Correct:** Amla contains approximately **600–700 mg of Vitamin C per 100g**, which is nearly 10 to 20 times higher than the concentration found in oranges. It also contains tannins that prevent the oxidation of the vitamin, making it more stable even when processed. * **Why Others are Incorrect:** * **Orange and Lemon:** These are classic citrus sources but contain significantly less Vitamin C (approx. 30–50 mg/100g) compared to Amla. * **Grapes:** While they contain antioxidants like resveratrol, their Vitamin C content is relatively low (approx. 4–10 mg/100g). **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **Biochemical Function:** Vitamin C acts as a coenzyme for **prolyl and lysyl hydroxylase**, which are essential for the post-translational modification of procollagen. Deficiency leads to **Scurvy** (characterized by spongy gums, petechiae, and poor wound healing). 2. **Iron Absorption:** Vitamin C enhances the absorption of **non-heme iron** by reducing it from the ferric ($Fe^{3+}$) to the ferrous ($Fe^{2+}$) state. 3. **Antioxidant Role:** It regenerates Vitamin E and acts as a scavenger of free radicals. 4. **Highest Source Overall:** While Amla is the highest among the given options, the **Barbados cherry (Acerola)** actually holds the highest concentration of Vitamin C in the plant kingdom.

Question 548: Which vitamin deficiency leads to lactic acidosis?

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

Explanation: **Explanation:** **Why Thiamine (Vitamin B1) is the correct answer:** Thiamine, in its active form **Thiamine Pyrophosphate (TPP)**, is a critical coenzyme for the **Pyruvate Dehydrogenase (PDH) complex**. The PDH complex is responsible for converting Pyruvate into Acetyl-CoA, which then enters the TCA cycle for aerobic respiration. In thiamine deficiency, the PDH complex cannot function. As a result, 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 a buildup of lactate, resulting in **Lactic Acidosis**. This is a hallmark feature of severe thiamine deficiency, such as in Wet Beriberi or Wernicke-Korsakoff Syndrome. **Why the other options are incorrect:** * **Riboflavin (B2):** Precursor to FAD/FMN. While involved in the PDH complex, its deficiency primarily causes cheilosis, glossitis, and corneal vascularization rather than acute lactic acidosis. * **Niacin (B3):** Precursor to NAD+/NADP+. Deficiency leads to Pellagra (Dermatitis, Diarrhea, Dementia, Death). * **Pantothenic acid (B5):** A component of Coenzyme A. While essential for metabolism, clinical deficiency is extremely rare and does not typically present with isolated lactic acidosis. **High-Yield NEET-PG Pearls:** * **Enzymes requiring TPP:** Pyruvate Dehydrogenase, Alpha-ketoglutarate dehydrogenase (TCA cycle), Branched-chain ketoacid dehydrogenase (Maple Syrup Urine Disease), and Transketolase (HMP Shunt). * **Clinical Correlation:** In chronic alcoholics with suspected thiamine deficiency, **always give Thiamine before Glucose**. Administering glucose first can precipitate Wernicke’s encephalopathy by consuming the remaining thiamine stores during glycolysis. * **Diagnostic Test:** Measurement of **Erythrocyte Transketolase activity** is the gold standard for diagnosing thiamine deficiency.

Question 549: The laboratory diagnosis of thiamine deficiency is by:

A. Transaldolase assay measured before and after the addition of thiamine pyrophosphate
B. Transketolase assay measured before and after the addition of thiamine pyrophosphate (Correct Answer)
C. Thiamine pyrophosphate assay in RBC after adding sugar
D. Serum LDH levels

Explanation: **Explanation:** The laboratory diagnosis of thiamine (Vitamin B1) deficiency relies on assessing the functional activity of thiamine-dependent enzymes. **Why Option B is correct:** Thiamine pyrophosphate (TPP) is a vital cofactor for the enzyme **Transketolase**, which functions in the Hexose Monophosphate (HMP) Shunt. The gold standard for diagnosing thiamine deficiency is the **Erythrocyte Transketolase Activation Test (ETKA)**. In this test, transketolase activity is measured in red blood cells before and after the addition of exogenous TPP. * If the enzyme activity increases significantly (usually >15-25%), it indicates that the original enzymes were "unsaturated" due to a lack of endogenous thiamine, confirming a deficiency. **Why other options are incorrect:** * **Option A:** Transaldolase is also an enzyme in the HMP shunt, but it does **not** require thiamine pyrophosphate as a cofactor. * **Option C:** While RBCs are used for the test, the assay measures enzyme activity (transketolase), not the TPP molecule itself after adding sugar. * **Option D:** Serum Lactate Dehydrogenase (LDH) is a marker of cell turnover or hemolysis (e.g., in megaloblastic anemia) and has no diagnostic value for thiamine deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Thiamine-dependent enzymes:** (Mnemonic: **ATP**) **A**lpha-ketoglutarate dehydrogenase, **T**ransketolase, **P**yruvate dehydrogenase, and Branched-chain ketoacid dehydrogenase. * **Clinical manifestations:** Dry Beriberi (polyneuropathy), Wet Beriberi (high-output heart failure), and Wernicke-Korsakoff Syndrome (ataxia, ophthalmoplegia, and confabulation). * **Management Rule:** Always administer thiamine **before** glucose in malnourished or alcoholic patients to prevent precipitating Wernicke encephalopathy (as glucose oxidation consumes remaining thiamine).

Question 550: Hangover following alcohol consumption can be treated with which of the following vitamins?

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

Explanation: **Explanation:** **Why Thiamine (Vitamin B1) is the Correct Answer:** Alcohol consumption leads to a transient deficiency of Thiamine through several mechanisms: it inhibits intestinal absorption, reduces hepatic storage, and impairs the activation of Thiamine into its active form, **Thiamine Pyrophosphate (TPP)**. Crucially, Thiamine is a mandatory coenzyme for the **Pyruvate Dehydrogenase (PDH)** and **alpha-ketoglutarate dehydrogenase** complexes. Alcohol metabolism increases the NADH/NAD+ ratio, shifting pyruvate to lactate and inhibiting gluconeogenesis. Administering Thiamine helps restore aerobic glucose metabolism and prevents the accumulation of toxic metabolites. In clinical practice, Thiamine is the first-line treatment to prevent **Wernicke-Korsakoff Syndrome**, a severe neurological consequence of chronic alcohol-induced B1 deficiency. **Analysis of Incorrect Options:** * **A. Pyridoxine (B6):** Primarily involved in transamination and heme synthesis. While it is used to treat sideroblastic anemia or isoniazid toxicity, it has no direct role in alcohol metabolism. * **C. Riboflavin (B2):** A precursor for FAD/FMN. While it participates in the TCA cycle, its deficiency is not the acute limiting factor in alcohol-related metabolic distress. * **D. Niacin (B3):** Alcohol metabolism actually produces an excess of NADH (derived from Niacin). Adding more Niacin does not alleviate a hangover and may cause flushing. **High-Yield Clinical Pearls for NEET-PG:** * **The "Banana Bag":** Intravenous fluids for alcoholics typically contain Thiamine, Folate, and Magnesium. * **Gold Standard Rule:** Always administer **Thiamine BEFORE Glucose** in a malnourished or alcoholic patient. Giving glucose first can precipitate acute Wernicke Encephalopathy by exhausting the remaining tiny reserves of Thiamine during sudden oxidative metabolism. * **Enzyme Marker:** Erythrocyte **transketolase** activity is the most reliable diagnostic test for Thiamine 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

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Vitamin K and Blood Coagulation

Practice Questions

Water-Soluble Vitamins: B Complex and C

Practice Questions

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

Practice Questions

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