Vitamins and Coenzymes — MCQs
On this page
Which of the following is the poorest source of vitamin C?
Which of the following vitamins may have an anti-infective action?
A young man from a poor socioeconomic family presented with symmetric distal peripheral neuropathy involving both sensory and motor systems along with features of congestive heart failure. Investigations revealed thiamine deficiency. All of the following are predisposing factors for thiamine deficiency, EXCEPT?
Which vitamin deficiency causes excretion of xanthurenic acid?
Which of the following factors are activated by vitamin K?
Thiamine pyrophosphate is a coenzyme required for which of the following enzymatic reactions?
25-dihydroxycholecalciferol is converted to 1,25-dihydroxycholecalciferol in which organ?
Which water-soluble vitamin is synthesized in the body?
Beta-alanine is formed by which of the following precursors?
An environmentalist attempting to live in a desolate forest for 6 months experienced health issues including bleeding gums, teeth falling out, and red spots on the thighs and legs, necessitating the termination of the experiment. This individual's symptoms suggest an inability to properly synthesize which of the following proteins?
Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs
Question 241: Which of the following is the poorest source of vitamin C?
- A. Milk (Correct Answer)
- B. Cabbage
- C. Guava
- D. Radish
Explanation: **Explanation:** Vitamin C (Ascorbic acid) is a water-soluble vitamin primarily found in fresh citrus fruits and vegetables. The correct answer is **Milk**, as it is notoriously deficient in Vitamin C and Iron. **1. Why Milk is the correct answer:** Animal-derived products, particularly bovine milk, contain negligible amounts of Vitamin C. Furthermore, any small amount present is often destroyed during the pasteurization or boiling process due to the vitamin's heat-labile nature. In pediatric medicine, infants fed exclusively on unfortified cow's milk are at a high risk of developing infantile scurvy. **2. Analysis of Incorrect Options:** * **Guava:** This is one of the richest sources of Vitamin C, containing significantly higher concentrations than even citrus fruits like oranges. * **Cabbage and Radish:** These are cruciferous and root vegetables, respectively. All fresh green leafy vegetables and tubers contain substantial amounts of ascorbic acid, making them good dietary sources. **3. High-Yield Clinical Pearls for NEET-PG:** * **Richest Source:** The richest natural source of Vitamin C is the **Amla (Indian Gooseberry)**, followed by Guava. * **Biological Function:** It acts as a coenzyme for the **hydroxylation of proline and lysine** residues during collagen synthesis. * **Clinical Deficiency:** Scurvy presents with "corkscrew hair," petechiae, and **swollen, bleeding gums**. * **Iron Absorption:** Vitamin C enhances the absorption of **non-heme iron** by reducing it from the ferric ($Fe^{3+}$) to the ferrous ($Fe^{2+}$) state in the stomach.
Question 242: Which of the following vitamins may have an anti-infective action?
- A. Vitamin K
- B. Vitamin D
- C. Vitamin A (Correct Answer)
- D. Vitamin E
Explanation: **Explanation:** **Vitamin A (Retinol)** is famously known as the **"Anti-infective Vitamin"** because it plays a critical role in maintaining the structural and functional integrity of epithelial tissues (skin and mucous membranes). These tissues serve as the body's first line of defense against pathogens. Vitamin A deficiency leads to squamous metaplasia and keratinization of the respiratory, gastrointestinal, and urinary tracts, which compromises the mucosal barrier and increases susceptibility to infections (especially measles and respiratory tract infections). Additionally, Vitamin A is essential for the optimal functioning of neutrophils, macrophages, and natural killer cells. **Why the other options are incorrect:** * **Vitamin K:** Primarily involved in the post-translational modification (gamma-carboxylation) of clotting factors II, VII, IX, and X. Its deficiency leads to bleeding diathesis, not increased infection risk. * **Vitamin D:** While it has immunomodulatory effects, its primary role is calcium and phosphate homeostasis. It is not classically labeled as the "anti-infective" vitamin in standard biochemical nomenclature. * **Vitamin E:** Acts as a potent lipid-soluble antioxidant, protecting cell membranes from free radical damage. While it supports the immune system, it lacks the specific barrier-maintenance role of Vitamin A. **High-Yield Clinical Pearls for NEET-PG:** * **WHO Recommendation:** High-dose Vitamin A supplementation is recommended for children with **Measles** to reduce mortality and complications like blindness. * **Epithelial Integrity:** Vitamin A is required for the synthesis of **glycoproteins** (like mucin), which keep membranes moist and resistant to bacterial invasion. * **Retinoic Acid:** This metabolite of Vitamin A acts like a hormone, binding to nuclear receptors (RAR/RXR) to regulate gene expression for cell differentiation.
Question 243: A young man from a poor socioeconomic family presented with symmetric distal peripheral neuropathy involving both sensory and motor systems along with features of congestive heart failure. Investigations revealed thiamine deficiency. All of the following are predisposing factors for thiamine deficiency, EXCEPT?
- A. Chronic diarrhea
- B. Chronic alcoholism
- C. Homocysteinemia (Correct Answer)
- D. Food faddism
Explanation: **Explanation:** The clinical presentation described—symmetric peripheral neuropathy (Dry Beriberi) combined with congestive heart failure (Wet Beriberi)—is classic for **Thiamine (Vitamin B1) deficiency**. Thiamine is a water-soluble vitamin with limited body stores (lasting only 2–3 weeks), making its levels highly dependent on continuous dietary intake and intestinal absorption. **Why Homocysteinemia is the Correct Answer:** **Homocysteinemia** (elevated homocysteine levels) is typically associated with deficiencies of **Vitamin B12 (Cobalamin), Vitamin B9 (Folate), or Vitamin B6 (Pyridoxine)**, as these are essential cofactors in the methionine-homocysteine metabolic cycle. Thiamine is not involved in this pathway; therefore, homocysteinemia is neither a cause nor a direct consequence of thiamine deficiency. **Analysis of Other Options:** * **Chronic Diarrhea:** Leads to malabsorption and increased loss of water-soluble vitamins from the GI tract. * **Chronic Alcoholism:** The most common cause in developed nations. Alcohol interferes with thiamine absorption (inhibits the ThHT-1/2 transporters) and impairs its conversion to the active form, Thiamine Pyrophosphate (TPP). * **Food Faddism:** Diets high in polished rice (lacks the husk containing thiamine) or those involving "fads" that exclude whole grains and legumes lead to inadequate intake. **High-Yield Clinical Pearls for NEET-PG:** * **Key Enzymes requiring TPP:** Pyruvate Dehydrogenase, $\alpha$-Ketoglutarate Dehydrogenase, and **Transketolase** (used for diagnosis via the Erythrocyte Transketolase Activation assay). * **Wernicke-Korsakoff Syndrome:** A neuropsychiatric complication seen in alcoholics (Triad: Ataxia, Ophthalmoplegia, Confusion). * **Rule of Thumb:** Always administer Thiamine **before** Glucose in malnourished patients to prevent precipitating acute Wernicke encephalopathy.
Question 244: Which vitamin deficiency causes excretion of xanthurenic acid?
- A. Folic acid
- B. Pyridoxine (Correct Answer)
- C. Niacin
- D. Vitamin B12
Explanation: **Explanation:** The correct answer is **Pyridoxine (Vitamin B6)**. **1. Why Pyridoxine is correct:** The amino acid **Tryptophan** is metabolized via the **Kynurenine pathway** to produce Niacin. A key enzyme in this pathway is **Kynureninase**, which requires **Pyridoxal Phosphate (PLP)**—the active form of Vitamin B6—as a cofactor. * In Vitamin B6 deficiency, Kynureninase activity is impaired. * This leads to the accumulation of metabolic intermediates, specifically **3-hydroxykynurenine**. * This intermediate is alternatively shunted to form **Xanthurenic acid**, which is then excreted in the urine. * *Clinical Note:* The "Tryptophan Load Test" was historically used to diagnose B6 deficiency by measuring urinary xanthurenic acid levels. **2. Why other options are incorrect:** * **Folic acid (B9):** Deficiency leads to megaloblastic anemia and increased excretion of **FIGLU** (Formiminoglutamic acid) after a histidine load. * **Niacin (B3):** While Tryptophan is a precursor to Niacin, a deficiency of Niacin itself (Pellagra) does not cause xanthurenic acid excretion; rather, B6 deficiency can *lead* to Niacin deficiency. * **Vitamin B12:** Deficiency causes megaloblastic anemia and subacute combined degeneration of the spinal cord. It is characterized by the excretion of **Methylmalonic acid (MMA)** in the urine. **High-Yield NEET-PG Pearls:** * **Isoniazid (INH):** This anti-tubercular drug inhibits pyridoxine, leading to peripheral neuropathy and xanthurenic aciduria. Always co-administer B6 with INH. * **B6-dependent enzymes:** Transaminases (ALT/AST), ALA synthase (heme synthesis), and DOPA decarboxylase. * **Sideroblastic Anemia:** Can be a manifestation of B6 deficiency due to impaired heme synthesis.
Question 245: Which of the following factors are activated by vitamin K?
- A. Factor II
- B. Protein C
- C. Protein S
- D. All of the above (Correct Answer)
Explanation: **Explanation:** 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** of specific glutamate residues into gamma-carboxyglutamate (Gla) residues on certain clotting factors. This carboxylation allows these proteins to bind calcium ions ($Ca^{2+}$), which is necessary for their attachment to phospholipid membranes during the coagulation cascade. The Vitamin K-dependent proteins include: * **Procoagulant Factors:** Factor II (Prothrombin), VII, IX, and X. * **Anticoagulant Proteins:** Protein C and Protein S. **Analysis of Options:** * **Option A (Factor II):** Correct. Prothrombin requires gamma-carboxylation to be converted into its active form, thrombin. * **Option B & C (Protein C & S):** Correct. Although these are natural anticoagulants (they inactivate Factors Va and VIIIa), they are synthesized in the liver via the same vitamin K-dependent mechanism. * **Option D (All of the above):** Since all listed factors require Vitamin K for functional activation, this is the correct choice. **High-Yield NEET-PG Pearls:** 1. **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K and thus inhibiting the activation of these factors. 2. **Early Effect of Warfarin:** Protein C has the shortest half-life among these factors. Its rapid depletion leads to a transient hypercoagulable state, which is why "heparin bridging" is required. 3. **Newborn Prophylaxis:** Neonates are born with sterile guts and low Vitamin K stores; they receive a prophylactic IM injection of Vitamin K to prevent **Hemorrhagic Disease of the Newborn (VKDB)**. 4. **Mnemonic:** Remember **"1972, C and S"** (Factors 10, 9, 7, 2, Protein C, and Protein S).
Question 246: Thiamine pyrophosphate is a coenzyme required for which of the following enzymatic reactions?
- A. Branched-chain amino acid dehydrogenase complex
- B. 2-hydroxy phytanoyl-CoA lyase
- C. Transketolase
- D. Transamination (Correct Answer)
Explanation: **Explanation** The question asks for the reaction that **requires** Thiamine Pyrophosphate (TPP). However, there is a discrepancy in the provided key: **Transamination (Option D) does NOT require TPP; it requires Pyridoxal Phosphate (Vitamin B6).** In clinical biochemistry, **Thiamine (Vitamin B1)** in its active form, TPP, serves as a coenzyme for enzymes involved in oxidative decarboxylation and the pentose phosphate pathway. **Why the other options are TPP-dependent:** * **Transketolase (Option C):** A key enzyme in the HMP shunt that requires TPP to transfer two-carbon units. Measuring erythrocyte transketolase activity is the gold standard for diagnosing Thiamine deficiency. * **Branched-chain α-ketoacid dehydrogenase (Option A):** Required for the metabolism of Leucine, Isoleucine, and Valine. Deficiency of this enzyme leads to Maple Syrup Urine Disease (MSUD). * **2-hydroxy phytanoyl-CoA lyase (Option B):** Involved in the alpha-oxidation of fatty acids (specifically Phytanic acid) in peroxisomes. * **Pyruvate Dehydrogenase & α-ketoglutarate Dehydrogenase:** Other critical TPP-dependent complexes. **Why Transamination (Option D) is the outlier:** Transamination is the transfer of an amino group from an amino acid to a ketoacid. This process is catalyzed by aminotransferases (e.g., ALT, AST), which strictly require **Pyridoxal Phosphate (PLP/Vitamin B6)** as a cofactor, not TPP. **High-Yield NEET-PG Pearls:** 1. **TPP-Dependent Enzymes Mnemonic:** "**T**he **P**arty **P**eople **A**re **B**ack" (**T**ransketolase, **P**yruvate Dehydrogenase, **α**-ketoglutarate Dehydrogenase, **B**ranched-chain ketoacid Dehydrogenase). 2. **Clinical Correlation:** Thiamine deficiency leads to **Beriberi** (Dry/Wet) and **Wernicke-Korsakoff Syndrome**. 3. **The "Glucose Rule":** Always administer Thiamine before Glucose in malnourished/alcoholic patients to prevent precipitating Wernicke encephalopathy, as glucose oxidation consumes remaining TPP stores.
Question 247: 25-dihydroxycholecalciferol is converted to 1,25-dihydroxycholecalciferol in which organ?
- A. Skin
- B. Lung
- C. Liver
- D. Kidney (Correct Answer)
Explanation: ### Explanation The synthesis of active Vitamin D (Calcitriol) is a multi-step process involving the skin, liver, and kidneys. **Why Kidney is the Correct Answer:** The final and rate-limiting step of Vitamin D activation occurs in the **proximal convoluted tubules of the kidney**. Here, the enzyme **1-α-hydroxylase** adds a hydroxyl group to the carbon-1 position of 25-hydroxycholecalciferol (Calcidiol) to form **1,25-dihydroxycholecalciferol (Calcitriol)**, which is the most potent biological form of the vitamin. This enzyme is strictly regulated by Parathyroid Hormone (PTH), low serum calcium, and low phosphate levels. **Why Other Options are Incorrect:** * **Skin:** This is where the synthesis begins. Under the influence of UV-B light, **7-dehydrocholesterol** is converted into Cholecalciferol (Vitamin D3). * **Liver:** This is the site of the first hydroxylation. The enzyme **25-hydroxylase** converts Vitamin D3 into 25-hydroxycholecalciferol (the major storage form). * **Lung:** The lungs are not involved in the physiological activation of Vitamin D, though certain granulomatous diseases (like Sarcoidosis) can show ectopic 1-α-hydroxylase activity in alveolar macrophages. **High-Yield Clinical Pearls for NEET-PG:** * **Storage Form:** 25-hydroxycholecalciferol (Calcidiol) has the longest half-life and is the form measured to assess a patient's Vitamin D status. * **Rate-limiting Enzyme:** 1-α-hydroxylase (Kidney). * **Chronic Kidney Disease (CKD):** Patients with CKD fail to activate Vitamin D, leading to secondary hyperparathyroidism and renal osteodystrophy. * **Inhibitor:** High levels of Calcitriol exert negative feedback by inducing the enzyme **24-hydroxylase**, which converts Vitamin D into inactive metabolites.
Question 248: Which water-soluble vitamin is synthesized in the body?
- A. Pantothenic acid (Correct Answer)
- B. Vitamin B complex
- C. Vitamin C
- D. Vitamin D
Explanation: **Explanation:** The correct answer is **Pantothenic acid (Vitamin B5)**. While vitamins are traditionally defined as essential micronutrients that cannot be synthesized by the body, Pantothenic acid is a unique exception in human physiology. It is synthesized by the **normal bacterial flora (microbiota) in the colon**. Although dietary intake is the primary source, this endogenous production contributes to the body's pool, making clinical deficiency of Vitamin B5 extremely rare. **Analysis of Options:** * **Vitamin B complex (Option B):** This is a group of vitamins. While some (like B12 or Biotin) are produced by gut bacteria, others (like B1/Thiamine) are not synthesized in significant amounts. Pantothenic acid is the most specific answer among the B-complex group often cited in this context. * **Vitamin C (Option C):** Humans lack the enzyme **L-gulonolactone oxidase**, which is essential for the final step of Vitamin C synthesis from glucose. Therefore, it is strictly essential in the diet. * **Vitamin D (Option D):** While Vitamin D is synthesized in the skin via UV light, it is a **fat-soluble** vitamin, not water-soluble. The question specifically asks for a water-soluble vitamin. **NEET-PG High-Yield Pearls:** * **Active Form:** Pantothenic acid is a precursor for **Coenzyme A (CoA)** and **Acyl Carrier Protein (ACP)**, essential for the TCA cycle and fatty acid synthesis. * **Deficiency:** Rare, but classically associated with **"Burning Foot Syndrome"** (Gopalan’s syndrome). * **Other Synthesized Vitamins:** Vitamin K (fat-soluble) and Biotin (B7) are also synthesized by intestinal bacteria. Niacin (B3) can be synthesized from the amino acid **Tryptophan** (60mg Tryptophan = 1mg Niacin).
Question 249: Beta-alanine is formed by which of the following precursors?
- A. Biotin
- B. Pyridoxal phosphate
- C. Pantothenic acid (Correct Answer)
- D. Folic acid
Explanation: **Explanation:** **Correct Option: C. Pantothenic acid** Pantothenic acid (Vitamin B5) is chemically composed of **pantoic acid** and **beta-alanine** joined by a peptide bond. In the body, pantothenic acid serves as the essential precursor for the synthesis of **Coenzyme A (CoA)** and the **Acyl Carrier Protein (ACP)**. While most amino acids in the body are alpha-amino acids, beta-alanine is a unique structural component of this vitamin. It is also found in the dipeptides carnosine and anserine, primarily located in muscle tissue. **Analysis of Incorrect Options:** * **A. Biotin (B7):** Acts as a coenzyme for carboxylation reactions (e.g., Pyruvate carboxylase). It consists of an imidazole ring fused with a thiophene ring and a valeric acid side chain. * **B. Pyridoxal Phosphate (B6):** The active form of Vitamin B6, essential for transamination, decarboxylation, and deamination. While it is involved in amino acid metabolism, it does not contain beta-alanine. * **D. Folic Acid (B9):** Composed of Pteridine ring, PABA (Para-aminobenzoic acid), and Glutamic acid. It functions in one-carbon metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Coenzyme A Function:** Essential for the TCA cycle (Acetyl-CoA), fatty acid synthesis, and cholesterol synthesis. * **Deficiency:** Pantothenic acid deficiency is rare but classically presents as **"Burning Feet Syndrome"** (Gopalan’s syndrome). * **Biochemical Precursor:** Beta-alanine is also a breakdown product of **uracil** (pyrimidine catabolism). * **Key Association:** Remember the "4'-phosphopantetheine" moiety in the Fatty Acid Synthase complex, which is derived from pantothenic acid.
Question 250: An environmentalist attempting to live in a desolate forest for 6 months experienced health issues including bleeding gums, teeth falling out, and red spots on the thighs and legs, necessitating the termination of the experiment. This individual's symptoms suggest an inability to properly synthesize which of the following proteins?
- A. Myoglobin
- B. Hemoglobin
- C. Collagen (Correct Answer)
- D. Insulin
Explanation: **Explanation:** The clinical presentation of bleeding gums, loose teeth (weakness of periodontal ligaments), and petechiae/ecchymoses (red spots) on the lower limbs is a classic description of **Scurvy**, caused by a deficiency of **Vitamin C (Ascorbic acid)**. **Why Collagen is the correct answer:** Vitamin C is 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 pre-procollagen. Hydroxyproline is essential for stabilizing the collagen triple helix via hydrogen bonding. In the absence of Vitamin C, non-hydroxylated collagen chains are unstable and easily degraded, leading to defective connective tissue and capillary fragility, which manifests as the symptoms described. **Why the other options are incorrect:** * **Myoglobin & Hemoglobin:** While Vitamin C aids in iron absorption (reducing $Fe^{3+}$ to $Fe^{2+}$), these proteins do not require Vitamin C-dependent hydroxylation for their structural synthesis. Deficiency would lead to anemia, not the structural connective tissue failure seen here. * **Insulin:** Insulin is a peptide hormone synthesized via standard ribosomal translation and proteolytic processing in the Golgi apparatus; its synthesis is independent of Vitamin C. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy Triad:** Anemia, Myalgia, and Dermatological signs (Perifollicular hemorrhages, Corkscrew hair). * **Osteoid Matrix:** Vitamin C is also needed for bone matrix formation; deficiency in children leads to **Barlow’s disease**. * **Key Enzyme:** Prolyl hydroxylase contains **Iron ($Fe^{2+}$)**; Vitamin C keeps the iron in its reduced state to maintain enzyme activity.
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