Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 121: For which one of the following has the cytotoxic effect of vitamin on human lymphocytes in vitro at high concentration been reported?

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

Explanation: **Explanation:** The correct answer is **Vitamin E (Tocopherol)**. While Vitamin E is primarily known as a potent lipid-soluble antioxidant that protects cell membranes from oxidative stress, research has demonstrated that at **high concentrations in vitro**, it can exert a **cytotoxic effect** on human lymphocytes. **Why Vitamin E is correct:** At supra-physiological doses, Vitamin E (specifically alpha-tocopherol) can interfere with the cell cycle and induce apoptosis in lymphocytes. This paradoxical effect is attributed to its ability to modulate signal transduction pathways and inhibit protein kinase C (PKC) activity, which is essential for lymphocyte proliferation and survival. **Analysis of Incorrect Options:** * **Vitamin A:** While Vitamin A toxicity (Hypervitaminosis A) is clinically significant (causing increased intracranial pressure and bone changes), its primary in vitro effects are related to cell differentiation and teratogenicity rather than direct lymphocyte cytotoxicity. * **Vitamin D:** Vitamin D acts more like a hormone. It is known for its **immunomodulatory** effects (inhibiting Th1 responses) rather than being directly cytotoxic to lymphocytes at high concentrations. * **Vitamin K:** Vitamin K is essential for the gamma-carboxylation of clotting factors (II, VII, IX, X). While high doses can interfere with warfarin therapy, it does not exhibit specific cytotoxic profiles against lymphocytes in the same manner as Vitamin E. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin E Toxicity:** The most significant clinical risk of high-dose Vitamin E is an **increased risk of hemorrhage**, as it can antagonize Vitamin K and interfere with the clotting cascade. * **Antioxidant Paradox:** Vitamin E is the most important chain-breaking antioxidant in the body, preventing the peroxidation of Polyunsaturated Fatty Acids (PUFAs). * **Deficiency Sign:** Look for **hemolytic anemia** (in premature infants) and **posterior column signs** (ataxia, loss of vibration/position sense) mimicking Vitamin B12 deficiency but without megaloblastic changes.

Question 122: Which vitamin is involved in one-carbon metabolism?

A. Folic acid (Correct Answer)
B. Thiamine
C. Pyridoxine
D. All of the above

Explanation: **Explanation:** **1. Why Folic Acid is Correct:** Folic acid (Vitamin B9) is the central coenzyme in **one-carbon metabolism**. Its active form, **Tetrahydrofolate (THF)**, acts as a carrier for one-carbon units (such as methyl, methylene, and formyl groups). These units are essential for the synthesis of purines and thymidine (DNA synthesis) and the conversion of homocysteine to methionine. Without THF, the cell cannot replicate its DNA or perform vital methylation reactions. **2. Why Other Options are Incorrect:** * **Thiamine (Vitamin B1):** Its active form, Thiamine Pyrophosphate (TPP), is involved in **oxidative decarboxylation** (e.g., Pyruvate Dehydrogenase complex) and the **transketolase** reaction in the HMP shunt. It does not carry one-carbon units. * **Pyridoxine (Vitamin B6):** Its active form, Pyridoxal Phosphate (PLP), is primarily involved in **transamination**, decarboxylation of amino acids, and heme synthesis. While it is a cofactor for *Serine Hydroxymethyltransferase* (which transfers a carbon to folate), PLP itself is not the one-carbon carrier. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Folate Trap":** A deficiency in Vitamin B12 leads to a functional folate deficiency because folate becomes "trapped" as N5-methyl-THF, unable to participate in DNA synthesis. * **Megaloblastic Anemia:** Deficiency of either Folate or B12 leads to impaired DNA synthesis, resulting in macrocytic cells. * **Neural Tube Defects (NTDs):** Maternal folate deficiency is a major risk factor; supplementation is recommended periconceptionally. * **Antimetabolites:** Drugs like **Methotrexate** inhibit *Dihydrofolate Reductase (DHFR)*, blocking the regeneration of THF and halting one-carbon metabolism.

Question 123: Select the TRUE statement about vitamin B12:

A. Does not act as a cofactor
B. Involved in the transfer of amino groups
C. Requires a specific glycoprotein for its absorption (Correct Answer)
D. Dietary requirement is met by intake of plant products

Explanation: **Explanation:** **Why Option C is correct:** Vitamin B12 (Cobalamin) absorption is a complex physiological process. It requires **Intrinsic Factor (IF)**, a specific glycoprotein secreted by the **parietal cells** of the gastric mucosa. In the small intestine, the B12-IF complex binds to specific receptors in the **terminal ileum** for absorption. A deficiency of Intrinsic Factor (often due to autoimmune destruction of parietal cells) leads to **Pernicious Anemia**. **Analysis of Incorrect Options:** * **Option A:** Incorrect. Vitamin B12 acts as a vital cofactor for two key enzymes: **Methionine synthase** (converting homocysteine to methionine) and **Methylmalonyl-CoA mutase** (converting methylmalonyl-CoA to succinyl-CoA). * **Option B:** Incorrect. The transfer of amino groups (transamination) is the primary function of **Vitamin B6 (Pyridoxal Phosphate)**, not B12. * **Option D:** Incorrect. Vitamin B12 is synthesized exclusively by microorganisms. It is found naturally only in **animal-source foods** (meat, eggs, dairy). Strict vegetarians (vegans) are at high risk of deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Storage:** Unlike other water-soluble vitamins, B12 is stored in the **liver** in significant amounts (3–5 mg), enough to last 3–5 years. * **Metabolic Markers:** B12 deficiency leads to elevated levels of both **Homocysteine** and **Methylmalonic acid (MMA)**. (Note: Folate deficiency only elevates homocysteine). * **Neurological Impact:** Deficiency causes **Subacute Combined Degeneration (SCD)** of the spinal cord due to defective myelin synthesis. * **Transport:** In the blood, B12 is primarily bound to **Transcobalamin II**.

Question 124: Which vitamin acts as an antioxidant, preventing oxidation?

A. Pyridoxine
B. Biotin
C. Retinol
D. Vitamin A (Correct Answer)

Explanation: **Explanation:** The correct answer is **Vitamin A**. This vitamin (specifically in the form of carotenoids like beta-carotene) acts as a potent **antioxidant** by quenching singlet oxygen and neutralizing free radicals. This process protects cellular membranes and DNA from lipid peroxidation and oxidative damage, which is crucial in preventing carcinogenesis and atherosclerosis. **Why the other options are incorrect:** * **Pyridoxine (Vitamin B6):** Primarily functions as a coenzyme (Pyridoxal Phosphate - PLP) in amino acid metabolism, including transamination, decarboxylation, and heme synthesis. It does not possess direct antioxidant properties. * **Biotin (Vitamin B7):** Acts as a coenzyme for **carboxylation reactions** (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). It is involved in gluconeogenesis and fatty acid synthesis, not antioxidant defense. * **Retinol:** While Retinol is a form of Vitamin A, the question asks for the vitamin group. In many competitive exams, if both a specific form and the general vitamin name are provided, the broader category or the form most associated with the specific function (like beta-carotene for antioxidants) is prioritized. However, in this specific MCQ structure, "Vitamin A" is the standard textbook answer for antioxidant function among the choices. **High-Yield Clinical Pearls for NEET-PG:** * **The Antioxidant Trio:** Remember **ACE** (Vitamin **A**, Vitamin **C**, and Vitamin **E**) as the primary antioxidant vitamins. * **Vitamin E** is the most powerful chain-breaking antioxidant in lipid membranes. * **Vitamin A Toxicity:** Chronic ingestion can lead to pseudotumor cerebri (idiopathic intracranial hypertension) and hepatomegaly. * **Beta-carotene** is the precursor of Vitamin A found in plants; it is the specific fraction responsible for the antioxidant effect.

Question 125: What amount of tryptophan produces 1 gm of niacin?

A. 40 mg
B. 50 mg
C. 60 mg (Correct Answer)
D. 70 mg

Explanation: ### Explanation **Correct Answer: C. 60 mg** **1. Understanding the Concept:** Niacin (Vitamin B3) can be synthesized endogenously from the essential amino acid **Tryptophan** via the **Kynurenine pathway**. This process is relatively inefficient: it takes approximately **60 mg of dietary Tryptophan to produce 1 mg of Niacin**. This ratio is the basis for calculating "Niacin Equivalents" (NE) in nutrition. The conversion requires several cofactors, making it a high-yield topic for biochemistry: * **Vitamin B6 (Pyridoxine):** Required for the enzyme kynureninase. * **Vitamin B2 (Riboflavin):** Required for kynurenine hydroxylase. * **Iron:** Required for tryptophan pyrrolase. **2. Analysis of Options:** * **A, B, and D:** These values are incorrect. While individual metabolic rates may vary slightly, the internationally accepted nutritional standard for the conversion ratio is strictly **60:1**. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Pellagra:** A deficiency of Niacin characterized by the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Hartnup Disease:** An autosomal recessive disorder involving defective transport of neutral amino acids (including Tryptophan) in the gut and kidneys. This leads to a secondary Niacin deficiency and Pellagra-like symptoms. * **Carcinoid Syndrome:** In this condition, up to 60% of Tryptophan is diverted to synthesize **Serotonin** (5-HT), leaving insufficient amounts for Niacin synthesis, which can result in Pellagra. * **Corn/Maize Diets:** Diets based primarily on corn are pellagragenic because corn is low in Tryptophan and its Niacin is in a bound, unabsorbable form called **Niacytin**.

Question 126: Which vitamin prevents the peroxidation of lipids at the cellular level?

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

Explanation: **Explanation:** **Vitamin E (α-tocopherol)** is the primary lipid-soluble antioxidant in the body. Its specific role is to protect polyunsaturated fatty acids (PUFAs) within cellular and organelle membranes 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 lipid bilayer. This prevents the propagation of oxidative damage that leads to membrane fragility. **Why other options are incorrect:** * **Vitamin A:** Primarily involved in vision (rhodopsin synthesis), cell differentiation, and immune function. While carotenoids have some antioxidant properties, Vitamin A is not the principal defender against lipid peroxidation. * **Vitamin C (Ascorbic Acid):** A potent water-soluble antioxidant. It functions in the aqueous compartments of the cell (cytosol) and is essential for collagen synthesis. Crucially, Vitamin C helps **regenerate** oxidized Vitamin E back to its active form. * **Vitamin K:** Essential for the post-translational carboxylation of clotting factors (II, VII, IX, X) and proteins C and S. It does not play a significant role in antioxidant defense. **High-Yield Clinical Pearls for NEET-PG:** * **Glutathione Peroxidase:** This enzyme works synergistically with Vitamin E to reduce lipid peroxides; it requires **Selenium** as a cofactor. * **Deficiency Manifestations:** Vitamin E deficiency leads to **hemolytic anemia** (due to fragile RBC membranes), posterior column degeneration, and ataxia. * **Toxicity:** High doses of Vitamin E can interfere with Vitamin K metabolism, leading to an increased risk of bleeding (prolonged PT/INR).

Question 127: What is the daily requirement of vitamin B1 per 100 kilocalories?

A. 0.05 mg (Correct Answer)
B. 0.5 mg
C. 5.0 mg
D. 1.0 gm

Explanation: **Explanation:** The daily requirement of **Vitamin B1 (Thiamine)** is directly proportional to the caloric intake, specifically carbohydrate metabolism. Thiamine, in its active form **Thiamine Pyrophosphate (TPP)**, serves as a crucial coenzyme for the oxidative decarboxylation of $\alpha$-keto acids (e.g., Pyruvate Dehydrogenase complex). 1. **Why 0.05 mg is correct:** According to nutritional standards (ICMR and WHO), the recommended dietary allowance (RDA) for Thiamine is **0.5 mg per 1000 kcal** of energy intake. When calculated per **100 kcal**, this value becomes **0.05 mg**. This ensures sufficient TPP is available to handle the metabolic load of glucose oxidation. 2. **Why other options are incorrect:** * **0.5 mg:** This is the requirement per **1000 kcal**, not 100 kcal. * **5.0 mg:** This is significantly higher than the physiological requirement; such doses are typically used for therapeutic supplementation in deficiency states. * **1.0 gm:** This is a massive, pharmacological dose and is irrelevant to daily nutritional requirements. **High-Yield Clinical Pearls for NEET-PG:** * **Minimum Intake:** Regardless of low caloric intake, a minimum of **1 mg/day** of Thiamine is recommended for adults. * **Wernicke-Korsakoff Syndrome:** Classically seen in alcoholics due to impaired absorption and storage. Always administer Thiamine **before** Glucose in malnourished patients to prevent precipitating acute Wernicke’s encephalopathy. * **Erythrocyte Transketolase Activity:** Measuring this enzyme's activity is the most reliable biochemical test to assess Thiamine status. * **Deficiency Manifestations:** Dry Beriberi (Polyneuritis), Wet Beriberi (High-output cardiac failure/Edema), and Infantile Beriberi.

Question 128: All vitamins are transported by specific proteins, except for which of the following?

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

Explanation: **Explanation** In biochemistry, most water-soluble vitamins require specific carrier proteins or active transport systems for systemic circulation and cellular uptake. However, **Riboflavin (Vitamin B2)** is the exception in this context. **Why Riboflavin is the Correct Answer:** Riboflavin is unique because it does not have a dedicated, specific transport protein for its systemic circulation in the blood. Instead, it is transported primarily in its free form or bound non-specifically to **albumin** and **immunoglobulins**. While there are specific riboflavin-binding proteins (RfBP) found in pregnant women (to facilitate fetal transfer), they are not the standard transport mechanism in the general adult population. **Analysis of Incorrect Options:** * **Niacin (B3):** Transported via specific sodium-dependent transporters (SLC5A8) and specific carriers in the blood. * **Thiamine (B1):** Utilizes specific high-affinity transporters (THTR-1 and THTR-2). Deficiencies in these transporters lead to Rogers Syndrome. * **Cyanocobalamin (B12):** This is the most "protein-dependent" vitamin. It requires **Intrinsic Factor** for absorption, **Haptocorrin** (Transcobalamin I) for protection in the gut, and **Transcobalamin II** for systemic transport to tissues. **High-Yield Clinical Pearls for NEET-PG:** * **Riboflavin Deficiency:** Characterized by **Cheilosis**, glossitis (magenta tongue), and corneal vascularization. * **B12 Transport:** Transcobalamin II is the primary functional transporter that delivers B12 to the liver and other tissues. * **Thiamine Transport:** THTR-1 is encoded by the *SLC19A2* gene; mutations cause Thiamine-responsive megaloblastic anemia (TRMA). * **Key Concept:** Most water-soluble vitamins are absorbed via sodium-dependent active transport, but their plasma transport varies significantly.

Question 129: Which disease is due to deficiency of B complex vitamins?

A. Pellagra (Correct Answer)
B. Night blindness
C. Osteomalacia
D. Spinocerebellar ataxia

Explanation: **Explanation:** **Pellagra** is the correct answer because it is caused by a deficiency of **Vitamin B3 (Niacin)**. Niacin is a precursor to the coenzymes NAD and NADP, which are essential for oxidation-reduction reactions. Clinically, Pellagra is characterized by the **"4 Ds"**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and, if untreated, Death. **Analysis of Incorrect Options:** * **Night Blindness (Nyctalopia):** This is the earliest clinical sign of **Vitamin A (Retinol)** deficiency. Vitamin A is essential for the synthesis of rhodopsin in the retina. * **Osteomalacia:** This is caused by a deficiency of **Vitamin D** in adults (Rickets in children). It involves inadequate mineralization of the bone osteoid, leading to "soft bones." * **Spinocerebellar Ataxia:** While ataxia can occur in B12 deficiency (Subacute Combined Degeneration), isolated spinocerebellar ataxia is most classically associated with **Vitamin E (Tocopherol)** deficiency, which mimics Friedreich’s ataxia due to oxidative damage to the posterior columns and spinocerebellar tracts. **High-Yield Clinical Pearls for NEET-PG:** * **Hartnup Disease:** An autosomal recessive disorder involving defective transport of **Tryptophan** (a precursor to Niacin), which can lead to Pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause Niacin deficiency because dietary tryptophan is diverted to overproduce Serotonin. * **Corn-based diets:** Maize is low in tryptophan and contains niacin in a bound, non-bioavailable form (niacytin), often leading to Pellagra in endemic areas.

Question 130: Vitamin B1 is named so because?

A. It was the first vitamin to be discovered in the B complex group. (Correct Answer)
B. It was the first vitamin to be classified in the B complex group.
C. It is the most important vitamin for the body.
D. None of the above.

Explanation: **Explanation:** The nomenclature of vitamins is primarily based on the chronological order of their discovery. **Vitamin B1 (Thiamine)** was the first member of the "B complex" group to be isolated and identified. In the early 20th century, researchers like Casimir Funk isolated a substance from rice polishings that could cure Beriberi; this "vital amine" was the first of the B vitamins to be discovered, hence the designation "B1." **Analysis of Options:** * **Option A (Correct):** As stated, the numbering (B1, B2, B3, etc.) reflects the historical sequence in which these water-soluble compounds were discovered and chemically characterized. * **Option B (Incorrect):** Classification refers to the grouping of substances based on chemical properties or functions. While it is part of the B complex, the "1" specifically denotes its priority in discovery, not its rank in a classification system. * **Option C (Incorrect):** No single vitamin is considered the "most important." All vitamins are essential micronutrients, and a deficiency in any (e.g., B12 or C) can lead to fatal or debilitating conditions. * **Option D (Incorrect):** Option A is the historically accurate reason. **High-Yield Clinical Pearls for NEET-PG:** * **Active Form:** Thiamine Pyrophosphate (TPP). * **Key Enzymes:** TPP is a coenzyme for **Pyruvate Dehydrogenase**, **$\alpha$-Ketoglutarate Dehydrogenase**, and **Transketolase** (HMP Shunt). * **Clinical Deficiency:** * **Dry Beriberi:** Polyneuritis and muscle wasting. * **Wet Beriberi:** High-output cardiac failure and edema. * **Wernicke-Korsakoff Syndrome:** Common in alcoholics; characterized by the triad of ataxia, ophthalmoplegia, and confusion. * **Diagnostic Test:** Measurement of **Erythrocyte Transketolase activity** is the most reliable method to assess thiamine status.

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free