Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 361: What is the cofactor of glutathione peroxidase?

A. Selenium (Correct Answer)
B. Magnesium
C. Manganese
D. Zinc

Explanation: **Explanation:** **1. Why Selenium is Correct:** Glutathione Peroxidase (GPx) is a critical antioxidant enzyme that protects cells from oxidative damage by reducing lipid hydroperoxides and free hydrogen peroxide ($H_2O_2$) into water. The functional activity of this enzyme is strictly dependent on **Selenium**. Specifically, selenium is incorporated into the enzyme's active site as the 21st amino acid, **Selenocysteine**. This makes GPx a "selenoprotein." Without selenium, the enzyme cannot neutralize reactive oxygen species, leading to cellular membrane damage. **2. Why the Other Options are Incorrect:** * **Magnesium (B):** Acts as a cofactor for enzymes involving ATP (kinases) and DNA/RNA polymerases. It is not involved in the glutathione redox cycle. * **Manganese (C):** This is the cofactor for **Manganese Superoxide Dismutase (Mn-SOD)**, found in the mitochondria. It is also a cofactor for Arginase and Pyruvate Carboxylase. * **Zinc (D):** A structural component of "Zinc fingers" in transcription factors and a cofactor for **Cytosolic SOD (Cu-Zn SOD)**, Carbonic Anhydrase, and Alcohol Dehydrogenase. **3. High-Yield Clinical Pearls for NEET-PG:** * **Keshan Disease:** A cardiomyopathy caused by Selenium deficiency, leading to decreased Glutathione Peroxidase activity. * **Glutathione Reductase:** Unlike the peroxidase, the *reductase* enzyme requires **Riboflavin (Vitamin B2)** in the form of FAD. * **The Redox Cycle:** Glutathione peroxidase uses **Reduced Glutathione (GSH)** as an electron donor, converting it to **Oxidized Glutathione (GSSG)**. * **Other Selenoproteins:** Deiodinase (converts T4 to T3) and Thioredoxin reductase also require Selenium.

Question 362: A patient presents with a confirmed deficiency of thiamine. What is a potential clinical manifestation of this deficiency?

A. Delayed wound healing
B. Cardiac abnormalities (Correct Answer)
C. Memory loss
D. Gingival bleeding

Explanation: **Explanation:** Thiamine (Vitamin B1) serves as a vital cofactor (Thiamine Pyrophosphate - TPP) for key enzymes in carbohydrate metabolism, including **Pyruvate Dehydrogenase**, **$\alpha$-Ketoglutarate Dehydrogenase**, and **Branched-chain $\alpha$-ketoacid Dehydrogenase**. **Why Cardiac Abnormalities is Correct:** Thiamine deficiency leads to **Beriberi**. **Wet Beriberi** specifically affects the cardiovascular system. The biochemical impairment of energy production leads to peripheral vasodilation, high-output cardiac failure, and peripheral edema. Clinical signs include cardiomegaly, tachycardia, and dyspnea. **Analysis of Incorrect Options:** * **A. Delayed wound healing:** Primarily associated with **Vitamin C** deficiency (impaired collagen synthesis) or Zinc deficiency. * **C. Memory loss:** While thiamine deficiency causes Wernicke-Korsakoff syndrome, the classic triad is ataxia, ophthalmoplegia, and confusion. While "memory loss" (confabulation) occurs in Korsakoff, **Cardiac abnormalities** is a more direct, systemic manifestation of the "Wet" form of the deficiency. * **D. Gingival bleeding:** A hallmark of **Scurvy** (Vitamin C deficiency) due to defective collagen cross-linking in capillary walls. **High-Yield NEET-PG Pearls:** 1. **Enzyme Marker:** Erythrocyte **transketolase** activity is the most reliable diagnostic test for thiamine status. 2. **Wernicke’s Encephalopathy:** Characterized by the triad of Global confusion, Ophthalmoplegia, and Ataxia. 3. **Clinical Caution:** Always administer thiamine **before** glucose in malnourished/alcoholic patients to prevent precipitating acute Wernicke’s (glucose oxidation consumes remaining thiamine). 4. **Dry Beriberi:** Presents as symmetrical peripheral neuropathy (sensory and motor deficits).

Question 363: Which of the following is a component of the visual pigment rhodopsin?

A. 13-Carotene
B. Retinal (Correct Answer)
C. Retinol
D. Retinoic acid

Explanation: **Explanation:** **1. Why Retinal is Correct:** Rhodopsin (visual purple) is the primary light-sensitive pigment found in the rod cells of the retina, responsible for vision in low light (scotopic vision). It is a conjugated protein consisting of a protein moiety called **Opsin** and a prosthetic group, **11-cis-retinal**. * **Mechanism:** When light hits the retina, 11-cis-retinal undergoes photoisomerization to **all-trans-retinal**. This conformational change triggers a nerve impulse via the G-protein transducin, which the brain interprets as light. **2. Why Other Options are Incorrect:** * **$\beta$-Carotene:** This is a provitamin A (precursor) found in plants. While it is cleaved in the intestine to form retinal, it is not the direct component of the rhodopsin molecule. * **Retinol (Vitamin A alcohol):** This is the transport and storage form of Vitamin A. While retinal can be reduced to retinol in the visual cycle, retinol itself does not bind to opsin. * **Retinoic Acid:** This is the oxidized form of Vitamin A involved in gene expression, epithelial maintenance, and growth. Crucially, **retinoic acid cannot be converted back to retinal** and therefore plays no role in the visual cycle. **3. NEET-PG High-Yield Clinical Pearls:** * **Wald’s Visual Cycle:** The process of regeneration of 11-cis-retinal from all-trans-retinal is known as the Wald cycle. * **Nyctalopia (Night Blindness):** The earliest symptom of Vitamin A deficiency due to impaired rhodopsin regeneration. * **Bitot’s Spots:** Keratin debris on the conjunctiva, a classic sign of Vitamin A deficiency. * **Specific Isomer:** Always remember it is **11-cis-retinal** that binds with opsin; "all-trans" is the inactive form post-light exposure.

Question 364: Which of the following enzymes requires zinc as a cofactor?

A. Tyrosinase
B. Carbonic anhydrase (Correct Answer)
C. Hexokinase
D. Oxidoreductase

Explanation: **Explanation:** **Carbonic anhydrase** is a classic example of a metalloenzyme that requires **Zinc ($Zn^{2+}$)** as a mandatory cofactor for its catalytic activity. The zinc ion is coordinated to three histidine residues and a water molecule/hydroxyl group at the enzyme's active site. This arrangement facilitates the rapid interconversion of carbon dioxide and water into bicarbonate and protons ($H^+$), a process vital for $CO_2$ transport in RBCs and acid-base regulation in the kidneys. **Analysis of Incorrect Options:** * **Tyrosinase:** This is a **Copper (Cu)** containing enzyme. It is essential for melanin synthesis; its deficiency leads to Albinism. * **Hexokinase:** This glycolytic enzyme requires **Magnesium ($Mg^{2+}$)**. Magnesium acts by complexing with ATP (Mg-ATP) to shield negative charges, facilitating the nucleophilic attack. * **Oxidoreductase:** This is a broad class of enzymes. While some may use zinc (like Alcohol Dehydrogenase), the term is too general. Many use NAD/FAD or Heme (Iron) as cofactors. **High-Yield Clinical Pearls for NEET-PG:** * **Other Zinc-containing enzymes:** Alcohol dehydrogenase, Alkaline phosphatase (ALP), Carboxypeptidase, and RNA/DNA polymerases. * **Zinc Finger Motifs:** Zinc is crucial for the structural stability of DNA-binding proteins (transcription factors). * **Clinical Deficiency:** Zinc deficiency leads to **Acrodermatitis enteropathica**, characterized by periorificial dermatitis, alopecia, and diarrhea. It also causes poor wound healing and hypogeusia (decreased taste acuity).

Question 365: Glossitis and cheilitis are clinical manifestations of which vitamin deficiency?

A. Vitamin B2 (Riboflavin) (Correct Answer)
B. Vitamin B12 (Cobalamin)
C. Vitamin K
D. Vitamin D

Explanation: ### Explanation **1. Why Vitamin B2 (Riboflavin) is Correct:** Riboflavin is a precursor for the coenzymes **FMN (Flavin Mononucleotide)** and **FAD (Flavin Adenine Dinucleotide)**, which are essential for oxidation-reduction reactions in the mitochondrial respiratory chain. Deficiency of Riboflavin leads to a clinical constellation known as **Ariboflavinosis**. The hallmark signs are localized to the oral cavity and skin: * **Glossitis:** Inflammation of the tongue (often appearing magenta-colored). * **Cheilitis/Cheilosis:** Fissuring and inflammation of the lips. * **Angular Stomatitis:** Fissures at the corners of the mouth. * **Corneal Neovascularization:** Increased blood vessel growth in the cornea. **2. Why the Other Options are Incorrect:** * **Vitamin B12 (Cobalamin):** Deficiency primarily causes **Megaloblastic Anemia** and subacute combined degeneration of the spinal cord. While it can cause glossitis (beefy red tongue), it is not the classic association for cheilitis compared to B2. * **Vitamin K:** Essential for the γ-carboxylation of clotting factors II, VII, IX, and X. Deficiency leads to **bleeding diathesis** and increased Prothrombin Time (PT). * **Vitamin D:** Crucial for calcium and phosphate homeostasis. Deficiency leads to **Rickets** in children and **Osteomalacia** in adults. **3. NEET-PG High-Yield Pearls:** * **The "Magenta Tongue":** A classic buzzword for Vitamin B2 deficiency. * **Enzyme Assay:** Riboflavin status is best assessed by measuring the activity of **Erythrocyte Glutathione Reductase**. * **Light Sensitivity:** Riboflavin is highly sensitive to UV light; this is why newborns undergoing phototherapy for jaundice may develop B2 deficiency. * **Metabolic Role:** FAD is a required cofactor for Succinate Dehydrogenase (TCA cycle) and Acyl-CoA Dehydrogenase (Beta-oxidation).

Question 366: Which form of tetrahydrofolate (THF) is used in treatment?

A. N5 formyl THF (Correct Answer)
B. N5 methyl THF
C. N10 formyl THF
D. N5 formimino THF

Explanation: **Explanation:** The correct answer is **N5 formyl THF**, also known as **Folinic acid** or **Leucovorin**. **1. Why N5 formyl THF is correct:** In clinical practice, N5 formyl THF is the stable, active form of folate used for "Leucovorin Rescue." Unlike dietary folic acid, which requires the enzyme **Dihydrofolate Reductase (DHFR)** to be converted into its active form, Folinic acid bypasses this step. This is critical when treating patients with high-dose **Methotrexate** (a DHFR inhibitor). By providing N5 formyl THF, clinicians can "rescue" normal cells from toxicity by restoring the folate pool necessary for DNA synthesis, even in the presence of Methotrexate. **2. Why the other options are incorrect:** * **N5 methyl THF:** This is the primary form of folate found in the **plasma** and the "trap" form in Vitamin B12 deficiency. It is not used as a standard pharmacological treatment for rescue therapy. * **N10 formyl THF:** This is the specific form used in **purine synthesis** (donating carbons 2 and 8 of the purine ring), but it is not the stable pharmaceutical preparation used for treatment. * **N5 formimino THF:** This is an intermediate in the catabolism of **Histidine**. It is a metabolic byproduct rather than a therapeutic agent. **3. Clinical Pearls for NEET-PG:** * **Leucovorin Rescue:** Used to mitigate the bone marrow toxicity of Methotrexate. * **5-Fluorouracil (5-FU) Potentiation:** Folinic acid is also used to *increase* the efficacy of 5-FU in colorectal cancer by stabilizing the complex between 5-FU and thymidylate synthase. * **FIGLU Test:** A deficiency in THF leads to the excretion of Formiminoglutamic acid (FIGLU) in urine after a histidine load.

Question 367: One mg of Niacin is produced by how many mg of Tryptophan?

A. 22
B. 37
C. 55
D. 60 (Correct Answer)

Explanation: **Explanation:** The correct answer is **60 mg**. This is a classic high-yield biochemical conversion ratio essential for understanding nutritional requirements. **The Underlying Concept:** Niacin (Vitamin B3) can be obtained directly from the diet or synthesized endogenously in the liver from the essential amino acid **Tryptophan**. This metabolic pathway (the Kynurenine pathway) is relatively inefficient. On average, it takes **60 mg of dietary Tryptophan to synthesize 1 mg of Niacin**. This relationship is expressed as **1 Niacin Equivalent (NE)**. **Analysis of Options:** * **D (60 mg):** This is the established physiological ratio used by the WHO and nutritional boards to calculate Niacin Equivalents. * **A, B, and C (22, 37, 55 mg):** These values are incorrect and do not represent any recognized physiological conversion factor for Niacin synthesis in humans. **Clinical Pearls for NEET-PG:** 1. **Cofactor Requirements:** The conversion of Tryptophan to Niacin requires **Vitamin B6 (Pyridoxine)**, **Riboflavin (B2)**, and **Iron**. A deficiency in B6 can lead to secondary Niacin deficiency. 2. **Pellagra:** A deficiency of Niacin (or Tryptophan) leads to Pellagra, characterized by the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. 3. **Hartnup Disease:** An autosomal recessive disorder involving defective transport of neutral amino acids (including Tryptophan) in the gut and kidneys, often presenting with Pellagra-like symptoms. 4. **Carcinoid Syndrome:** In this condition, up to 60% of Tryptophan is diverted to produce **Serotonin**, leading to a secondary Niacin deficiency and Pellagra.

Question 368: Vitamin A intoxication causes injury to which of the following cellular organelles?

A. Lysosomes (Correct Answer)
B. Mitochondria
C. Endoplasmic reticulum
D. Microtubules

Explanation: **Explanation:** The correct answer is **Lysosomes**. **Mechanism of Action:** Vitamin A (Retinol) is a fat-soluble vitamin that plays a critical role in vision and epithelial integrity. However, in states of hypervitaminosis A (intoxication), the excessive amounts of retinol and retinyl esters exceed the binding capacity of Retinol Binding Protein (RBP). This "free" vitamin A acts as a potent **labilizer of lysosomal membranes**. It increases the permeability and fragility of the lysosomal membrane, leading to the leakage of hydrolytic enzymes (acid hydrolases) into the cytosol. This intracellular release of enzymes results in cellular autolysis and tissue damage, particularly in the liver and bones. **Analysis of Incorrect Options:** * **B. Mitochondria:** While some toxins affect the electron transport chain or induce apoptosis via cytochrome c release, Vitamin A does not primarily target mitochondrial membrane stability. * **C. Endoplasmic Reticulum (ER):** ER stress is associated with protein misfolding (unfolded protein response), but it is not the primary site of injury in Vitamin A toxicity. * **D. Microtubules:** These are targeted by drugs like Colchicine or Vinca alkaloids. Vitamin A does not interfere with tubulin polymerization or microtubule stability. **High-Yield Clinical Pearls for NEET-PG:** * **Acute Toxicity:** Presents with **Pseudotumor Cerebri** (idiopathic intracranial hypertension), characterized by headache, vomiting, and papilledema. * **Chronic Toxicity:** Presents with hepatomegaly, bone pain, alopecia, and dry, scaly skin (desquamation). * **Teratogenicity:** Vitamin A is highly teratogenic; it is contraindicated in pregnancy as it can cause craniofacial and cardiac defects in the fetus. * **Antagonist:** Vitamin E is considered a "stabilizer" of membranes, often acting in opposition to the labilizing effect of Vitamin A.

Question 369: All of the following statements regarding niacin-induced flushing and pruritus are true EXCEPT?

A. Occurs due to release of prostaglandins by niacin.
B. More likely to occur if taken with ethanol-containing beverages.
C. Warrants immediate stoppage of niacin. (Correct Answer)
D. Aspirin alleviates flushing in most patients.

Explanation: ### Explanation **Core Concept:** Niacin (Vitamin B3), used in high doses to treat dyslipidemia, commonly causes **cutaneous flushing and pruritus**. This is a well-known, dose-dependent side effect mediated by the release of **Prostaglandin D2 (PGD2) and PGE2** in the skin, leading to vasodilation. **Why Option C is the Correct Answer (The False Statement):** Flushing is a **benign, self-limiting side effect** and does not indicate drug toxicity or an allergic reaction. Therefore, it **does not warrant immediate stoppage** of the medication. Most patients develop tolerance to this effect within 1–2 weeks of continued therapy. **Analysis of Incorrect Options:** * **Option A:** Niacin binds to G protein-coupled receptors (GPR109A) on dermal macrophages and keratinocytes, triggering the release of **prostaglandins**. This is the primary mechanism of flushing. * **Option B:** Ethanol is a vasodilator. Consuming **alcohol** or hot beverages alongside niacin exacerbates cutaneous vasodilation, making flushing more severe. * **Option C:** Since the mechanism is prostaglandin-mediated, taking **Aspirin (325 mg)** or other NSAIDs 30 minutes before the niacin dose significantly reduces the severity of flushing. --- ### High-Yield Clinical Pearls for NEET-PG: * **Mechanism of Niacin in Dyslipidemia:** It inhibits adipose tissue lipolysis (via hormone-sensitive lipase inhibition), leading to decreased VLDL and LDL, and is the most potent agent for **increasing HDL**. * **The "Niacin Flush" Prevention:** Start with low doses, take with meals, avoid hot/alcoholic drinks, and pre-treat with **Aspirin**. * **Other Side Effects:** Hyperuricemia (can precipitate **Gout**), Hyperglycemia (caution in Diabetics), and Hepatotoxicity. * **Hartnup Disease:** A deficiency of Tryptophan transport leading to Niacin deficiency (Pellagra-like symptoms: Dermatitis, Diarrhea, Dementia).

Question 370: Vitamin B12 acts as a coenzyme to which one of the following enzymes?

A. Isocitrate dehydrogenase
B. Homocysteine methyl transferase (Correct Answer)
C. Glycogen synthase
D. G-6-P dehydrogenase

Explanation: **Explanation:** Vitamin B12 (Cobalamin) serves as an essential cofactor for only two enzymes in the human body. The correct option, **Homocysteine methyltransferase** (also known as Methionine Synthase), utilizes Methylcobalamin to transfer a methyl group from N5-methyltetrahydrofolate to homocysteine, converting it into Methionine. This reaction is critical as it links the folate cycle with B12 metabolism; a deficiency leads to the "Folate Trap," resulting in megaloblastic anemia. **Analysis of Incorrect Options:** * **Isocitrate dehydrogenase:** This is a rate-limiting enzyme of the TCA cycle and requires **NAD+** (derived from Vitamin B3/Niacin) as a cofactor, not B12. * **Glycogen synthase:** This enzyme regulates glycogenesis and is controlled by covalent modification (phosphorylation/dephosphorylation) and allosteric effectors (G-6-P). It does not require a B-complex vitamin cofactor. * **G-6-P dehydrogenase (G6PD):** This is the rate-limiting enzyme of the Pentose Phosphate Pathway (HMP Shunt) and requires **NADP+** (Vitamin B3) as a cofactor. **High-Yield Clinical Pearls for NEET-PG:** 1. **The Second Enzyme:** The only other B12-dependent enzyme is **Methylmalonyl-CoA mutase** (uses Adenosylcobalamin), which converts Methylmalonyl-CoA to Succinyl-CoA. 2. **Diagnostic Marker:** In B12 deficiency, both Homocysteine and Methylmalonic acid (MMA) levels are elevated. In Folate deficiency, only Homocysteine is elevated. 3. **Neurological Symptoms:** Subacute Combined Degeneration (SCD) of the spinal cord in B12 deficiency is attributed to the failure of the Methylmalonyl-CoA mutase reaction, leading to abnormal fatty acid incorporation into myelin.

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