Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 221: All of the following vitamins are anti-oxidants EXCEPT:

A. Beta-carotene
B. Ascorbic acid
C. Vitamin E
D. Vitamin K (Correct Answer)

Explanation: **Explanation** Antioxidants are substances that neutralize free radicals (Reactive Oxygen Species) to prevent cellular damage. In biochemistry, the primary antioxidant vitamins are **A, C, and E** (often remembered by the mnemonic **"ACE"**). **Why Vitamin K is the Correct Answer:** Vitamin K (Phylloquinone/Menaquinone) is primarily a co-factor for the **gamma-carboxylation of glutamate residues** on clotting factors II, VII, IX, and X. Its physiological role is centered on blood coagulation and bone metabolism, not the scavenging of free radicals. Therefore, it is not classified as an antioxidant. **Analysis of Other Options:** * **Beta-carotene (Option A):** A precursor to Vitamin A, it is a potent lipid-soluble antioxidant that quenches singlet oxygen and prevents lipid peroxidation in membranes. * **Ascorbic acid (Option B):** Vitamin C is the most important **water-soluble** antioxidant. It directly neutralizes ROS and is essential for regenerating the reduced (active) form of Vitamin E. * **Vitamin E (Option C):** Specifically $\alpha$-tocopherol, it is the most powerful **lipid-soluble** antioxidant. It protects LDL and polyunsaturated fatty acids (PUFAs) in cell membranes from oxidative damage. **High-Yield Clinical Pearls for NEET-PG:** * **Chain Breaker:** Vitamin E is known as the "Chain-breaking antioxidant" because it interrupts the self-propagating cycle of lipid peroxidation. * **Synergy:** Vitamin C and Vitamin E work in a redox cycle; Vitamin C acts as a reducing agent to "recharge" Vitamin E. * **Selenium Connection:** Selenium acts as a mineral antioxidant because it is a vital component of the enzyme **Glutathione Peroxidase**. * **Vitamin K Antagonist:** Warfarin inhibits Vitamin K Epoxide Reductase, preventing the recycling of Vitamin K.

Question 222: Pyridoxine is involved in which of the following biochemical processes?

A. Carboxylation
B. Trans-sulfuration (Correct Answer)
C. Oxidation-reduction
D. Transketolation

Explanation: **Explanation:** **Pyridoxine (Vitamin B6)**, in its active form **Pyridoxal Phosphate (PLP)**, serves as a versatile coenzyme primarily involved in amino acid metabolism. **1. Why Trans-sulfuration is correct:** The trans-sulfuration pathway is the process by which **Homocysteine** is converted into **Cysteine**. This pathway involves two key enzymes, both of which are **PLP-dependent**: * **Cystathionine β-synthase:** Converts Homocysteine + Serine to Cystathionine. * **Cystathionine γ-lyase:** Converts Cystathionine to Cysteine. A deficiency in B6 leads to an accumulation of homocysteine (Hyperhomocysteinemia), a significant risk factor for cardiovascular disease. **2. Why other options are incorrect:** * **A. Carboxylation:** This process requires **Biotin (Vitamin B7)** (e.g., Pyruvate carboxylase). Note: PLP is required for *decarboxylation*, not carboxylation. * **C. Oxidation-reduction:** These reactions typically require **Niacin (B3/NAD+)** or **Riboflavin (B2/FAD)**. * **D. Transketolation:** This is a key step in the Pentose Phosphate Pathway catalyzed by Transketolase, which requires **Thiamine (Vitamin B1)** as a cofactor. **High-Yield Clinical Pearls for NEET-PG:** * **PLP Functions:** Transamination (ALT/AST), Decarboxylation (GABA, Histamine, Serotonin synthesis), and Heme synthesis (ALA synthase). * **Drug Interaction:** **Isoniazid (INH)** therapy for TB induces B6 deficiency by binding to PLP, leading to peripheral neuropathy. Always co-administer B6 with INH. * **Glycogenolysis:** PLP is a unique cofactor for **Glycogen Phosphorylase**, where it plays a structural rather than catalytic role.

Question 223: A 45-year-old woman with longstanding Crohn disease and severe fat malabsorption experiences a fracture of the femoral neck after a minor contusion. This woman most likely has a deficiency of which of the following vitamins?

A. Vitamin B1 (thiamine)
B. Vitamin C
C. Vitamin D (Correct Answer)
D. Vitamin K

Explanation: **Explanation:** The correct answer is **Vitamin D**. **1. Why Vitamin D is correct:** This patient has **Crohn disease**, a chronic inflammatory bowel disease that often involves the terminal ileum, leading to **fat malabsorption**. Vitamins A, D, E, and K are fat-soluble and require bile salts and intact intestinal mucosa for absorption. A deficiency in Vitamin D leads to decreased intestinal calcium and phosphate absorption. In adults, this results in **osteomalacia**, characterized by poor mineralization of the osteoid matrix. This weakens the bones, making them susceptible to "pathological" fractures even after minor trauma (like a femoral neck fracture from a minor contusion). **2. Why the other options are incorrect:** * **Vitamin B1 (Thiamine):** A water-soluble vitamin. Deficiency typically causes Beriberi (dry or wet) or Wernicke-Korsakoff syndrome, primarily affecting the neurological and cardiovascular systems, not bone density. * **Vitamin C:** Deficiency causes Scurvy. While it affects collagen synthesis and can lead to bone pain or subperiosteal hemorrhage, it is water-soluble and less likely to be the primary result of fat malabsorption compared to Vitamin D. * **Vitamin K:** Also fat-soluble and would be deficient in this patient; however, its deficiency manifests as **coagulopathy** (increased PT/INR and bleeding tendencies) rather than bone fractures. **3. NEET-PG High-Yield Pearls:** * **Fat-Soluble Vitamins (A, D, E, K):** Always suspect deficiency in conditions like Celiac disease, Crohn’s, Chronic Pancreatitis, or Biliary obstruction. * **Osteomalacia vs. Osteoporosis:** Osteomalacia is a *qualitative* defect (poor mineralization/soft bones), whereas Osteoporosis is a *quantitative* defect (decreased bone mass/brittle bones). * **Radiological Sign:** Look for **Looser’s zones** (pseudofractures) in cases of Osteomalacia. * **Biochemical Profile:** In Vitamin D deficiency, expect **Low Serum Calcium, Low Serum Phosphate, and High Alkaline Phosphatase (ALP).**

Question 224: Why does Vitamin B6 deficiency lead to anemia?

A. Heme synthesis is affected due to reduced activity of ALA synthase (Correct Answer)
B. Iron absorption is impaired
C. Red blood cell life span is reduced
D. Vitamin B6 deficiency causes bleeding tendencies

Explanation: **Explanation:** The correct answer is **A. Heme synthesis is affected due to reduced activity of ALA synthase.** **Mechanism:** Vitamin B6 (Pyridoxine) is converted into its active form, **Pyridoxal Phosphate (PLP)**. PLP serves as an essential coenzyme for **$\delta$-aminolevulinic acid (ALA) synthase**, which is the rate-limiting enzyme in heme synthesis. This enzyme catalyzes the condensation of Succinyl CoA and Glycine to form ALA. In B6 deficiency, the activity of ALA synthase decreases, leading to impaired heme production. This results in **Sideroblastic Anemia**, characterized by microcytic hypochromic RBCs and the presence of ringed sideroblasts (iron-laden mitochondria) in the bone marrow. **Analysis of Incorrect Options:** * **B. Iron absorption is impaired:** Iron absorption occurs primarily in the duodenum and is regulated by hepcidin and ferroportin; it is not dependent on Vitamin B6. * **C. Red blood cell life span is reduced:** While B6 deficiency causes anemia, it is due to production failure (heme synthesis), not premature destruction (hemolysis). * **D. Bleeding tendencies:** Bleeding disorders are typically associated with Vitamin K deficiency (clotting factors) or Vitamin C deficiency (collagen/scurvy), not Vitamin B6. **High-Yield NEET-PG Pearls:** * **Isoniazid (INH) Connection:** The anti-tubercular drug INH is a structural analog of B6 and inhibits pyridoxine kinase. Patients on INH must receive B6 supplementation to prevent peripheral neuropathy and sideroblastic anemia. * **PLP Functions:** Beyond heme synthesis, PLP is a coenzyme for all **transamination** reactions (ALT/AST), **decarboxylation** (e.g., Glutamate to GABA), and the **cystathionine synthase** enzyme (homocysteine metabolism). * **Diagnostic Clue:** Sideroblastic anemia shows **increased** serum iron and ferritin levels despite low hemoglobin, as iron cannot be incorporated into heme.

Question 225: Erythrocyte glutathione reductase deficiency is seen in deficiency of which vitamin?

A. Riboflavin (Correct Answer)
B. Folic acid
C. Vitamin C
D. Vitamin E

Explanation: **Explanation:** The correct answer is **Riboflavin (Vitamin B2)**. **Why Riboflavin is correct:** Riboflavin is the precursor for two essential coenzymes: **Flavin Mononucleotide (FMN)** and **Flavin Adenine Dinucleotide (FAD)**. The enzyme **Erythrocyte Glutathione Reductase (EGR)** requires FAD as a mandatory cofactor to convert oxidized glutathione (GSSG) back to its reduced form (GSH). Reduced glutathione is critical for protecting red blood cells against oxidative damage. In clinical biochemistry, measuring the "EGR activation coefficient" (the increase in enzyme activity after adding FAD in vitro) is the gold standard functional assay to diagnose Riboflavin deficiency. **Why other options are incorrect:** * **Folic Acid (B9):** Primarily involved in one-carbon metabolism and DNA synthesis. Deficiency leads to megaloblastic anemia, not glutathione reductase dysfunction. * **Vitamin C (Ascorbic Acid):** Acts as a water-soluble antioxidant and a cofactor for collagen hydroxylation. While it aids in reducing oxidative stress, it is not a cofactor for glutathione reductase. * **Vitamin E (Tocopherol):** A lipid-soluble antioxidant that protects cell membranes from peroxidation. It works synergistically with glutathione but does not serve as a coenzyme for the reductase enzyme. **High-Yield NEET-PG Pearls:** * **EGR Activity Test:** The most sensitive marker for B2 deficiency. * **Clinical Triad of B2 Deficiency:** Cheilosis (fissures at corners of mouth), Glossitis (magenta tongue), and Corneal vascularization. * **Other FAD-dependent enzymes:** Succinate dehydrogenase (TCA cycle), Acyl-CoA dehydrogenase (Beta-oxidation), and Xanthine oxidase.

Question 226: Which of the following vitamins acts as a cofactor for the metabolism of sulphur-containing amino acids?

A. Folic acid (Correct Answer)
B. Biotin
C. Vitamin K
D. Pyridoxine

Explanation: **Explanation:** The metabolism of sulfur-containing amino acids (Methionine and Cysteine) is intricately linked to the **One-Carbon Metabolism** pathway, where **Folic acid (Vitamin B9)** plays a central role. **Why Folic Acid is Correct:** The conversion of **Homocysteine** back to **Methionine** (remethylation) requires **5-methyltetrahydrofolate** (a derivative of folic acid) as a methyl donor. This reaction is catalyzed by Methionine Synthase, which also requires Vitamin B12 as a co-factor. Without folic acid, homocysteine levels rise, leading to hyperhomocysteinemia, a known risk factor for cardiovascular disease and neural tube defects. **Analysis of Incorrect Options:** * **B. Biotin (B7):** Acts as a cofactor for **carboxylation** reactions (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). It is not directly involved in sulfur amino acid pathways. * **C. Vitamin K:** Essential for the **gamma-carboxylation** of clotting factors (II, VII, IX, X) and proteins like Osteocalcin. * **D. Pyridoxine (B6):** While B6 *is* involved in the transsulfuration pathway (converting Homocysteine to Cysteine via Cystathionine beta-synthase), the question specifically identifies Folic acid as the primary answer in this context. *Note: In many exam patterns, if both B6 and Folic acid are options, Folic acid is prioritized for its role in the methionine cycle.* **High-Yield Clinical Pearls for NEET-PG:** * **The Methyl Trap:** A deficiency of Vitamin B12 leads to folate being "trapped" as 5-methyl THF, causing a functional folate deficiency and megaloblastic anemia. * **Homocystinuria:** Most commonly due to a deficiency in **Cystathionine beta-synthase (B6 dependent)**, presenting with ectopia lentis, intellectual disability, and thromboembolism. * **FIGLU Test:** Histidine load test used to diagnose Folate deficiency (FIGLU is excreted in urine when folate is low).

Question 227: A 30-year-old female patient complains of irritability and nervousness. She reports a history of consuming oral contraceptives. Laboratory investigations reveal the presence of xanthurenic acid in her urine. This patient may be suffering from a deficiency of:

A. Vitamin C
B. Vitamin B1
C. Vitamin B2
D. Vitamin B6 (Correct Answer)

Explanation: **Explanation:** The correct answer is **Vitamin B6 (Pyridoxine)**. **1. Why Vitamin B6 is correct:** The presence of **xanthurenic acid** in urine is a classic biochemical marker for Vitamin B6 deficiency. Vitamin B6 (as Pyridoxal Phosphate - PLP) is a crucial coenzyme in the **Kynurenine pathway** of Tryptophan metabolism. Specifically, the enzyme **Kynureninase** requires PLP to convert 3-hydroxykynurenine into 3-hydroxyanthranilic acid. When B6 is deficient, this pathway is blocked, leading to the shunting of metabolites toward the formation of xanthurenic acid, which is then excreted in the urine. Furthermore, oral contraceptives are a well-known cause of B6 deficiency as they interfere with its metabolism. **2. Why other options are incorrect:** * **Vitamin C:** Deficiency leads to Scurvy (collagen synthesis defect), characterized by bleeding gums and petechiae, not xanthurenic aciduria. * **Vitamin B1 (Thiamine):** Deficiency causes Beriberi or Wernicke-Korsakoff syndrome. It is a cofactor for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase), not tryptophan metabolism. * **Vitamin B2 (Riboflavin):** Deficiency causes cheilosis, glossitis, and corneal vascularization. While it is involved in the kynurenine pathway (Kynurenine hydroxylase), it does not typically result in the specific excretion of xanthurenic acid. **3. High-Yield Clinical Pearls for NEET-PG:** * **Drug-induced B6 deficiency:** Always look for history of **Isoniazid (INH)**, Penicillamine, or Oral Contraceptive Pills. * **Tryptophan Load Test:** Used to diagnose B6 deficiency by measuring urinary xanthurenic acid after a tryptophan dose. * **Neurological symptoms:** B6 is essential for the synthesis of neurotransmitters (GABA, Serotonin, Dopamine); hence, deficiency presents with irritability or seizures. * **Sideroblastic Anemia:** B6 is a cofactor for **ALA synthase**, the rate-limiting step in heme synthesis.

Question 228: Post-translational modification of hydroxylases is mediated by?

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

Explanation: **Explanation:** The correct answer is **Vitamin C (Ascorbic Acid)**. **Why Vitamin C is correct:** Vitamin C acts 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 during **collagen synthesis**. This modification is essential for the cross-linking of collagen fibers, which provides structural integrity and tensile strength to connective tissues. Mechanistically, Vitamin C maintains the iron cofactor of these enzymes in its active **ferrous (Fe²⁺) state** by acting as a reducing agent. **Why other options are incorrect:** * **Vitamin K:** Involved in the post-translational **gamma-carboxylation** of glutamic acid residues, primarily in clotting factors (II, VII, IX, X) and proteins C and S. It is not involved in hydroxylation. * **Vitamin E:** Functions primarily as a lipid-soluble **antioxidant**, protecting cell membranes from free radical damage (peroxidation). It does not serve as a cofactor for hydroxylases. * **Vitamin D:** Acts as a steroid hormone regulating calcium and phosphate homeostasis. It does not mediate post-translational modifications of enzymes. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to defective collagen synthesis, manifesting as "corkscrew hair," petechiae, splinter hemorrhages, and **bleeding gums**. * **Wound Healing:** Vitamin C is critical for the proliferative phase of wound healing due to its role in collagen formation. * **Other Hydroxylations:** Vitamin C is also a cofactor for **Dopamine β-hydroxylase**, which converts dopamine to norepinephrine.

Question 229: Which of the following is NOT a feature of Riboflavin deficiency?

A. Angular Stomatitis
B. Magenta Coloured tongue
C. Peripheral Neuropathy (Correct Answer)
D. Sore throat

Explanation: **Explanation:** Riboflavin (Vitamin B2) deficiency, known as **Ariboflavinosis**, primarily manifests as lesions of the mucocutaneous surfaces of the mouth and skin. **Why Peripheral Neuropathy is the correct answer:** Peripheral neuropathy is **not** a feature of Riboflavin deficiency. It is a classic manifestation of deficiencies in **Thiamine (B1)** (Dry Beriberi), **Pyridoxine (B6)**, or **Cobalamin (B12)**. Riboflavin deficiency typically lacks severe systemic or neurological involvement, focusing instead on epithelial integrity. **Analysis of Incorrect Options:** * **Angular Stomatitis:** This is a hallmark sign of B2 deficiency, characterized by fissuring and inflammation at the corners of the mouth. * **Magenta Tongue:** Riboflavin deficiency causes glossitis where the tongue takes on a characteristic purplish-red or "magenta" hue due to vascular proliferation and atrophy of the papillae. * **Sore Throat:** Early ariboflavinosis often presents with hyperemia and edema of the pharyngeal mucous membranes, leading to a sore throat and painful swallowing. **High-Yield NEET-PG Pearls:** * **Coenzyme Forms:** Riboflavin is the precursor for **FMN** (Flavin Mononucleotide) and **FAD** (Flavin Adenine Dinucleotide), essential for redox reactions (e.g., Succinate dehydrogenase in TCA cycle). * **Cheilosis:** Vertical fissuring of the lips is another common feature. * **Corneal Neovascularization:** A specific ocular finding in B2 deficiency. * **Diagnostic Test:** Measurement of **Erythrocyte Glutathione Reductase activity** (activity increases upon adding FAD in vitro if the patient is deficient).

Question 230: Which of the following is an uncommon symptom of vitamin A deficiency?

A. Night blindness
B. Xerophthalmia
C. Keratomalacia
D. Polyneuropathy (Correct Answer)

Explanation: **Explanation:** Vitamin A (Retinol) is essential for the maintenance of epithelial integrity, immune function, and the visual cycle. **Polyneuropathy** is the correct answer because it is not a feature of Vitamin A deficiency; rather, it is a classic manifestation of **Vitamin B1 (Thiamine)** deficiency (Dry Beriberi) or **Vitamin B12** deficiency. **Analysis of Options:** * **Night Blindness (Nyctalopia):** This is the earliest clinical symptom. Vitamin A is a precursor to rhodopsin; its deficiency impairs rod function in low light. * **Xerophthalmia:** This refers to the spectrum of ocular manifestations of Vitamin A deficiency, starting with conjunctival xerosis and progressing to **Bitot's spots** (keratin debris on the conjunctiva). * **Keratomalacia:** This is a late-stage manifestation involving liquefactive necrosis and ulceration of the cornea, often leading to permanent blindness. **NEET-PG High-Yield Pearls:** 1. **Sequence of Ocular Signs:** Night blindness → Conjunctival xerosis → Bitot’s spots → Corneal xerosis → Keratomalacia (the most severe stage). 2. **Epithelial Changes:** Vitamin A deficiency leads to **Squamous Metaplasia** and follicular hyperkeratosis (Phrynoderma or "Toad skin"). 3. **WHO Classification:** Bitot’s spots are classified as **X1B**, while Keratomalacia is **X3**. 4. **Toxicity:** Acute Vitamin A toxicity can cause **Pseudotumor cerebri** (idiopathic intracranial hypertension).

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

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