Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 561: Antibiotic abuse can induce the deficiency of which of the following vitamins?

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

Explanation: **Explanation:** **Correct Option: C (Vitamin K)** The primary reason for this deficiency is the disruption of the **intestinal microflora**. While Vitamin K1 (Phylloquinone) is obtained from green leafy vegetables, **Vitamin K2 (Menaquinone)** is synthesized by the symbiotic bacteria (such as *E. coli* and *Bacteroides*) residing in the human colon. Prolonged or indiscriminate use of broad-spectrum antibiotics (e.g., cephalosporins, sulfonamides) sterilizes the gut, eliminating these commensal bacteria. This significantly reduces the endogenous production of Vitamin K, leading to a deficiency that manifests as a bleeding tendency due to impaired gamma-carboxylation of clotting factors II, VII, IX, and X. **Why other options are incorrect:** * **A. Thiamine (B1):** Primarily sourced from dietary intake (whole grains, legumes). While some gut synthesis occurs, it is insufficient to meet human requirements; deficiency is usually due to alcoholism or malnutrition. * **B. Niacin (B3):** Primarily derived from the diet or synthesized endogenously from the amino acid **Tryptophan**. It is not significantly dependent on gut flora. * **D. Vitamin E:** A fat-soluble antioxidant obtained strictly from dietary oils and nuts. Its levels are affected by fat malabsorption syndromes, not by gut bacterial colonization. **High-Yield Clinical Pearls for NEET-PG:** * **Newborns** are naturally deficient in Vitamin K because they have a **sterile gut** and breast milk is a poor source. This is why a prophylactic Vitamin K injection is given at birth to prevent Hemorrhagic Disease of the Newborn. * **Warfarin** acts as an antagonist to Vitamin K by inhibiting the enzyme **Vitamin K Epoxide Reductase (VKOR)**. * Always suspect Vitamin K deficiency in a patient on long-term antibiotics who presents with an increased **Prothrombin Time (PT)**.

Question 562: Which of the following enzymatic conversions is irreversible?

A. Formyl tetrahydrofolate (THFA) to Methenyl tetrahydrofolate (THFA)
B. Methenyl tetrahydrofolate (THFA) to Methylene tetrahydrofolate (THFA)
C. Methylene tetrahydrofolate (THFA) to Methyl tetrahydrofolate (THFA) (Correct Answer)
D. All of the above

Explanation: ### Explanation The correct answer is **C: Methylene tetrahydrofolate (THFA) to Methyl tetrahydrofolate (THFA).** #### 1. Why Option C is Correct The conversion of **N5, N10-methylene THF** to **N5-methyl THF** is catalyzed by the enzyme **Methylene Tetrahydrofolate Reductase (MTHFR)**. This reaction requires NADPH as a reducing agent. In biological systems, this specific step is **physiologically irreversible**. Once N5-methyl THF is formed, it represents a "metabolic dead-end" unless it is utilized by the enzyme **Methionine Synthase** (which requires Vitamin B12) to convert homocysteine to methionine. This irreversibility is the basis of the "Folate Trap" hypothesis. #### 2. Why Other Options are Incorrect * **Options A and B:** The interconversions between Formyl-THF, Methenyl-THF, and Methylene-THF are **reversible** reactions catalyzed by the trifunctional enzyme complex (MTHFD1). These forms can easily shift back and forth depending on the metabolic needs of the cell (e.g., purine synthesis vs. thymidylate synthesis). #### 3. High-Yield Clinical Pearls for NEET-PG * **The Folate Trap:** If **Vitamin B12** is deficient, N5-methyl THF cannot be converted back to other folate forms. Folate becomes "trapped" in the methyl form, leading to a functional folate deficiency and **megaloblastic anemia**, even if dietary folate intake is adequate. * **MTHFR Polymorphism:** Mutations in the MTHFR gene (e.g., C677T) are associated with hyperhomocysteinemia, which increases the risk of neural tube defects (NTDs) and cardiovascular thrombosis. * **Carbon Units:** * **N10-Formyl THF:** Used in Purine synthesis. * **N5, N10-Methylene THF:** Used in Thymidylate (dTMP) synthesis. * **N5-Methyl THF:** Used in Homocysteine remethylation.

Question 563: Which of the following supplements is used in the treatment of multiple carboxylase deficiency?

A. Biotin (Correct Answer)
B. Pyridoxin
C. Thiamine
D. Folic acid

Explanation: **Explanation:** **Multiple Carboxylase Deficiency (MCD)** is a metabolic disorder caused by defects in either **holocarboxylase synthetase** (early-onset) or **biotinidase** (late-onset). Both enzymes are essential for the normal cycling and utilization of **Biotin (Vitamin B7)**. **Why Biotin is the correct answer:** Biotin serves as a vital coenzyme for four major carboxylases in the body: 1. **Pyruvate carboxylase** (Gluconeogenesis) 2. **Acetyl-CoA carboxylase** (Fatty acid synthesis) 3. **Propionyl-CoA carboxylase** (Amino acid catabolism) 4. **3-Methylcrotonyl-CoA carboxylase** (Leucine catabolism) In MCD, these enzymes cannot function because biotin is not properly attached to them or recycled. High-dose pharmacological supplementation of **Biotin** bypasses these defects, restoring enzyme activity and resolving clinical symptoms like dermatitis, alopecia, and metabolic acidosis. **Why other options are incorrect:** * **Pyridoxine (B6):** Coenzyme for transamination and decarboxylation (e.g., ALT, AST, GAD). Deficiency leads to sideroblastic anemia and seizures. * **Thiamine (B1):** Coenzyme for oxidative decarboxylation (e.g., Pyruvate dehydrogenase). Deficiency causes Beriberi and Wernicke-Korsakoff syndrome. * **Folic acid (B9):** Involved in one-carbon metabolism and DNA synthesis. Deficiency causes megaloblastic anemia. **High-Yield Clinical Pearls for NEET-PG:** * **The "CO2 Fixer":** Biotin is always the coenzyme for carboxylases (except Vitamin K-dependent carboxylation). * **Egg White Injury:** Raw egg whites contain **Avidin**, which binds biotin tightly and prevents its absorption, potentially leading to deficiency. * **Clinical Triad:** Suspect MCD in infants presenting with **alopecia, skin rash, and lactic acidosis.**

Question 564: Which of the following is least affected in Vitamin C deficiency?

A. Gingival fibres
B. Periodontal ligament
C. Blood vessels of the gingiva
D. Epithelial lining of the mucosa (Correct Answer)

Explanation: ### Explanation The core biochemical defect in Vitamin C (Ascorbic acid) deficiency is the **impaired hydroxylation of Proline and Lysine residues** during collagen synthesis. Vitamin C acts as a co-factor for the enzymes *prolyl hydroxylase* and *lysyl hydroxylase*, which are essential for the cross-linking and stabilization of the collagen triple helix. **Why the Epithelial Lining is Least Affected:** The epithelial lining of the mucosa consists of **stratified squamous epithelium**, which is primarily composed of **keratinocytes** and held together by desmosomes. Unlike connective tissues, the integrity of the epithelium itself does not depend on collagen. Therefore, while the underlying basement membrane may be weakened, the epithelial cells are not directly structural targets of Vitamin C deficiency. **Analysis of Incorrect Options:** * **Gingival Fibres & Periodontal Ligament (PDL):** These structures are composed almost entirely of **Type I Collagen**. They have a high turnover rate. In Scurvy, the inability to form stable collagen leads to the breakdown of these fibers, resulting in the hallmark symptom of **loosening of teeth**. * **Blood Vessels of the Gingiva:** The tunica adventitia and the basement membrane of capillaries rely heavily on collagen for structural integrity. Deficiency leads to **capillary fragility**, manifesting as petechiae, ecchymosis, and "spongy," bleeding gums. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy Presentation:** Remember the **4 H’s**: **H**emorrhage (petechiae, bleeding gums), **H**yperkeratosis (corkscrew hairs), **H**ematologic (anemia), and **H**ypochondriasis. * **Osteoid Matrix:** Vitamin C is also essential for the formation of the bone matrix; deficiency in children leads to **Barlow’s disease** (infantile scurvy). * **Wound Healing:** Vitamin C is critical for the proliferative phase of wound healing (collagen deposition); deficiency leads to wound dehiscence.

Question 565: Vitamin E is responsible for all the following functions EXCEPT:

A. It prevents lipid peroxidation in biological membranes.
B. It prevents RBCs from hemolysis.
C. It is considered to be the anti-sterility vitamin.
D. It cures the oxidation of low-density lipoproteins. (Correct Answer)

Explanation: **Explanation:** Vitamin E (Tocopherol) is a potent lipid-soluble antioxidant that functions primarily as a free radical scavenger. **Why Option D is the correct answer:** The statement "It cures the oxidation of low-density lipoproteins" is incorrect because Vitamin E **prevents** or inhibits the oxidation of LDL; it does not "cure" or reverse oxidation once it has occurred. Oxidized LDL is a major pro-atherogenic factor; by preventing its formation, Vitamin E helps reduce the risk of atherosclerosis. **Analysis of other options:** * **Option A:** Vitamin E is the primary defense against **lipid peroxidation**. It protects polyunsaturated fatty acids (PUFAs) in biological membranes from being damaged by reactive oxygen species (ROS). * **Option B:** By protecting the erythrocyte membrane from oxidative stress, Vitamin E **prevents hemolysis**. Deficiency of Vitamin E is a known cause of hemolytic anemia, especially in premature infants. * **Option C:** In animal models (specifically rats), Vitamin E deficiency leads to germinal epithelium degeneration and sterility. Thus, it is historically labeled the **"anti-sterility vitamin,"** though this effect is less clearly defined in humans. **High-Yield Clinical Pearls for NEET-PG:** * **Most active form:** $\alpha$-tocopherol is the most biologically active form. * **Synergy:** Vitamin E works in tandem with **Vitamin C** (which regenerates reduced tocopherol) and **Selenium** (as part of glutathione peroxidase). * **Deficiency:** Presents with posterior column signs (loss of proprioception/vibration), ataxia, and hemolytic anemia, mimicking Friedreich’s ataxia. * **Toxicity:** High doses can interfere with Vitamin K action, leading to an increased risk of hemorrhages.

Question 566: The pro-oxidant action of Vitamin C is potentiated by:

A. Selenium
B. Copper (Correct Answer)
C. Calcium
D. Iron

Explanation: ### Explanation **Core Concept: The Dual Nature of Vitamin C** While Vitamin C (Ascorbic acid) is primarily known as a potent antioxidant, it can act as a **pro-oxidant** under specific conditions, particularly in the presence of free transition metal ions. **Why Copper is the Correct Answer:** Vitamin C can reduce cupric ions ($Cu^{2+}$) to cuprous ions ($Cu^{+}$). These reduced metal ions then react with hydrogen peroxide ($H_2O_2$) via the **Fenton-type reaction** to generate the **hydroxyl radical (•OH)**—the most reactive and damaging reactive oxygen species (ROS). This process accelerates oxidative stress and lipid peroxidation. While Iron ($Fe^{3+}$) also participates in this reaction, **Copper** is traditionally recognized in biochemistry as a more potent catalyst for Vitamin C-induced pro-oxidant activity in specific experimental models. **Analysis of Incorrect Options:** * **Iron (D):** Iron does facilitate pro-oxidant activity via the Fenton reaction. However, in the context of standard medical examinations, Copper is often highlighted as the specific potentiator for Vitamin C’s pro-oxidant effect. (Note: In some clinical contexts, both are relevant, but Copper is the classic academic answer). * **Selenium (A):** Selenium is a component of **Glutathione Peroxidase** and works synergistically with Vitamin C as an **antioxidant**, not a pro-oxidant. * **Calcium (C):** Calcium is a divalent cation involved in signaling and bone mineralization; it does not possess the redox-cycling capacity required to act as a pro-oxidant. **High-Yield Clinical Pearls for NEET-PG:** * **Fenton Reaction:** $Fe^{2+} / Cu^{+} + H_2O_2 \rightarrow Fe^{3+} / Cu^{2+} + \bullet OH + OH^-$ * **Scurvy:** Vitamin C deficiency leads to defective collagen synthesis (impaired hydroxylation of Proline and Lysine). * **Therapeutic Note:** High-dose intravenous Vitamin C is sometimes explored in cancer research specifically for its pro-oxidant effect to kill tumor cells. * **Absorption:** Vitamin C enhances the absorption of **Non-heme Iron** by reducing it from the ferric ($Fe^{3+}$) to the ferrous ($Fe^{2+}$) state.

Question 567: Vitamin K deficiency is indicated by which of the following laboratory findings?

A. Low platelet count
B. Increased prothrombin time (Correct Answer)
C. Decreased prothrombin time
D. None of the above

Explanation: **Explanation:** **1. Why the correct answer is right:** Vitamin K is an essential fat-soluble vitamin that acts as a cofactor for the enzyme **$\gamma$-glutamyl carboxylase**. This enzyme is responsible for the post-translational modification (carboxylation of glutamate residues) of specific clotting factors: **Factors II (Prothrombin), VII, IX, and X**, as well as Proteins C and S. In Vitamin K deficiency, these factors are synthesized but remain inactive (known as PIVKAs—Proteins Induced by Vitamin K Absence). Since Factor VII has the shortest half-life and is part of the **extrinsic pathway**, its deficiency rapidly leads to an **increased Prothrombin Time (PT)**. While the Activated Partial Thromboplastin Time (aPTT) may also increase in severe cases, PT is the most sensitive clinical marker for Vitamin K status. **2. Why the incorrect options are wrong:** * **Option A (Low platelet count):** Vitamin K is involved in the coagulation cascade (secondary hemostasis), not platelet production or survival (primary hemostasis). Thrombocytopenia is typically seen in bone marrow disorders or hypersplenism. * **Option C (Decreased prothrombin time):** A decreased PT indicates a hypercoagulable state, whereas Vitamin K deficiency leads to a bleeding diathesis due to the inability to form clots. **3. NEET-PG High-Yield Pearls:** * **Warfarin Mechanism:** Warfarin acts as a Vitamin K antagonist by inhibiting **Vitamin K epoxide reductase**, preventing the recycling of Vitamin K. * **Newborns:** They are prone to Vitamin K deficiency (Hemorrhagic Disease of the Newborn) due to sterile guts and poor placental transfer; hence, a prophylactic IM dose of Vitamin K is given at birth. * **Absorption:** Being fat-soluble, its deficiency is often secondary to fat malabsorption (e.g., obstructive jaundice, celiac disease).

Question 568: Diagnosis of vitamin B1 deficiency is made by?

A. Thiamine level
B. Transketolase activity (Correct Answer)
C. Carboxylase activity
D. Transaminase activity

Explanation: **Explanation:** The diagnosis of Vitamin B1 (Thiamine) deficiency is most reliably confirmed by measuring **erythrocyte transketolase activity (ETKA)**. **1. Why Transketolase Activity is Correct:** Thiamine pyrophosphate (TPP) is a vital coenzyme for the enzyme **transketolase** in the Pentose Phosphate Pathway (HMP Shunt). In clinical practice, measuring the serum thiamine level is often inaccurate as it does not reflect intracellular stores. Instead, a functional assay is used: transketolase activity is measured before and after the addition of exogenous TPP. An increase in enzyme activity (>15–25%) indicates a deficiency, as it proves the enzyme was "hungry" for its missing cofactor. **2. Why Other Options are Incorrect:** * **Thiamine level:** While blood levels can be measured, they are transient and do not accurately represent the total body nutritional status or metabolic function. * **Carboxylase activity:** This is used to assess **Biotin (B7)** deficiency. Biotin is a cofactor for enzymes like pyruvate carboxylase and acetyl-CoA carboxylase. (Note: While TPP is a cofactor for pyruvate *dehydrogenase*, it is not a carboxylase). * **Transaminase activity:** This is used to assess **Vitamin B6 (Pyridoxine)** status. Enzymes like ALT and AST require Pyridoxal Phosphate (PLP) as a cofactor. **High-Yield Clinical Pearls for NEET-PG:** * **Key TPP-dependent enzymes:** Pyruvate Dehydrogenase, $\alpha$-ketoglutarate Dehydrogenase, and Branched-chain $\alpha$-ketoacid Dehydrogenase. * **Clinical Triad:** Wernicke’s Encephalopathy presents with **A**taxia, **C**onfusion, and **O**phthalmoplegia (mnemonic: **ACO**). * **Gold Standard:** Erythrocyte transketolase activity is the "gold standard" biochemical marker for thiamine status.

Question 569: Casals Necklace pattern is seen in which of the following conditions?

A. Thiamine deficiency
B. Riboflavin deficiency
C. Niacin deficiency (Correct Answer)
D. All of the above

Explanation: **Explanation:** **Casal’s Necklace** is a pathognomonic clinical sign of **Pellagra**, which is caused by a deficiency of **Niacin (Vitamin B3)** or its precursor amino acid, **Tryptophan**. The underlying mechanism involves the role of Niacin as a precursor for NAD and NADP, which are essential for DNA repair and energy metabolism. In its absence, keratinocytes become highly sensitive to UV radiation. Upon exposure to sunlight, a well-demarcated, hyperpigmented, scaly erythematous rash develops. When this rash occurs specifically around the lower neck in a collar-like distribution, it is termed "Casal’s Necklace." **Analysis of Options:** * **Thiamine (B1) deficiency:** Leads to Beriberi (Dry/Wet) or Wernicke-Korsakoff syndrome. It does not cause photosensitive dermatosis. * **Riboflavin (B2) deficiency:** Characterized by the "Oral-Ocular-Genital" syndrome (cheilosis, angular stomatitis, glossitis, and corneal neovascularization), but not Casal's necklace. * **Niacin (B3) deficiency:** Correct. It presents with the classic **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and if untreated, Death. **High-Yield Clinical Pearls for NEET-PG:** * **Hartnup Disease:** An autosomal recessive disorder involving defective transport of neutral amino acids (Tryptophan), leading to pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause Niacin deficiency because Tryptophan is diverted toward the overproduction of Serotonin. * **Corn-based diets:** Maize is low in tryptophan and contains niacin in a bound, unabsorbable form (niacytin), predisposing populations to Pellagra. * **Isoniazid (INH) therapy:** Can precipitate B3 deficiency by inhibiting the conversion of tryptophan to niacin.

Question 570: White polished rice causes deficiency of which vitamin?

A. Thiamine (Correct Answer)
B. Tryptophan
C. Riboflavin
D. Protein

Explanation: **Explanation:** **Correct Answer: A. Thiamine (Vitamin B1)** Thiamine is primarily located in the outer layer (pericarp) and the germ of cereal grains. The process of **milling and polishing rice** removes these outer layers (bran), stripping the grain of its thiamine content. Consequently, populations that rely heavily on white polished rice as a staple food are at high risk for Thiamine deficiency, classically manifesting as **Beriberi**. **Analysis of Incorrect Options:** * **B. Tryptophan:** This is an essential amino acid, not a vitamin. Its deficiency is associated with maize (corn)-based diets, as maize is deficient in tryptophan, leading to Pellagra (since tryptophan is a precursor for Niacin). * **C. Riboflavin (Vitamin B2):** While milling reduces riboflavin levels, the clinical hallmark of polished rice consumption is specifically thiamine deficiency. Riboflavin deficiency (Ariboflavinosis) typically presents with cheilosis and glossitis. * **D. Protein:** While polished rice is low in protein, its consumption specifically triggers micronutrient deficiency (B1) rather than primary Protein-Energy Malnutrition (PEM), unless the overall caloric intake is also insufficient. **High-Yield Clinical Pearls for NEET-PG:** * **Dry Beriberi:** Characterized by peripheral neuropathy and muscle wasting. * **Wet Beriberi:** Characterized by high-output heart failure and edema. * **Wernicke-Korsakoff Syndrome:** Thiamine deficiency often seen in chronic alcoholics; presents with the triad of ataxia, ophthalmoplegia, and confusion. * **Enzyme Link:** Thiamine pyrophosphate (TPP) is a vital coenzyme for **Pyruvate Dehydrogenase** and **Alpha-ketoglutarate dehydrogenase**. Deficiency leads to impaired aerobic metabolism and lactic acidosis.

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

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Riboflavin (B2) and Flavin Coenzymes

Practice Questions

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

Practice Questions

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