Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 661: Xanthurenic acid in the urine is suggestive of deficiency of which vitamin?

A. Vitamin B6 (Correct Answer)
B. Vitamin B12
C. Folic acid
D. Thiamine

Explanation: **Explanation:** The presence of **Xanthurenic acid** in urine is a classic biochemical marker for **Vitamin B6 (Pyridoxine) deficiency**. **Why Vitamin B6 is the correct answer:** Vitamin B6, in its active form **Pyridoxal Phosphate (PLP)**, is a vital cofactor in the **Kynurenine pathway**, which converts the amino acid Tryptophan to Niacin (Vitamin B3). A key enzyme in this pathway, **Kynureninase**, is highly dependent on PLP. When B6 is deficient, Kynureninase activity is impaired, leading to the accumulation of an intermediate metabolite called 3-hydroxykynurenine. This excess metabolite is diverted into an alternative side pathway where it is converted into **Xanthurenic acid**, which is then excreted in the urine. **Why other options are incorrect:** * **Vitamin B12 & Folic Acid:** Deficiency of these vitamins leads to Megaloblastic anemia. B12 deficiency specifically causes an increase in **Methylmalonic acid (MMA)** and Homocysteine, but not Xanthurenic acid. * **Thiamine (B1):** Deficiency leads to Beriberi or Wernicke-Korsakoff syndrome. It is a cofactor for oxidative decarboxylation (e.g., Pyruvate dehydrogenase); its deficiency is typically monitored via **Erythrocyte transketolase activity**. **High-Yield Clinical Pearls for NEET-PG:** * **Tryptophan Load Test:** Historically used to diagnose B6 deficiency by measuring urinary Xanthurenic acid after an oral dose of Tryptophan. * **Drug-Induced Deficiency:** **Isoniazid (INH)**, used in TB treatment, inhibits Pyridoxine kinase, leading to B6 deficiency and peripheral neuropathy. Always co-administer B6 with INH. * **Sideroblastic Anemia:** B6 is a cofactor for **ALA synthase** (the rate-limiting step in heme synthesis); thus, B6 deficiency can cause microcytic hypochromic anemia with ringed sideroblasts.

Question 662: Which of the following is NOT a dietary source of vitamin B12?

A. Fish
B. Meat
C. Soya bean (Correct Answer)
D. Liver

Explanation: **Explanation:** The core concept tested here is the unique biological origin of **Vitamin B12 (Cobalamin)**. Unlike most other vitamins, B12 is synthesized exclusively by microorganisms (bacteria and archaea). These microbes are found in the soil and the digestive tracts of animals. Consequently, Vitamin B12 is found **naturally only in foods of animal origin**. * **Why Soya bean is the correct answer:** Soya bean is a plant-based legume. Plants do not require B12 for their metabolism and do not possess the machinery to synthesize it. Therefore, natural plant products are devoid of Vitamin B12 unless they are artificially fortified or contaminated with soil bacteria. * **Why Fish, Meat, and Liver are incorrect:** These are all animal tissues. Animals ingest B12-producing bacteria or consume other animals in the food chain, storing the vitamin in their tissues. The **liver** is the primary storage site for B12 in the body, making it the richest dietary source. **NEET-PG High-Yield Clinical Pearls:** 1. **Strict Vegans:** Individuals who consume no animal products (vegans) are at high risk for B12 deficiency and must rely on fortified foods or supplements. 2. **Absorption:** B12 requires **Intrinsic Factor (IF)**, secreted by gastric parietal cells, for absorption in the **terminal ileum**. 3. **Storage:** Unlike other water-soluble vitamins, the liver stores significant amounts of B12 (enough for 3–5 years), which is why deficiency symptoms take years to manifest after stopping intake. 4. **Diagnostic Marker:** In B12 deficiency, both **Homocysteine** and **Methylmalonic Acid (MMA)** levels are elevated (distinguishing it from Folate deficiency, where only Homocysteine is raised).

Question 663: Rickets is due to which of the following?

A. Loss of calcium in urine
B. Increased mobilization of calcium from bone
C. Decreased absorption of calcium
D. Loss of phosphate in urine (Correct Answer)

Explanation: **Explanation:** The pathophysiology of Rickets is primarily centered on a failure of mineralization of the osteoid matrix. While Vitamin D deficiency is the most common cause, the biochemical hallmark that leads to the clinical manifestation of Rickets is **hypophosphatemia**. **Why "Loss of phosphate in urine" is correct:** In Vitamin D deficiency, there is decreased intestinal absorption of calcium, leading to transient hypocalcemia. This triggers the parathyroid glands to secrete **Parathyroid Hormone (PTH)** (Secondary Hyperparathyroidism). PTH acts on the proximal renal tubules to **inhibit phosphate reabsorption**, leading to significant phosphaturia (loss of phosphate in urine). Low serum phosphate levels are the critical limiting factor that prevents the formation of hydroxyapatite crystals, directly causing the softening of bones seen in Rickets. **Analysis of Incorrect Options:** * **A & C:** While decreased absorption of calcium occurs, the body’s compensatory rise in PTH prevents severe hypocalcemia at the expense of phosphate. It is the low phosphate, not just low calcium, that prevents mineralization. * **B:** Increased mobilization of calcium from bone is an *effect* of high PTH levels to maintain serum calcium, but this process (resorption) is what leads to bone demineralization, not the primary cause of the rickets pathology itself. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Profile:** Low/Normal Calcium, **Low Phosphate**, **High Alkaline Phosphatase (ALP)**, and High PTH. * **Radiological Signs:** Cupping, splaying, and fraying of metaphyses (most prominent at the wrist and knee). * **Key Feature:** The most sensitive biochemical marker for rickets activity is an **elevated serum ALP**. * **Vitamin D Resistant Rickets:** Usually due to X-linked hypophosphatemia (mutation in PHEX gene), where the primary defect is also renal phosphate wasting.

Question 664: Which of the following can lead to osteomalacia?

A. Vitamin D deficiency
B. Phosphorus deficiency in diet (Correct Answer)
C. Gastrectomy
D. Lack of exposure to sunlight

Explanation: **Explanation:** Osteomalacia is a metabolic bone disease characterized by **impaired mineralization** of the osteoid (the organic bone matrix), leading to "soft bones." Bone mineralization requires an adequate **Calcium-Phosphorus product**. **Why Option B is the Correct Answer:** While Vitamin D deficiency is the most common cause, the question asks which of the following *can* lead to osteomalacia. **Phosphorus deficiency** (Hypophosphatemia) is a direct cause because phosphate is a mandatory structural component of **Hydroxyapatite crystals** $[Ca_{10}(PO_4)_6(OH)_2]$. Without sufficient inorganic phosphate, the osteoid cannot mineralize, even if calcium levels are normal. This is frequently seen in renal phosphate wasting or chronic use of phosphate-binding antacids. **Analysis of Incorrect Options:** * **Options A, C, and D:** These are all classic causes of **Vitamin D deficiency**. Vitamin D deficiency leads to secondary hyperparathyroidism, which maintains serum calcium at the expense of bone, eventually causing osteomalacia. * **Why B is "more" correct in this specific MCQ context:** In many competitive exams (like NEET-PG), when multiple options are physiologically linked to a condition, the option that represents a **direct biochemical requirement** for the process (mineralization) or a "lesser-known" specific cause is often highlighted to test deeper conceptual knowledge. **High-Yield Clinical Pearls for NEET-PG:** * **Radiological Hallmark:** **Looser’s zones** (Pseudofractures or Milkman’s lines) are pathognomonic for osteomalacia. * **Biochemical Profile:** Low/Normal Calcium, **Low Phosphate**, and **Elevated Alkaline Phosphatase (ALP)**. * **Rickets vs. Osteomalacia:** Rickets occurs in children (before epiphyseal closure); Osteomalacia occurs in adults (after epiphyseal closure). * **Fanconi Syndrome:** A common cause of phosphate-wasting osteomalacia due to proximal renal tubular dysfunction.

Question 665: What is the Vitamin A activity of beta-carotene compared to that of retinol?

A. 1:2
B. 1:4
C. 1:6 (Correct Answer)
D. 1:8

Explanation: **Explanation** The correct answer is **1:6**. This ratio represents the biological conversion efficiency of dietary beta-carotene into active Vitamin A (retinol) in the human body. **1. Why 1:6 is Correct:** Vitamin A exists in two forms: **Preformed Vitamin A** (Retinol, found in animal sources) and **Provitamin A** (Carotenoids like beta-carotene, found in plant sources). Biochemically, one molecule of beta-carotene is symmetrically cleaved by the enzyme **$\beta$-carotene 15,15'-dioxygenase** in the intestinal mucosa to yield two molecules of retinal. However, due to poor intestinal absorption and inefficient conversion, the biological activity is significantly lower. According to the WHO and traditional nutritional standards, **6 $\mu$g of beta-carotene is required to produce 1 $\mu$g of retinol**, making the activity ratio 1:6. **2. Analysis of Incorrect Options:** * **A (1:2):** This is a theoretical chemical ratio. While one molecule of beta-carotene can theoretically yield two molecules of retinol, the physiological conversion efficiency is much lower. * **B (1:4):** This ratio does not correspond to any standard nutritional conversion factor for Vitamin A. * **D (1:8):** While some modern studies suggest conversion might be even less efficient (e.g., 1:12 for mixed diets), the classic textbook and exam-standard value remains 1:6. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Retinol Equivalent (RE):** 1 RE = 1 $\mu$g of Retinol = 6 $\mu$g of Beta-carotene = 12 $\mu$g of other Provitamin A carotenoids. * **International Units (IU):** 1 IU of Vitamin A = 0.3 $\mu$g of Retinol. * **Storage:** Vitamin A is stored in the liver as **retinyl palmitate** within **Ito cells** (Stellate cells). * **Toxicity:** Excessive intake of preformed Vitamin A is toxic (Hypervitaminosis A), but excessive beta-carotene only causes **carotenemia** (yellowish skin) and is not toxic because the conversion to retinol is rate-limited.

Question 666: Biotin deficiency is due to which of the following?

A. Avidin (Correct Answer)
B. Flavoproteins
C. Metallo flavoproteins
D. Oxireductase

Explanation: **Explanation:** **Biotin (Vitamin B7)** serves as a vital coenzyme for carboxylation reactions. The correct answer is **Avidin** because it is a heat-labile protein found in **raw egg whites** that has an extremely high affinity for biotin. When consumed, avidin binds irreversibly to biotin in the gastrointestinal tract, preventing its absorption and leading to deficiency. This is clinically referred to as "Egg White Injury." **Analysis of Options:** * **Avidin (Correct):** It acts as an antinutrient. Cooking denatures avidin, making cooked eggs safe; however, chronic consumption of raw eggs is a classic cause of biotin deficiency. * **Flavoproteins:** These are proteins containing FAD or FMN (derived from Riboflavin/B2). They are involved in redox reactions, not biotin inhibition. * **Metallo flavoproteins:** These are flavoproteins that require a metal ion (like Iron or Molybdenum) for catalytic activity (e.g., Xanthine oxidase). They have no role in biotin deficiency. * **Oxidoreductases:** This is a broad class of enzymes that catalyze electron transfer. While some biotin-dependent enzymes are involved in metabolic pathways, oxidoreductases themselves do not cause deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Biotin-Dependent Enzymes:** Remember the "ABC" carboxylases: **A**cetyl-CoA carboxylase, **B**ropionyl-CoA carboxylase, and **P**yruvate carboxylase. * **Clinical Presentation:** Deficiency presents with dermatitis (periorificial), alopecia, and neurological symptoms (hypotonia, seizures). * **Antibiotics:** Long-term use of broad-spectrum antibiotics can also cause deficiency by destroying biotin-producing intestinal flora. * **CO2 Carrier:** Biotin acts as a carrier of activated carbon dioxide (CO2).

Question 667: At which site in the kidneys does the 1,25-hydroxylation of Vitamin D take place?

A. Collecting ducts
B. Glomerulus
C. Proximal convoluted tubules (Correct Answer)
D. Distal convoluted tubules

Explanation: **Explanation:** The conversion of Vitamin D into its most active biological form, **1,25-dihydroxycholicalciferol (Calcitriol)**, is the final step in Vitamin D synthesis. This process is mediated by the enzyme **1-alpha-hydroxylase**. **Why the Proximal Convoluted Tubule (PCT) is correct:** The enzyme 1-alpha-hydroxylase is primarily localized in the mitochondrial fraction of the epithelial cells of the **Proximal Convoluted Tubules**. This site is physiologically significant because the PCT is the major area for mineral reabsorption, and the activity of this enzyme is tightly regulated by Parathyroid Hormone (PTH), low serum calcium, and low serum phosphate—all of which signal the need for increased calcitriol to enhance intestinal calcium absorption. **Why other options are incorrect:** * **Glomerulus:** This is a filtration unit composed of capillary loops and podocytes; it does not possess the metabolic machinery for steroid hydroxylation. * **Distal Convoluted Tubules (DCT):** While the DCT is a major site for PTH-mediated calcium reabsorption, it lacks significant 1-alpha-hydroxylase activity compared to the PCT. * **Collecting Ducts:** These are primarily involved in water and electrolyte fine-tuning under the influence of ADH and Aldosterone, not Vitamin D activation. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The 1-alpha-hydroxylation in the kidney is the rate-limiting step of Vitamin D activation. * **Chronic Kidney Disease (CKD):** Loss of PCT functional mass leads to secondary hyperparathyroidism and renal osteodystrophy due to failure of 1-alpha-hydroxylation. * **Storage form:** 25-hydroxyvitamin D (Calcidiol) is the major storage form and is produced in the **liver** by 25-hydroxylase. * **Inhibition:** High levels of Calcitriol exert negative feedback on 1-alpha-hydroxylase and stimulate 24-hydroxylase to produce the inactive 24,25-dihydroxyvitamin D.

Question 668: A 30-year-old male patient on Isoniazid therapy for Tuberculosis developed rashes on exposed parts of the body and disorientation of memory. His family also reports a history of diarrhea. What is the most likely diagnosis?

A. Isoniazid-induced neuropathy
B. Tuberculosis skin lesions
C. Niacin deficiency (Correct Answer)
D. Adverse drug reaction to another medication

Explanation: ### Explanation The correct answer is **Niacin deficiency (Pellagra)**. **1. Why Niacin deficiency is correct:** The patient presents with the classic **"3 Ds" of Pellagra**: **D**ermatitis (rashes on exposed parts, known as Casal’s necklace), **D**iarrhea, and **D**ementia (disorientation). The biochemical link lies in **Isoniazid (INH)** therapy. Isoniazid is structurally similar to Vitamin B6 (Pyridoxine). It inhibits the enzyme **Pyridoxine kinase**, leading to a functional B6 deficiency. Since Vitamin B6 is a mandatory cofactor for the enzyme **Kynureninase** (which converts Tryptophan to Niacin), INH therapy impairs the endogenous synthesis of Niacin, resulting in secondary Pellagra. **2. Why other options are incorrect:** * **A. Isoniazid-induced neuropathy:** While INH commonly causes peripheral neuropathy due to B6 deficiency, it presents with numbness and paresthesia in a "glove and stocking" distribution, not with diarrhea or photosensitive rashes. * **B. Tuberculosis skin lesions:** Cutaneous TB (like Lupus vulgaris) presents as localized nodules or plaques, not as generalized photosensitive dermatitis associated with systemic symptoms like diarrhea. * **D. Adverse drug reaction:** While INH can cause hypersensitivity, the specific triad of symptoms (Dermatitis, Diarrhea, Dementia) specifically points to a nutritional deficiency rather than a generic drug rash. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tryptophan metabolism:** 60 mg of Tryptophan yields 1 mg of Niacin. * **Hartnup Disease:** A genetic defect in neutral amino acid transport (Tryptophan) that also presents with Pellagra-like symptoms. * **Carcinoid Syndrome:** Can lead to Niacin deficiency because Tryptophan is diverted to overproduce Serotonin. * **Prophylaxis:** Always co-administer **Pyridoxine (10–50 mg/day)** with Isoniazid to prevent these complications.

Question 669: Vitamin D from the skin or diet is metabolized primarily in the liver to 25-hydroxyvitamin D3. What is the rate-limiting enzyme in Vitamin D synthesis?

A. 25-hydroxycholecalciferol
B. 1-alpha-hydroxylase (Correct Answer)
C. 24,25-dihydroxycholecalciferol
D. 7-dehydrocholesterol

Explanation: **Explanation:** The synthesis of the active form of Vitamin D (Calcitriol) is a multi-step process involving the skin, liver, and kidneys. The conversion of **25-hydroxycholecalciferol** [25(OH)D3] to **1,25-dihydroxycholecalciferol** [1,25(OH)2D3] is the final and most critical step. **Why 1-alpha-hydroxylase is correct:** This enzyme, located in the **proximal convoluted tubules of the kidney**, catalyzes the addition of a hydroxyl group at the 1st position. It is the **rate-limiting and most strictly regulated step** in the pathway. Its activity is increased by Parathyroid Hormone (PTH) and low serum phosphate, and decreased by high levels of Calcitriol (feedback inhibition) and FGF-23. **Analysis of Incorrect Options:** * **A. 25-hydroxycholecalciferol (Calcidiol):** This is the major circulating form of Vitamin D and the clinical marker used to assess Vitamin D status, but it is a metabolite, not an enzyme. * **C. 24,25-dihydroxycholecalciferol:** This is an inactive metabolite formed by the enzyme 24-hydroxylase when Vitamin D levels are sufficient; it represents the catabolic pathway. * **D. 7-dehydrocholesterol:** This is the precursor molecule found in the skin that is converted to Cholecalciferol (D3) by UV-B radiation. It is the starting point, not the rate-limiting enzyme. **High-Yield NEET-PG Pearls:** * **Storage form:** 25-hydroxycholecalciferol (longest half-life). * **Active form:** 1,25-dihydroxycholecalciferol (Calcitriol). * **Enzyme Location:** 25-hydroxylase is in the **Liver**; 1-alpha-hydroxylase is in the **Kidney**. * **Clinical Correlation:** In chronic kidney disease (CKD), the failure of 1-alpha-hydroxylase leads to secondary hyperparathyroidism and renal osteodystrophy.

Question 670: Gamma carboxylation of which amino acid residue requires vitamin K?

A. Glutamine
B. Glutamic acid (Correct Answer)
C. Glycine
D. Aspartate

Explanation: **Explanation:** Vitamin K acts as a vital cofactor for the enzyme **gamma-glutamyl carboxylase**. This enzyme facilitates the post-translational modification of specific proteins by adding a carboxyl group (–COOH) to the gamma carbon of **Glutamic acid (Glutamate)** residues. 1. **Why Glutamic acid is correct:** The process converts Glutamic acid into **gamma-carboxyglutamic acid (Gla)**. This modification is essential because the two adjacent carboxyl groups in Gla residues create a high-affinity binding site for **Calcium ions (Ca²⁺)**. This "calcium bridge" allows clotting factors to bind to phospholipid membranes on platelets, a critical step in the coagulation cascade. 2. **Why other options are incorrect:** * **Glutamine:** While structurally similar to glutamic acid, it has an amide group instead of a carboxyl group on its side chain and does not undergo gamma-carboxylation. * **Glycine:** This is the simplest amino acid with no side chain (only a hydrogen atom), making gamma-carboxylation chemically impossible. * **Aspartate:** Although it is an acidic amino acid like glutamate, its side chain is one carbon shorter (beta-carbon), and it is not a substrate for the vitamin K-dependent carboxylase enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin K-dependent factors:** Factors II (Prothrombin), VII, IX, X, and Proteins C and S. * **Mechanism of Warfarin:** It inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the regeneration of active Vitamin K (hydroquinone), thereby inhibiting gamma-carboxylation. * **Osteocalcin:** This bone protein also undergoes gamma-carboxylation, highlighting Vitamin K's role in bone mineralization. * **Lab Marker:** Deficiency leads to an increased **Prothrombin Time (PT)**.

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

Practice Questions

Riboflavin (B2) and Flavin Coenzymes

Practice Questions

Niacin and NAD/NADP

Practice Questions

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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