Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 621: Pyridoxal phosphate (PLP) is not a required cofactor for which of the following enzymes?

A. Cystathionine beta-synthase
B. Glutamate decarboxylase
C. Aspartate transaminase
D. Glucose-6-phosphate dehydrogenase (Correct Answer)

Explanation: **Explanation:** The correct answer is **Glucose-6-phosphate dehydrogenase (G6PD)**. **1. Why G6PD is the correct answer:** Glucose-6-phosphate dehydrogenase is the rate-limiting enzyme of the **Pentose Phosphate Pathway (HMP Shunt)**. It catalyzes the oxidation of Glucose-6-phosphate to 6-phosphogluconolactone. This enzyme requires **NADP⁺** as a cofactor (reducing it to NADPH), not Pyridoxal Phosphate (PLP). PLP is primarily involved in amino acid metabolism, whereas G6PD is involved in carbohydrate metabolism and antioxidant defense. **2. Analysis of incorrect options (PLP-dependent enzymes):** * **Cystathionine beta-synthase:** This enzyme converts homocysteine to cystathionine in the transsulfuration pathway. It is a classic PLP-dependent enzyme. Deficiency leads to **Homocystinuria**. * **Glutamate decarboxylase:** PLP is a mandatory cofactor for almost all **decarboxylation** reactions of amino acids. This enzyme converts Glutamate to **GABA** (an inhibitory neurotransmitter). * **Aspartate transaminase (AST):** All **transamination** reactions (transfer of an amino group from an amino acid to a ketoacid) require PLP as a carrier of the amino group. **3. High-Yield Clinical Pearls for NEET-PG:** * **PLP (Vitamin B6)** is involved in: Transamination, Decarboxylation, Deamination, Transsulfuration, and Heme synthesis (ALA synthase). * **Glycogen Phosphorylase** is a unique enzyme that requires PLP for glycogenolysis (the only major carbohydrate enzyme requiring B6). * **Isoniazid (INH)** therapy for TB can induce B6 deficiency by binding to PLP, leading to peripheral neuropathy and sideroblastic anemia. * **G6PD deficiency** is the most common enzymopathy, leading to episodic hemolytic anemia due to the inability to maintain reduced glutathione in RBCs.

Question 622: Erythrocyte transketolase activity is associated with which vitamin?

A. Riboflavin
B. Thiamine (Correct Answer)
C. Folic acid
D. Niacin

Explanation: **Explanation:** **Thiamine (Vitamin B1)** is the correct answer because its active form, **Thiamine Pyrophosphate (TPP)**, serves as an essential coenzyme for the enzyme **Transketolase**. This enzyme is a key component of the Non-oxidative phase of the **Hexose Monophosphate (HMP) Shunt**, facilitating the interconversion of sugars. Measuring **Erythrocyte Transketolase Activity (ETKA)** is the gold-standard functional assay to diagnose Thiamine deficiency; a significant increase in enzyme activity upon adding TPP in vitro indicates a deficiency state. **Analysis of Incorrect Options:** * **Riboflavin (B2):** Associated with **Erythrocyte Glutathione Reductase** activity. Its coenzymes are FMN and FAD. * **Folic Acid (B9):** Involved in one-carbon metabolism and DNA synthesis. Deficiency is typically assessed via serum folate or FIGLU excretion levels. * **Niacin (B3):** Functions as NAD/NADP in redox reactions (e.g., LDH, PDH). Deficiency leads to Pellagra, not alterations in transketolase. **High-Yield Clinical Pearls for NEET-PG:** * **TPP-Dependent Enzymes:** Remember the mnemonic **"ATP"**: **A**lpha-ketoglutarate dehydrogenase, **T**ransketolase, and **P**yruvate dehydrogenase. (Also Branched-chain ketoacid dehydrogenase). * **Wernicke-Korsakoff Syndrome:** Classically seen in alcoholics; characterized by the triad of ataxia, ophthalmoplegia, and confusion. * **Refeeding Syndrome:** Administering glucose before Thiamine in a malnourished patient can precipitate Wernicke’s encephalopathy because glucose oxidation rapidly consumes the remaining TPP stores.

Question 623: Which of the following vitamins is a component of the Electron Transport Chain (ETC)?

A. Vitamin B12
B. Riboflavin (Correct Answer)
C. Nicotinic acid
D. Thiamine

Explanation: **Explanation:** **Riboflavin (Vitamin B2)** is the correct answer because it is the precursor for the coenzymes **FMN (Flavin Mononucleotide)** and **FAD (Flavin Adenine Dinucleotide)**. These coenzymes are integral components of the Electron Transport Chain (ETC): * **Complex I (NADH Dehydrogenase):** Contains FMN as a prosthetic group to accept electrons from NADH. * **Complex II (Succinate Dehydrogenase):** Contains FAD to accept electrons from succinate. **Analysis of Incorrect Options:** * **Vitamin B12 (Cobalamin):** Primarily functions as a coenzyme for methionine synthase and methylmalonyl-CoA mutase. It is not a direct component of the ETC. * **Nicotinic Acid (Vitamin B3):** While it forms **NAD+**, which carries electrons *to* the ETC, NAD+ is considered a mobile electron carrier/substrate rather than a structural "component" of the mitochondrial membrane complexes themselves. (Note: In many competitive exams, if both B2 and B3 are options, B2 is preferred as FMN/FAD are tightly bound prosthetic groups of the complexes). * **Thiamine (Vitamin B1):** Functions as Thiamine Pyrophosphate (TPP), a coenzyme for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase), but has no role in the ETC. **High-Yield Clinical Pearls for NEET-PG:** * **Complex II** is the only enzyme that participates in both the TCA cycle and the ETC. * **Iron-Sulfur (Fe-S) centers** are present in Complexes I, II, and III. * **Cytochrome a3** (in Complex IV) contains copper ($Cu^{2+}$), which is a frequent target for cyanide poisoning. * Riboflavin deficiency is characterized by **cheilosis, glossitis, and corneal vascularization.**

Question 624: A constellation of neuropathy, muscle weakness and wasting, cardiomegaly, edema, ophthalmoplegia, and confabulation strongly suggests which of the following conditions?

A. Hysteria
B. Thiamine deficiency (Correct Answer)
C. Lead poisoning
D. Intracerebral hemorrhage

Explanation: ### Explanation The clinical presentation described is a classic combination of **Wet Beriberi**, **Dry Beriberi**, and **Wernicke-Korsakoff Syndrome**, all of which result from **Thiamine (Vitamin B1) deficiency**. **1. Why Thiamine Deficiency is Correct:** Thiamine pyrophosphate (TPP) is a crucial cofactor for key enzymes in carbohydrate metabolism: *Pyruvate Dehydrogenase*, *alpha-ketoglutarate dehydrogenase*, and *Transketolase*. * **Dry Beriberi:** Presents as symmetrical peripheral neuropathy, muscle wasting, and weakness. * **Wet Beriberi:** Involves high-output heart failure, leading to cardiomegaly and peripheral edema. * **Wernicke-Korsakoff Syndrome:** Characterized by the triad of ophthalmoplegia (extraocular muscle paralysis), ataxia, and confusion. **Confabulation** (making up stories to fill memory gaps) is the hallmark of Korsakoff psychosis, caused by damage to the mammillary bodies. **2. Why the Other Options are Incorrect:** * **Hysteria:** This is a psychological diagnosis (conversion disorder) and would not present with objective clinical findings like cardiomegaly, edema, or ophthalmoplegia. * **Lead Poisoning:** Typically presents with abdominal colic, "lead lines" on gums (Burton lines), wrist/foot drop, and microcytic anemia with basophilic stippling, but not cardiomegaly or confabulation. * **Intracerebral Hemorrhage:** Usually presents acutely with focal neurological deficits, sudden severe headache, and altered consciousness, rather than a chronic constellation of systemic symptoms. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Marker:** Erythrocyte **Transketolase activity** is the gold standard for diagnosing thiamine deficiency. * **The "Glucose Rule":** Always administer thiamine *before* intravenous glucose in malnourished/alcoholic patients to prevent precipitating acute Wernicke encephalopathy. * **Vulnerable Areas:** The **mammillary bodies** are the most characteristic site of brain lesions in Wernicke-Korsakoff syndrome.

Question 625: Which of the following vitamins is not required from the diet?

A. Nicotinic acid
B. Ascorbic acid
C. Vitamin A
D. Vitamin D (Correct Answer)

Explanation: ### Explanation The correct answer is **Vitamin D**. **Why Vitamin D is the correct answer:** Vitamin D is unique because it is technically a pro-hormone that can be synthesized endogenously. In the presence of ultraviolet B (UVB) light, **7-dehydrocholesterol** in the skin is converted to cholecalciferol (Vitamin D3). Because the human body can produce sufficient quantities given adequate sunlight exposure, it does not strictly meet the definition of a "vitamin" (an essential micronutrient that *must* be obtained from the diet). **Analysis of Incorrect Options:** * **Nicotinic Acid (Vitamin B3):** While a small amount can be synthesized from the amino acid Tryptophan (60 mg Tryptophan = 1 mg Niacin), this pathway is inefficient and insufficient to meet daily requirements. Therefore, dietary intake is essential. * **Ascorbic Acid (Vitamin C):** Humans lack the enzyme **L-gulonolactone oxidase**, making us unable to synthesize Vitamin C from glucose. It is a mandatory dietary requirement. * **Vitamin A:** Humans cannot synthesize the steroid nucleus of Vitamin A de novo. It must be ingested either as preformed Vitamin A (retinol) from animal sources or as provitamin carotenoids from plants. **High-Yield NEET-PG Pearls:** * **The "Sunlight Vitamin":** The rate-limiting step of Vitamin D synthesis occurs in the skin, but functional activation requires two hydroxylations: 25-hydroxylation in the **Liver** and 1-alpha-hydroxylation in the **Kidney** (via 1-alpha-hydroxylase). * **Tryptophan Connection:** Deficiency of Tryptophan (as seen in Hartnup disease or Maize-based diets) leads to **Pellagra** due to secondary Niacin deficiency. * **Scurvy:** Vitamin C is a co-factor for prolyl and lysyl hydroxylase, essential for collagen cross-linking. Its absence leads to capillary fragility and poor wound healing.

Question 626: Which vitamin participates as a coenzyme in carboxylation reactions of gluconeogenesis and fatty acid synthesis?

A. Biotin
B. Vitamin B7
C. Anti-egg white injury factor
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. All of the above** because Biotin, Vitamin B7, and Anti-egg white injury factor are all synonymous names for the same water-soluble vitamin. **1. Underlying Medical Concept:** Biotin acts as a vital coenzyme for **carboxylase enzymes**. It functions as a carrier of activated carbon dioxide ($CO_2$). In the context of the question: * **Gluconeogenesis:** Biotin is a cofactor for *Pyruvate Carboxylase*, which converts pyruvate to oxaloacetate. * **Fatty Acid Synthesis:** It is a cofactor for *Acetyl-CoA Carboxylase*, the rate-limiting step that converts Acetyl-CoA to Malonyl-CoA. **2. Analysis of Options:** * **Option A (Biotin):** The standard biochemical name for the cofactor used by carboxylases (ABC enzymes: Acetyl-CoA, Propionyl-CoA, and Pyruvate carboxylase). * **Option B (Vitamin B7):** The numerical designation for Biotin in the B-complex series. * **Option C (Anti-egg white injury factor):** Historically, Biotin was discovered as the factor that cured dermatitis and hair loss caused by consuming raw egg whites. Raw egg whites contain **Avidin**, a protein that binds biotin with high affinity, preventing its absorption. **High-Yield Clinical Pearls for NEET-PG:** * **The "ABC" Rule:** Biotin is required for enzymes involving **A**TP, **B**iotin, and **C**O₂. * **Avidin-Biotin Interaction:** This is the strongest known non-covalent biological bond, often exploited in laboratory techniques like ELISA. * **Clinical Deficiency:** Presents as dermatitis, alopecia, and lactic acidosis (due to failure of pyruvate carboxylase). It is rare except in cases of excessive raw egg white consumption or prolonged parenteral nutrition.

Question 627: Which of the following vitamins is considered a hormonal vitamin?

A. Niacin
B. Pyridoxine
C. Vitamin D (Correct Answer)
D. Riboflavin

Explanation: **Explanation:** **Vitamin D** is uniquely classified as a "hormonal vitamin" because it fulfills the criteria of a hormone rather than a traditional dietary cofactor. Unlike other vitamins, it is synthesized endogenously in the skin (via UV light), transported through the blood to distant target organs (intestines, bones, kidneys), and acts via specific intracellular receptors (**VDR**) to regulate gene expression. Its active form, **1,25-dihydroxycholicalciferol (Calcitriol)**, functions similarly to steroid hormones, maintaining calcium and phosphate homeostasis. **Analysis of Incorrect Options:** * **A. Niacin (B3):** Functions as a precursor to coenzymes **NAD and NADP**, which are essential for redox reactions. While it can be synthesized from Tryptophan, it does not act via hormonal signaling pathways. * **B. Pyridoxine (B6):** Its active form, **Pyridoxal Phosphate (PLP)**, is a vital coenzyme for transamination, decarboxylation, and heme synthesis. * **D. Riboflavin (B2):** A precursor for **FMN and FAD**, which serve as prosthetic groups for various oxidoreductases (e.g., Succinate dehydrogenase). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The conversion of 25(OH)D to 1,25(OH)₂D by **1-alpha-hydroxylase** in the kidney is the most strictly regulated step. * **Storage form:** 25-hydroxyvitamin D [Calcidiol] is the major circulating form used to clinically assess Vitamin D status. * **Vitamin A** is the only other vitamin that acts through a nuclear receptor mechanism (RAR/RXR), but Vitamin D is the classic answer for "hormonal vitamin."

Question 628: What condition is caused by niacin deficiency?

A. Pellagra (Correct Answer)
B. Scurvy
C. Rickets
D. Lathyrism

Explanation: **Explanation:** **Pellagra** is the clinical manifestation of **Niacin (Vitamin B3)** deficiency. Niacin is a precursor to the coenzymes **NAD and NADP**, which are essential for numerous oxidation-reduction reactions in energy metabolism. The deficiency typically occurs due to a diet reliant on maize (which contains bound niacin) or a lack of **Tryptophan**, an amino acid that can be converted into niacin in the body (60 mg Tryptophan = 1 mg Niacin). **Analysis of Options:** * **Pellagra (Correct):** Characterized by the **"4 Ds"**: Dermatitis (photosensitive "Casal’s necklace"), Diarrhea, Dementia, and Death if untreated. * **Scurvy:** Caused by **Vitamin C (Ascorbic acid)** deficiency, leading to defective collagen synthesis, bleeding gums, and petechiae. * **Rickets:** Caused by **Vitamin D** deficiency in children, resulting in impaired mineralization of bone osteoid (e.g., bow legs, rachitic rosary). * **Lathyrism:** A neurological condition (spastic paraplegia) caused by the chronic ingestion of *Lathyrus sativus* (Khesari dal), which contains the neurotoxin **BOAA**. **NEET-PG High-Yield Pearls:** 1. **Hartnup Disease:** An autosomal recessive disorder involving defective transport of neutral amino acids (Tryptophan), leading to pellagra-like symptoms. 2. **Carcinoid Syndrome:** Can cause secondary niacin deficiency because tryptophan is diverted to synthesize excessive **Serotonin** instead of Niacin. 3. **INH (Isoniazid) Therapy:** Used in TB, it inhibits the enzyme Pyridoxal phosphate (B6), which is a required cofactor for converting Tryptophan to Niacin, potentially inducing Pellagra.

Question 629: Vitamin D is not found in sufficient quantities in:

A. Fish fat
B. Egg
C. Halibut liver oil
D. Milk (Correct Answer)

Explanation: **Explanation:** The correct answer is **Milk**. While milk is often perceived as a complete food, it is naturally a **poor source of Vitamin D and Iron**. In many developed countries, milk is "fortified" with Vitamin D, but in its natural state, the concentration is insufficient to meet daily requirements (containing only about 0.1–1.0 IU/mL). **Analysis of Options:** * **Fish fat & Halibut liver oil (Options A & C):** These are among the richest natural sources of Vitamin D3 (Cholecalciferol). Fish liver oils (Cod, Halibut, Shark) are concentrated reservoirs of fat-soluble vitamins (A and D). * **Egg (Option B):** The egg yolk is a significant animal source of Vitamin D. One large egg yolk typically provides approximately 40 IU of Vitamin D. **Clinical Pearls for NEET-PG:** 1. **Synthesis:** Vitamin D is unique because it can be synthesized endogenously in the skin from **7-dehydrocholesterol** via UV-B radiation. 2. **Active Form:** The most active form is **1,25-dihydroxycholecalciferol (Calcitriol)**, synthesized in the kidney by the enzyme **1-alpha-hydroxylase**. 3. **Storage:** Unlike other fat-soluble vitamins stored primarily in the liver, Vitamin D is stored significantly in **adipose tissue**. 4. **Deficiency:** Leads to **Rickets** in children (characterized by craniotabes and rachitic rosary) and **Osteomalacia** in adults (characterized by bone pain and muscle weakness). 5. **Milk Deficiency Rule:** Remember the mnemonic: Milk is deficient in **"Vitamin C and Iron"** (and naturally low in Vitamin D).

Question 630: Glutathione peroxidase, which protects erythrocytes against hemolysis, contains which of the following elements?

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

Explanation: **Explanation:** **Glutathione Peroxidase (GPx)** is a critical antioxidant enzyme that protects cells, particularly erythrocytes, from oxidative damage. It catalyzes the reduction of hydrogen peroxide ($H_2O_2$) and lipid hydroperoxides into water and alcohols, respectively, using reduced glutathione (GSH) as a donor. 1. **Why Selenium is Correct:** Selenium is an essential trace element that is incorporated into the active site of Glutathione Peroxidase as the 21st amino acid, **Selenocysteine**. This "selenoprotein" structure is vital for the enzyme's catalytic activity. Without selenium, GPx cannot neutralize peroxides, leading to oxidative stress and subsequent hemolysis of red blood cells. 2. **Why Other Options are Incorrect:** * **Chromium (A):** Primarily functions as a component of the Glucose Tolerance Factor (GTF), enhancing the action of insulin. * **Manganese (B):** Acts as a cofactor for enzymes like Pyruvate Carboxylase and **Manganese Superoxide Dismutase (Mn-SOD)**, which is found in the mitochondria. * **Zinc (C):** A cofactor for over 300 enzymes, including Carbonic Anhydrase, Alcohol Dehydrogenase, and **Cytoplasmic SOD (Cu-Zn SOD)**. **High-Yield Clinical Pearls for NEET-PG:** * **Keshan Disease:** A cardiomyopathy caused by Selenium deficiency. * **Kashin-Beck Disease:** An osteoarthropathy associated with Selenium deficiency. * **Glutathione Reductase:** Unlike the peroxidase, this enzyme requires **Vitamin B2 (Riboflavin)** as FAD to regenerate reduced glutathione. * **G6PD Deficiency:** Leads to hemolysis because the cell cannot produce NADPH, which is required by Glutathione Reductase to keep Glutathione in its protective reduced state.

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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