Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 531: Hypervitaminosis of which of the following will cause bony abnormalities?

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

Explanation: **Explanation:** **Vitamin D (Correct Answer):** Hypervitaminosis D leads to excessive calcium absorption from the gut and increased bone resorption. This results in **hypercalcemia**, which causes metastatic calcification of soft tissues (like kidneys and blood vessels) and significant **bony abnormalities**, including bone pain, demineralization, and subperiosteal bone resorption. In children, chronic toxicity can lead to premature closure of epiphyses and growth retardation. **Why other options are incorrect:** * **Vitamin A:** While Vitamin A toxicity causes skeletal symptoms (like cortical thickening and painful periostitis), the classical presentation focuses more on **pseudotumor cerebri** (increased intracranial pressure), hepatomegaly, and skin peeling. In NEET-PG, if Vitamin D is an option, it is the preferred answer for systemic bony metabolic derangement. * **Vitamin C:** Toxicity is rare as it is water-soluble. Excessive intake is more commonly associated with **renal oxalate stones** and gastrointestinal upset, not bony abnormalities. (Deficiency, however, causes Scurvy, which affects bone matrix). * **Vitamin E:** This is the least toxic fat-soluble vitamin. High doses primarily interfere with Vitamin K metabolism, leading to an **increased risk of bleeding**, but do not affect bone structure. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin D Toxicity:** Look for the triad of hypercalcemia, hypercalciuria, and polyuria. * **Vitamin A Toxicity:** Key signs include papilledema (pseudotumor cerebri), alopecia, and cheilitis. * **Teratogenicity:** Vitamin A is highly teratogenic (causes craniofacial and cardiac defects); hence, isotretinoin is contraindicated in pregnancy. * **Storage:** Vitamin A is stored in **Ito cells** of the liver.

Question 532: What is the most potent form of vitamin D?

A. 1, 25-dihydroxycholecalciferol (Correct Answer)
B. 7-dihydrocholecalciferol
C. 25-hydroxycholecalciferol
D. Ergocalciferol (Vit. D2)

Explanation: **Explanation:** Vitamin D undergoes a two-step hydroxylation process to become biologically active. The correct answer is **1, 25-dihydroxycholecalciferol (Calcitriol)** because it is the active hormonal form of Vitamin D. It binds with high affinity to the Vitamin D Receptor (VDR) in the intestine and bones to regulate calcium and phosphate homeostasis. **Analysis of Options:** * **A. 1, 25-dihydroxycholecalciferol (Calcitriol):** Produced in the **proximal convoluted tubule** of the kidney by the enzyme **1-alpha-hydroxylase**. It is 100–1000 times more potent than its precursor, 25-hydroxycholecalciferol. * **B. 7-dehydrocholesterol:** This is the **provitamin** found in the skin. It is converted to cholecalciferol (Vitamin D3) upon exposure to UV-B light. It has no intrinsic biological activity. * **C. 25-hydroxycholecalciferol (Calcidiol):** This is the major **circulating form** and the primary storage form of Vitamin D. While it is used to clinically measure a patient's Vitamin D status, it is a pro-hormone with minimal biological potency compared to calcitriol. * **D. Ergocalciferol (Vit. D2):** This is the plant-derived form of Vitamin D. It must still undergo hepatic and renal hydroxylation to become active (1, 25-dihydroxyergocalciferol). **High-Yield NEET-PG Pearls:** 1. **Rate-limiting step:** The conversion of 25-OH-D3 to 1,25-(OH)2-D3 by **1-alpha-hydroxylase** in the kidney is the most strictly regulated step (stimulated by PTH and low phosphate). 2. **Storage:** 25-hydroxycholecalciferol has the longest half-life (approx. 2-3 weeks), making it the best indicator of Vitamin D stores. 3. **Clinical Correlation:** In chronic kidney disease (CKD), the failure of 1-alpha-hydroxylation leads to Vitamin D deficiency and secondary hyperparathyroidism (Renal Osteodystrophy).

Question 533: Which vitamin is synthesized from an amino acid?

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

Explanation: **Explanation:** The correct answer is **Niacin (Vitamin B3)**. Niacin is unique among vitamins because it can be synthesized endogenously from the essential amino acid **Tryptophan**. This conversion occurs primarily in the liver and requires a ratio of **60 mg of Tryptophan to produce 1 mg of Niacin**. This metabolic pathway is clinically significant as it requires Vitamin B6 (Pyridoxine), Vitamin B2 (Riboflavin), and Iron as cofactors. **Analysis of Options:** * **Thiamine (B1):** An essential vitamin that must be obtained from the diet (e.g., whole grains, legumes). It is not synthesized from amino acids. * **Riboflavin (B2):** Synthesized by plants and microorganisms, but humans lack the biosynthetic pathway to produce it from amino acids. * **Biotin (B7):** While synthesized by intestinal bacteria, it is not produced by human tissues from an amino acid precursor. **High-Yield Clinical Pearls for NEET-PG:** * **Pellagra:** A deficiency of Niacin characterized by the "4 Ds": Dermatitis, Diarrhea, Dementia, and Death. * **Hartnup Disease:** An autosomal recessive disorder involving defective transport of neutral amino acids (including Tryptophan) in the gut and kidneys, leading to Pellagra-like symptoms. * **Carcinoid Syndrome:** Can lead to Niacin deficiency because Tryptophan is diverted toward the overproduction of Serotonin (5-HT) instead of Niacin. * **Corn-based diets:** Maize is low in Tryptophan and contains Niacin in a bound, unabsorbable form (Niacytin), often leading to deficiency in populations where corn is a staple.

Question 534: What is the richest source of Vitamin A?

A. Halibut liver oil (Correct Answer)
B. Carrot
C. Butter
D. Margarine

Explanation: **Explanation:** Vitamin A (Retinol) is a fat-soluble vitamin essential for vision, epithelial integrity, and immune function. The correct answer is **Halibut liver oil** because it contains the highest concentration of preformed Vitamin A (Retinol). **1. Why Halibut liver oil is correct:** Animal-derived sources provide Vitamin A in the form of **Retinyl esters**. Among these, fish liver oils are the most concentrated sources. While Cod liver oil is a well-known source, **Halibut liver oil** and **Shark liver oil** contain significantly higher concentrations of the vitamin per unit weight, making Halibut the "richest" source among the options provided. **2. Analysis of Incorrect Options:** * **Carrot:** This is a rich source of **Beta-carotene** (a provitamin). However, the conversion of beta-carotene to active retinol in the human body is inefficient (6:1 ratio). Therefore, plant sources are never "richer" than concentrated animal oils. * **Butter:** While butter is a good source of Vitamin A, its concentration is much lower compared to fish liver oils. * **Margarine:** This is a vegetable oil-based product. It does not naturally contain Vitamin A and is usually **fortified** with it to match the nutritional profile of butter. **Clinical Pearls for NEET-PG:** * **Storage:** Vitamin A is stored in the **Ito cells** (Stellate cells) of the liver. * **Deficiency:** The earliest clinical sign is **Nyctalopia** (Night blindness). The earliest objective sign is **Conjunctival Xerosis**. * **Bitot’s Spots:** Triangular, pearly-white foamy plaques on the bulbar conjunctiva, pathognomonic for Vitamin A deficiency. * **Toxicity:** Hypervitaminosis A can lead to **Pseudotumor cerebri** (idiopathic intracranial hypertension). * **Golden Rice:** A genetically modified variety of rice produced to biosynthesize beta-carotene to prevent deficiency in developing countries.

Question 535: Korsakoff psychosis is typically seen in which of the following conditions?

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

Explanation: **Explanation:** **Korsakoff psychosis** is a late-stage manifestation of **Thiamine (Vitamin B1)** deficiency, often occurring as a sequel to Wernicke encephalopathy (together known as Wernicke-Korsakoff Syndrome). Thiamine, in its active form **Thiamine Pyrophosphate (TPP)**, is a crucial coenzyme for key enzymes in carbohydrate metabolism: Pyruvate Dehydrogenase, $\alpha$-ketoglutarate dehydrogenase, and Transketolase. In chronic deficiency (commonly due to alcoholism), the brain's inability to utilize glucose leads to neuronal damage, specifically in the **mammillary bodies** and dorsomedial nucleus of the thalamus. Korsakoff psychosis is characterized by anterograde amnesia and **confabulation** (filling memory gaps with fabricated stories). **Analysis of Incorrect Options:** * **Riboflavin (B2) deficiency:** Presents as "Ariboflavinosis," characterized by cheilosis, glossitis, corneal vascularization, and seborrheic dermatitis. * **Niacin (B3) deficiency:** Causes **Pellagra**, characterized by the "3 Ds": Dermatitis (Casal’s necklace), Diarrhea, and Dementia. While it involves cognitive decline, it does not present with the specific confabulatory amnesia of Korsakoff. * **Cyanocobalamin (B12) deficiency:** Leads to Megaloblastic anemia and **Subacute Combined Degeneration (SCD)** of the spinal cord, affecting the posterior and lateral columns. **High-Yield Clinical Pearls for NEET-PG:** * **Wernicke’s Encephalopathy Triad:** Confusion, Ataxia, and Ophthalmoplegia (reversible with Thiamine). * **Korsakoff Psychosis:** Irreversible memory loss and confabulation. * **Diagnostic Test:** Measurement of **Erythrocyte Transketolase activity** (increases after adding TPP). * **Management Rule:** Always administer Thiamine *before* Glucose in a malnourished patient to prevent precipitating Wernicke’s.

Question 536: Which vitamin deficiency may lead to keratinization?

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

Explanation: **Explanation:** **Vitamin A (Retinol)** is the correct answer because it plays a critical role in maintaining the integrity of epithelial tissues. Retinoic acid, the active metabolite of Vitamin A, acts like a hormone to regulate gene expression. It promotes the differentiation of epithelial cells into mucus-secreting goblet cells. In its absence, these cells undergo **squamous metaplasia**, leading to **keratinization** (the accumulation of keratin) in tissues that are normally non-keratinized. This manifests clinically as **Xerophthalmia** (dryness of the eyes) and **Bitot’s spots** on the conjunctiva. **Why other options are incorrect:** * **Vitamin B:** This is a complex of water-soluble vitamins. Deficiencies typically present with neurological symptoms (B1, B12), dermatitis/glossitis (B2, B3, B6), or megaloblastic anemia (B9, B12), but not systemic keratinization. * **Vitamin C:** Deficiency leads to **Scurvy**, characterized by defective collagen synthesis. Symptoms include bleeding gums, petechiae, and impaired wound healing, rather than keratinization. * **Vitamin D:** Primarily regulates calcium and phosphate metabolism. Deficiency leads to **Rickets** in children and **Osteomalacia** in adults, affecting bone mineralization rather than epithelial differentiation. **High-Yield NEET-PG Pearls:** * **Follicular Hyperkeratosis (Phrynoderma):** Also known as "Toad Skin," this is a classic cutaneous sign of Vitamin A deficiency. * **Mechanism:** Vitamin A maintains the "mucus-secreting" phenotype; deficiency shifts this to a "keratin-producing" phenotype. * **First Sign:** The earliest clinical sign of Vitamin A deficiency is **Nyctalopia** (Night blindness). * **Teratogenicity:** Excessive Vitamin A intake during pregnancy is highly teratogenic (craniofacial and cardiac defects).

Question 537: Which substance inhibits Vitamin A activity?

A. Linetin
B. Blackberries
C. Citral (Correct Answer)
D. Red cabbage

Explanation: **Explanation:** **Correct Option: C (Citral)** Citral (3,7-dimethyl-2,6-octadienal) is a known **antagonist of Vitamin A**. It acts as a competitive inhibitor of the enzyme **retinal dehydrogenase**, which is responsible for converting retinal to retinoic acid. Retinoic acid is the active form of Vitamin A required for gene expression and cellular differentiation. By blocking this conversion, citral can induce symptoms of Vitamin A deficiency, such as xerophthalmia and impaired epithelial integrity. **Analysis of Incorrect Options:** * **A. Linetin:** This is a term often associated with flaxseed derivatives or specific fatty acid formulations; it does not have a documented inhibitory effect on Vitamin A metabolism. * **B. Blackberries:** These are rich in antioxidants and anthocyanins. They do not inhibit Vitamin A; rather, fruits generally support vitamin absorption through synergistic antioxidant effects. * **D. Red Cabbage:** While red cabbage contains goitrogens (which can interfere with iodine/thyroid function), it does not inhibit Vitamin A. In fact, it contains beta-carotene, a precursor to Vitamin A. **High-Yield Clinical Pearls for NEET-PG:** * **Active Forms:** Remember that **Retinal** is essential for vision (Wald’s Visual Cycle), while **Retinoic Acid** is essential for growth and differentiation. * **Storage:** Vitamin A is stored in the liver as **retinyl palmitate** within the **Ito cells** (Stellate cells). * **Toxicity:** Hypervitaminosis A can lead to pseudotumor cerebri (idiopathic intracranial hypertension). * **Therapeutic Use:** All-trans retinoic acid (ATRA) is a high-yield treatment for **Acute Promyelocytic Leukemia (M3)**.

Question 538: Which of the following is NOT a characteristic symptom of pellagra?

A. Diarrhea
B. Dyspepsia (Correct Answer)
C. Dementia
D. Dermatitis

Explanation: Pellagra is a clinical deficiency syndrome caused by a lack of **Niacin (Vitamin B3)** or its precursor amino acid, **Tryptophan**. It is classically characterized by the **"4 Ds"**: Dermatitis, Diarrhea, Dementia, and, if untreated, Death. **Explanation of the Correct Answer:** * **B. Dyspepsia:** While pellagra involves significant gastrointestinal distress, "Dyspepsia" (indigestion/epigastric pain) is not a defining clinical feature of the syndrome. The GI involvement in pellagra specifically manifests as chronic, non-bloody diarrhea due to atrophy of the intestinal villi and inflammation of the mucosal lining. **Explanation of Incorrect Options (The 3 Ds):** * **A. Diarrhea:** This is the gastrointestinal hallmark. It results from widespread inflammation of the digestive tract, often accompanied by glossitis (magenta tongue) and stomatitis. * **C. Dementia:** This represents the neurological component. Early symptoms include irritability and insomnia, progressing to confusion, memory loss, hallucinations, and frank psychosis. * **D. Dermatitis:** The skin lesions are characteristic—bilateral, symmetrical, and found on sun-exposed areas. A high-yield manifestation is **Casal’s necklace**, a hyperpigmented rash around the neck. **High-Yield Clinical Pearls for NEET-PG:** * **The Tryptophan Connection:** 60 mg of Tryptophan is required to synthesize 1 mg of Niacin. * **Hartnup Disease:** A genetic defect in neutral amino acid transport (Tryptophan) that presents with pellagra-like symptoms. * **Carcinoid Syndrome:** Can lead to pellagra because Tryptophan is diverted to produce excessive Serotonin rather than Niacin. * **Corn/Maize Diets:** Pellagra is common in populations dependent on corn because the niacin in corn is bound (**niacytin**) and unavailable for absorption.

Question 539: A 53-year-old male with a history of chronic heavy alcohol consumption and documented vitamin B6 deficiency presents with a pigmented rash on sun-exposed areas, a bright red tongue, diarrhea, apathy, memory loss, and disorientation. What is the most likely cause for these features?

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

Explanation: ### Explanation The patient presents with the classic **"3 Ds" of Pellagra**: **D**ermatitis (pigmented rash on sun-exposed areas, often called Casal’s necklace), **D**iarrhea, and **D**ementia (apathy, memory loss, disorientation). **Why Niacin (Vitamin B3) deficiency is the correct answer:** Pellagra is caused by a deficiency of Niacin. In this case, the underlying cause is twofold: 1. **Chronic Alcoholism:** Leads to poor dietary intake and impaired absorption. 2. **Vitamin B6 Deficiency:** This is the "high-yield" link. The amino acid **Tryptophan** is converted into Niacin in the liver. This metabolic pathway requires **Pyridoxal Phosphate (PLP)**, the active form of **Vitamin B6**, as a cofactor. Therefore, a deficiency in B6 impairs Niacin synthesis, precipitating Pellagra even if some Niacin is present in the diet. **Why incorrect options are wrong:** * **Riboflavin (B2) deficiency:** Characterized by cheilosis, glossitis, and corneal vascularization, but does not cause the symmetric photosensitive dermatitis or dementia seen here. * **Folate (B9) deficiency:** Primarily presents with macrocytic megaloblastic anemia and glossitis, without the specific dermatological or neurological features of Pellagra. * **Thiamine (B1) deficiency:** While common in alcoholics, it presents as Wernicke-Korsakoff syndrome (ataxia, ophthalmoplegia, confabulation) or Beriberi (cardiac failure/peripheral neuropathy), not a pigmented rash. **NEET-PG High-Yield Pearls:** * **The 60:1 Rule:** 60 mg of Tryptophan produces 1 mg of Niacin. * **Hartnup Disease:** A genetic defect in neutral amino acid (Tryptophan) transporters that also leads to Pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause Niacin deficiency because Tryptophan is diverted toward massive Serotonin production. * **Isoniazid (INH) Therapy:** A common cause of B6 deficiency, which can secondarily lead to Pellagra.

Question 540: Vitamin B12 is required for which step in folate metabolism?

A. Tetrahydrofolate to folinic acid
B. Methyl tetrahydrofolate to tetrahydrofolate (Correct Answer)
C. Formyl tetrahydrofolate to tetrahydrofolate
D. Methylene tetrahydrofolate to methyl tetrahydrofolate

Explanation: ### Explanation **1. Why Option B is Correct: The Methionine Synthase Reaction** Vitamin B12 (Cobalamin) acts as a vital coenzyme for the enzyme **Methionine Synthase**. This enzyme catalyzes the transfer of a methyl group from **N5-methyl tetrahydrofolate (N5-methyl THF)** to homocysteine, converting it into methionine. * During this process, N5-methyl THF is converted back into **Tetrahydrofolate (THF)**. * THF is the "active" form of folate required for DNA synthesis. Without B12, folate remains "trapped" as N5-methyl THF, a phenomenon known as the **"Folate Trap."** This explains why B12 deficiency leads to megaloblastic anemia, as folate cannot be recycled for nucleotide synthesis. **2. Why Other Options are Incorrect:** * **Option A:** The conversion of THF to folinic acid (N5-formyl THF) is a synthetic pathway (Leucovorin) and does not require B12. * **Option C:** Formyl THF is used in purine synthesis; its conversion back to THF occurs during carbon donation in the purine pathway, independent of B12. * **Option D:** The conversion of N5,N10-methylene THF to N5-methyl THF is catalyzed by the enzyme **MTHFR** (Methylene tetrahydrofolate reductase) and is B12-independent. This step is irreversible and leads into the folate trap if B12 is absent. **3. Clinical Pearls for NEET-PG:** * **The Folate Trap:** B12 deficiency causes a functional folate deficiency. Giving high-dose folic acid can correct the anemia but will **not** stop the neurological damage (Subacute Combined Degeneration of the Spinal Cord) caused by B12 deficiency. * **Two B12-Dependent Enzymes:** 1. **Methionine Synthase** (Cytoplasm) – Links B12 to folate metabolism. 2. **Methylmalonyl-CoA Mutase** (Mitochondria) – Converts Methylmalonyl-CoA to Succinyl-CoA. Deficiency leads to elevated **Methylmalonic Acid (MMA)** levels, a specific marker for B12 deficiency. * **Homocysteine:** Levels are elevated in both B12 and Folate deficiencies.

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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