Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 301: Which of the following is a serum marker of rickets?

A. Acid phosphatase
B. Alkaline phosphatase (Correct Answer)
C. Decreased serum calcium
D. Urinary phosphates

Explanation: **Explanation:** In rickets (and osteomalacia), the primary biochemical hallmark is an **elevation in serum Alkaline Phosphatase (ALP)**. This occurs because rickets is characterized by a failure of osteoid mineralization. To compensate for the weakened bone structure, **osteoblastic activity** increases. Since ALP is a byproduct of osteoblast activity (reflecting bone turnover), its levels rise significantly. It is often the earliest and most sensitive biochemical marker of the disease, frequently rising before radiological changes appear. **Analysis of Incorrect Options:** * **Acid Phosphatase (ACP):** This is a marker of **osteoclast** activity and is primarily used as a marker for prostatic carcinoma or Gaucher’s disease, not metabolic bone disease like rickets. * **Decreased Serum Calcium:** While calcium can be low, it is often **normal** in early or moderate rickets due to compensatory secondary hyperparathyroidism, which mobilizes calcium from bones to maintain serum levels. Therefore, it is not as reliable a marker as ALP. * **Urinary Phosphates:** While phosphate metabolism is affected, urinary phosphate levels are variable and influenced by PTH; they are not considered a primary diagnostic "serum marker" for the condition. **NEET-PG High-Yield Pearls:** * **Earliest sign of Rickets:** Increase in serum Alkaline Phosphatase. * **Most specific radiological sign:** Cupping, splaying, and fraying of the metaphysis (best seen at the wrist). * **Biochemical Profile:** Low/Normal Calcium, Low Phosphate, **High ALP**, and High PTH (Secondary Hyperparathyroidism). * **Vitamin D-dependent Rickets Type II:** Characterized by end-organ resistance to 1,25(OH)₂D (mutated receptors) and often presents with alopecia.

Question 302: Scurvy is the result of deficiency of which vitamin?

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

Explanation: **Explanation:** **Vitamin C (Ascorbic Acid)** is the correct answer. It serves as a vital co-factor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes are responsible for the post-translational hydroxylation of proline and lysine residues in collagen synthesis. Hydroxyproline is essential for stabilizing the collagen triple helix via hydrogen bonding. In Vitamin C deficiency, defective collagen synthesis leads to capillary fragility and poor wound healing, manifesting clinically as **Scurvy**. **Analysis of Incorrect Options:** * **Vitamin A (Retinol):** Deficiency primarily affects vision and epithelial integrity, leading to Night Blindness, Xerophthalmia, and Bitot’s spots. * **Vitamin B:** This is a complex of vitamins. Specific deficiencies lead to Beriberi (B1), Cheilosis/Glossitis (B2), Pellagra (B3), or Megaloblastic anemia (B12/B9). * **Vitamin D (Calciferol):** Deficiency leads to impaired calcium absorption, resulting in Rickets in children and Osteomalacia in adults. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad of Scurvy:** Perifollicular hemorrhages, Corkscrew hair, and Bleeding gums (swollen, spongy gums). * **Radiological Signs:** Look for **Wimberger’s sign** (ring epiphysis), **Frankel’s line** (white line of Scorbutus), and **Pelkan spurs**. * **Key Function:** Vitamin C also aids in **Iron absorption** (maintains iron in the ferrous $Fe^{2+}$ state) and acts as a potent antioxidant. * **Synthesis:** Humans cannot synthesize Vitamin C due to the absence of the enzyme **L-gulonolactone oxidase**.

Question 303: Which of the following is not a form of vitamin D?

A. Cholecalciferol
B. Calcifediol
C. Ergosterol (Correct Answer)
D. Calcitriol

Explanation: ### Explanation The correct answer is **Ergosterol** because it is a **provitamin** (a precursor) found in plants and fungi, not an active or intermediate form of Vitamin D itself. #### Why Ergosterol is the Correct Choice: Ergosterol is the plant-derived precursor that, upon exposure to ultraviolet (UV) light, is converted into **Ergocalciferol (Vitamin D2)**. While it is essential for the synthesis of Vitamin D2, the molecule itself does not possess Vitamin D activity in the human body until it undergoes photochemical conversion. #### Analysis of Other Options: * **Cholecalciferol (Vitamin D3):** This is the form of Vitamin D synthesized in the skin of animals and humans from 7-dehydrocholesterol via UV radiation. It is also found in oily fish and egg yolks. * **Calcifediol (25-hydroxyvitamin D3):** This is the major **circulating form** of Vitamin D. It is produced in the **liver** by the action of the enzyme 25-hydroxylase. It is the clinical marker used to transition/measure a patient's Vitamin D status. * **Calcitriol (1,25-dihydroxyvitamin D3):** This is the **biologically active form** of Vitamin D. It is produced in the **kidneys** via the enzyme 1-alpha-hydroxylase. It acts as a hormone to increase intestinal calcium absorption. #### NEET-PG High-Yield Pearls: 1. **Rate-limiting step:** The conversion of Calcifediol to Calcitriol in the kidney (regulated by PTH). 2. **Storage:** Vitamin D is stored primarily in **adipose tissue**, but the liver produces the main circulating form (Calcifediol). 3. **Deficiency:** Leads to **Rickets** in children (delayed osteoid mineralization) and **Osteomalacia** in adults (remodeling defect). 4. **Activation Sequence:** Skin (D3) $\rightarrow$ Liver (25-OH) $\rightarrow$ Kidney (1,25-OH₂).

Question 304: Neurotoxicity is seen with excess of which vitamin?

A. Vitamin B6 (Correct Answer)
B. Vitamin B12
C. Vitamin B2
D. Vitamin C

Explanation: **Explanation:** The correct answer is **Vitamin B6 (Pyridoxine)**. While most water-soluble vitamins are excreted in the urine when consumed in excess, Vitamin B6 is a notable exception. Chronic ingestion of high doses (typically >200-500 mg/day) leads to **sensory neuropathy**. The mechanism involves the accumulation of pyridoxine in the dorsal root ganglia, leading to axonal degeneration. Clinically, this manifests as progressive sensory ataxia, numbness, and diminished proprioception. **Analysis of Incorrect Options:** * **Vitamin B12 (Cobalamin):** It has a very high safety profile. No known toxicity is associated with high doses; instead, its *deficiency* causes subacute combined degeneration of the spinal cord (SCD). * **Vitamin B2 (Riboflavin):** It is non-toxic even in large amounts. Excess intake simply results in "flavinuria," where the urine turns a bright fluorescent yellow. * **Vitamin C (Ascorbic Acid):** Massive doses are generally well-tolerated but can lead to gastrointestinal upset (diarrhea) or the formation of **oxalate kidney stones** due to its metabolic breakdown into oxalate. It does not cause neurotoxicity. **High-Yield Clinical Pearls for NEET-PG:** * **The B6 Paradox:** Both deficiency and toxicity of B6 can cause neuropathy. Deficiency typically presents with peripheral neuropathy and convulsions (due to decreased GABA synthesis), while toxicity presents as a pure sensory neuropathy. * **Isoniazid (INH) Connection:** INH therapy for TB induces B6 deficiency by forming pyridoxal-hydrazone complexes. Always co-prescribe B6 (10-50 mg) to prevent peripheral neuropathy. * **Sideroblastic Anemia:** B6 is a cofactor for ALA synthase; its deficiency can lead to microcytic hypochromic anemia with ringed sideroblasts.

Question 305: Which disease is NOT due to Vitamin B deficiency?

A. Scurvy (Correct Answer)
B. Acrodermatitis
C. Beri-Beri
D. Neuropathy

Explanation: ### Explanation **Correct Answer: A. Scurvy** **1. Why Scurvy is the correct answer:** Scurvy is caused by a deficiency of **Vitamin C (Ascorbic Acid)**, not Vitamin B. Vitamin C is a vital water-soluble antioxidant and a mandatory co-factor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes are responsible for the hydroxylation of proline and lysine residues during collagen synthesis. In the absence of Vitamin C, defective collagen formation leads to capillary fragility, gum bleeding, and impaired wound healing—the hallmarks of Scurvy. **2. Analysis of Incorrect Options (Vitamin B Deficiencies):** * **B. Acrodermatitis:** Specifically, **Acrodermatitis Enteropathica**-like skin lesions can occur in **Vitamin B7 (Biotin)** deficiency or **Vitamin B2 (Riboflavin)** deficiency (often presenting as cheilosis and dermatitis). *Note: While primarily associated with Zinc deficiency, B-complex deficiencies are a major differential for such dermatological presentations.* * **C. Beri-Beri:** This is the classic deficiency disease of **Vitamin B1 (Thiamine)**. It manifests as **Dry Beri-Beri** (peripheral neuropathy) or **Wet Beri-Beri** (high-output heart failure with edema). * **D. Neuropathy:** Peripheral neuropathy is a common manifestation of several B-vitamin deficiencies, most notably **B1 (Thiamine)**, **B6 (Pyridoxine)**, and **B12 (Cobalamin)**. B12 deficiency specifically causes Subacute Combined Degeneration (SCD) of the spinal cord. **3. NEET-PG High-Yield Pearls:** * **Vitamin C & Iron:** Vitamin C enhances the absorption of non-heme iron by maintaining it in the **ferrous (Fe²⁺) state**. * **B12 vs. Folate:** Both cause megaloblastic anemia, but only **B12 deficiency** causes neurological symptoms (due to methylmalonyl-CoA accumulation). * **Pellagra (B3/Niacin):** Remember the 4 Ds: Dermatitis, Diarrhea, Dementia, and Death. * **B6 (Pyridoxine):** Must be supplemented during **Isoniazid (INH)** therapy for TB to prevent peripheral neuropathy.

Question 306: What vitamin supplementation is advised when a person is on treatment for tuberculosis?

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

Explanation: **Explanation:** The correct answer is **Pyridoxine (Vitamin B6)**. **Why Pyridoxine is the correct answer:** The primary drug used in the treatment of tuberculosis is **Isoniazid (INH)**. Isoniazid is structurally similar to pyridoxine and interferes with its metabolism in two ways: 1. It inhibits the enzyme **pyridoxine phosphokinase**, which converts pyridoxine to its active form, **Pyridoxal-5-Phosphate (PLP)**. 2. It reacts with PLP to form an inactive hydrazone complex, which is then excreted in the urine. A deficiency of PLP leads to decreased synthesis of neurotransmitters (like GABA), resulting in **peripheral neuropathy**. Therefore, prophylactic supplementation of 10–50 mg/day of Pyridoxine is mandatory for patients on INH to prevent this neurological side effect. **Why other options are incorrect:** * **Thiamine (B1):** Deficiency causes Beriberi or Wernicke-Korsakoff syndrome, typically associated with chronic alcoholism, not anti-tubercular therapy. * **Biotin (B7):** Acts as a coenzyme for carboxylation reactions. Deficiency is rare but can be induced by excessive raw egg white consumption (avidin). * **Niacin (B3):** While INH can theoretically cause Pellagra (by inhibiting the conversion of Tryptophan to Niacin), Pyridoxine is the standard supplement specifically indicated to prevent the more common dose-related neuropathy. **High-Yield Clinical Pearls for NEET-PG:** * **Sideroblastic Anemia:** INH can also cause this because PLP is a cofactor for **ALA synthase**, the rate-limiting enzyme in heme synthesis. * **High-risk groups:** Diabetics, alcoholics, and malnourished patients are more prone to INH-induced neuropathy and must always receive B6. * **Drug-Nutrient Interaction:** Always remember the triad: **INH – B6 Deficiency – Peripheral Neuropathy.**

Question 307: The Histidine load test is indicated for which of the following conditions?

A. Histidine deficiency
B. Folic acid deficiency (Correct Answer)
C. Glutamate deficiency
D. Vitamin B12 deficiency

Explanation: **Explanation:** The **Histidine Load Test** (also known as the FIGLU excretion test) is a diagnostic biochemical test used to detect **Folic acid deficiency**. **The Underlying Concept:** In the normal catabolic pathway of the amino acid Histidine, it is converted into **Formiminoglutamic acid (FIGLU)**. Under normal conditions, the enzyme *formiminotransferase* transfers the formimino group from FIGLU to **Tetrahydrofolate (THF)**, converting FIGLU into Glutamate. * **In Folic acid deficiency:** There is a lack of THF to accept the formimino group. Consequently, FIGLU cannot be converted to glutamate and instead accumulates in the body and is excreted in large amounts in the urine. **Analysis of Options:** * **B. Folic acid deficiency (Correct):** As explained, the inability to metabolize FIGLU due to low THF levels leads to high urinary FIGLU levels following a histidine "load." * **A. Histidine deficiency:** This test requires an excess (load) of histidine to challenge the metabolic pathway; it does not measure a deficiency of the amino acid itself. * **C. Glutamate deficiency:** While glutamate is the end product of this pathway, its systemic levels are maintained by multiple other metabolic routes (like transamination of alpha-ketoglutarate). * **D. Vitamin B12 deficiency:** While B12 and Folate metabolism are linked (via the Methyl-trap hypothesis), the FIGLU test is specific to Folate. The specific marker for B12 deficiency is **Methylmalonic acid (MMA)**. **High-Yield Clinical Pearls for NEET-PG:** * **FIGLU Excretion:** Increased urinary FIGLU is a sensitive indicator of early folate deficiency. * **B12 vs. Folate:** If a question mentions **Methylmalonic Aciduria**, think **B12 deficiency**. If it mentions **FIGLU**, think **Folate deficiency**. * **The "Methyl Trap":** In B12 deficiency, folate is trapped as N5-methyl-THF, which can lead to a secondary functional folate deficiency, but FIGLU remains the classic marker for primary folate status.

Question 308: A 55-year-old man, a chronic alcoholic, presented with confusion, ataxia, and diplopia. What is the appropriate treatment protocol?

A. Thiamine infusion alone
B. Thiamine injection followed by glucose infusion (Correct Answer)
C. Glucose infusion alone
D. Glucose infusion followed by thiamine injection

Explanation: ### Explanation **Concept Overview:** The patient presents with the classic triad of **Wernicke Encephalopathy (WE)**: confusion, ataxia, and ophthalmoplegia (diplopia). This is caused by a deficiency of **Vitamin B1 (Thiamine)**, a critical cofactor for enzymes in glucose metabolism, specifically *Pyruvate Dehydrogenase* and *Alpha-ketoglutarate Dehydrogenase*. **Why Option B is Correct:** In thiamine-deficient states, the brain's ability to oxidize glucose is severely impaired. If a **glucose load** is administered first, it consumes the remaining meager stores of thiamine to fuel glycolysis and the TCA cycle. This sudden depletion can precipitate acute metabolic failure in the brain, potentially leading to irreversible brain damage or **Korsakoff Psychosis**. Therefore, thiamine must be administered **before or concurrently** with glucose to ensure the metabolic machinery is ready to process the sugar. **Analysis of Incorrect Options:** * **Option A:** While thiamine is the definitive treatment, these patients are often hypoglycemic or malnourished; glucose is eventually necessary for energy recovery. * **Option C:** Giving glucose alone is dangerous and can worsen the neurological symptoms or trigger a coma by exhausting residual thiamine. * **Option D:** This is the most common clinical error. Reversing the order (Glucose before Thiamine) is the classic "wrong" step that exacerbates Wernicke Encephalopathy. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Marker:** Erythrocyte **Transketolase** activity (decreased in B1 deficiency) is the most reliable biochemical indicator. * **Brain Pathology:** Characterized by symmetrical lesions/hemorrhages in the **mammillary bodies** and periaqueductal gray matter. * **Korsakoff Syndrome:** The chronic stage of WE, characterized by **confabulation** (making up stories) and anterograde amnesia. * **The "Banana Bag":** In clinical practice, IV fluids for alcoholics often contain thiamine, folate, and magnesium (a cofactor for thiamine-dependent enzymes).

Question 309: The level of circulating 1,25-dihydroxycholecalciferol is significantly reduced in patients with chronic liver disease because:

A. The liver can no longer efficiently convert 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol.
B. The liver can no longer efficiently convert vitamin D to cholecalciferol.
C. The liver can no longer efficiently convert vitamin D to 25-hydroxycholecalciferol. (Correct Answer)
D. The liver can no longer efficiently convert cholecalciferol to 1,25-dihydroxycholecalciferol.

Explanation: **Explanation:** The synthesis of the active form of Vitamin D (Calcitriol) involves a two-step hydroxylation process occurring in different organs. 1. **The Correct Answer (C):** Vitamin D (Cholecalciferol) obtained from the diet or skin synthesis is biologically inactive. The **first step** of activation occurs in the **liver**, where the enzyme **25-hydroxylase** converts Vitamin D into **25-hydroxycholecalciferol [25(OH)D]**, also known as Calcidiol. In chronic liver disease, the parenchymal damage leads to a deficiency of this enzyme, resulting in decreased production of 25(OH)D. Since 25(OH)D is the substrate for the next step, its reduction directly leads to low levels of circulating 1,25-dihydroxycholecalciferol. 2. **Analysis of Incorrect Options:** * **Options A & D:** These are incorrect because the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol (the **second step**) occurs in the **kidneys** via the enzyme **1-alpha-hydroxylase**, not the liver. * **Option B:** This is incorrect because Vitamin D3 *is* cholecalciferol; the liver does not convert one into the other, but rather hydroxylates the existing molecule. **NEET-PG High-Yield Pearls:** * **Storage Form:** 25-hydroxycholecalciferol is the major storage form and the best indicator of a patient's Vitamin D status. * **Active Form:** 1,25-dihydroxycholecalciferol (Calcitriol) is the most potent biological form. * **Rate-Limiting Step:** The renal 1-alpha-hydroxylation is the rate-limiting step, regulated by PTH and serum phosphate levels. * **Clinical Correlation:** Patients with liver cirrhosis often develop "Hepatic Osteodystrophy" due to this impaired hydroxylation and subsequent secondary hyperparathyroidism.

Question 310: Which of the following are coenzymes for pyruvate dehydrogenase?

A. Thiamine pyrophosphate (TPP)
B. Coenzyme A (CoA)
C. Lipoic acid
D. NADPH (Correct Answer)

Explanation: **Explanation:** The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme system that converts pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. This complex requires **five specific coenzymes**, often remembered by the mnemonic **"Tender Loving Care For Nancy."** 1. **T**hiamine pyrophosphate (TPP) – Derived from Vitamin B1 2. **L**ipoic acid (Lipoamide) 3. **C**oenzyme A (CoA) – Derived from Vitamin B5 (Pantothenic acid) 4. **F**AD – Derived from Vitamin B2 (Riboflavin) 5. **N**AD+ – Derived from Vitamin B3 (Niacin) **Why NADPH is the correct answer (the exception):** The question asks which of the following is a coenzyme for PDH. However, in the context of standard NEET-PG multiple-choice patterns, if the question asks for an "except" or identifies the outlier, **NADPH** is the correct choice because it is **not** involved in the PDH reaction. PDH uses **NAD+** (the oxidized form) as an electron acceptor to produce **NADH**. NADPH is primarily used in reductive biosynthesis (like fatty acid synthesis) and the HMP shunt, not in the oxidative decarboxylation of pyruvate. **Analysis of Incorrect Options:** * **A, B, and C:** TPP, CoA, and Lipoic acid are all essential prosthetic groups or substrates for the three subunits of the PDH complex (E1, E2, and E3 respectively). **High-Yield Clinical Pearls for NEET-PG:** * **Arsenic Poisoning:** Arsenite inhibits PDH by binding to the -SH groups of **Lipoic acid**, leading to lactic acidosis and neurological symptoms. * **Thiamine Deficiency:** Leads to Beriberi and Wernicke-Korsakoff syndrome because PDH and Alpha-ketoglutarate dehydrogenase cannot function without TPP. * **Regulation:** PDH is inhibited by its products (**Acetyl-CoA and NADH**) and activated by **Calcium** (important in skeletal muscle).

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

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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

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