Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 441: All of the following statements are true regarding Vitamin A deficiency except?

A. Growth retardation is common
B. Frequent infections can occur
C. Hydrocephalus is infrequent
D. Anterior segment of the eye is initially involved (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Anterior segment of the eye is initially involved** **Why Option D is the "Except" (Incorrect Statement):** In Vitamin A deficiency, the **posterior segment** of the eye (the retina) is involved **initially**, not the anterior segment. The earliest clinical symptom is **Nyctalopia (Night Blindness)**, which results from the failure to regenerate **Rhodopsin** (visual purple) in the retinal rods. The anterior segment (conjunctiva and cornea) is involved later in the progression of the disease, leading to Xerophthalmia (Bitot’s spots, corneal xerosis, and keratomalacia). **Analysis of Other Options:** * **A. Growth retardation:** Vitamin A (Retinoic acid) acts like a hormone, regulating gene expression essential for skeletal growth and cell differentiation. Deficiency leads to stunted bone growth. * **B. Frequent infections:** Vitamin A is known as the **"Anti-infective vitamin."** It maintains epithelial integrity and enhances immune function. Deficiency causes squamous metaplasia of mucosal surfaces (respiratory and GI tracts), leading to recurrent infections. * **C. Hydrocephalus:** While rare, Vitamin A deficiency can lead to an increase in cerebrospinal fluid (CSF) pressure due to impaired resorption of CSF by arachnoid villi and altered skull bone growth, potentially causing secondary hydrocephalus. Therefore, saying it is "infrequent" is a true statement. **High-Yield Clinical Pearls for NEET-PG:** * **Earliest Symptom:** Night Blindness (Nyctalopia). * **Earliest Sign:** Conjunctival Xerosis. * **WHO Classification (Xerophthalmia):** * **X1A:** Conjunctival xerosis * **X1B:** Bitot’s spots (foamy patches on the bulbar conjunctiva) * **X2:** Corneal xerosis * **X3A/B:** Keratomalacia (liquefactive necrosis of the cornea; a medical emergency). * **Storage:** Vitamin A is stored in the **Ito cells** (Stellate cells) of the liver as Retinyl palmitate.

Question 442: Oxidation of lactate to pyruvate requires which vitamin?

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

Explanation: **Explanation:** The conversion of **Lactate to Pyruvate** is catalyzed by the enzyme **Lactate Dehydrogenase (LDH)**. This reaction is a critical step in anaerobic glycolysis and gluconeogenesis. **1. Why Niacin (Vitamin B3) is correct:** Niacin is the precursor for the coenzymes **NAD+ (Nicotinamide Adenine Dinucleotide)** and NADP+. In the oxidation of lactate to pyruvate, NAD+ acts as the electron acceptor (oxidizing agent), being reduced to NADH + H⁺. Dehydrogenase enzymes that involve the transfer of a hydride ion (H⁻) typically require NAD+ or NADP+ as a coenzyme. **2. Why the other options are incorrect:** * **Riboflavin (B2):** Precursor for FAD and FMN. While FAD is also involved in redox reactions (e.g., Succinate Dehydrogenase), it is generally involved in reactions forming double bonds (C=C) rather than the oxidation of alcohols to ketones. * **Folic Acid (B9):** Functions in **one-carbon metabolism** (transfer of methyl, formyl groups). It is essential for DNA synthesis and amino acid metabolism, not simple redox reactions of organic acids. * **Biotin (B7):** Acts as a coenzyme for **carboxylation reactions** (e.g., Pyruvate Carboxylase). It facilitates the addition of CO₂ and requires ATP. **High-Yield Clinical Pearls for NEET-PG:** * **LDH Isoenzymes:** LDH has 5 isoenzymes. LDH-1 (Heart) and LDH-5 (Liver/Muscle) are clinically significant biomarkers for myocardial infarction and liver injury, respectively. * **Niacin Deficiency:** Leads to **Pellagra**, characterized by the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Hartnup Disease:** A defect in tryptophan transport that can lead to niacin deficiency because tryptophan is a precursor for NAD synthesis.

Question 443: Pyridoxine hypervitaminosis causes which of the following?

A. Polyneuropathy (Correct Answer)
B. Hypercalcemia
C. Liver damage
D. Renal failure

Explanation: **Explanation:** **Pyridoxine (Vitamin B6)** is unique among water-soluble vitamins because, unlike most others that are easily excreted in the urine, chronic high-dose intake can lead to significant toxicity. **Why Polyneuropathy is correct:** Hypervitaminosis B6 (typically occurring at doses >200 mg/day over long periods) leads to **sensory polyneuropathy**. The underlying mechanism involves the accumulation of pyridoxine in the dorsal root ganglia, leading to neuronal degeneration. Patients typically present with progressive sensory loss, paresthesia, and ataxia (sensory ataxia), while motor strength usually remains intact. This is a high-yield "paradox" because while B6 deficiency causes neuropathy, its excess does as well. **Why the other options are incorrect:** * **Hypercalcemia:** This is characteristic of **Vitamin D toxicity**, where excessive calcium absorption from the gut and bone resorption occur. * **Liver damage:** While associated with high doses of **Vitamin B3 (Niacin)**—specifically hepatotoxicity and flushing—it is not a primary feature of Pyridoxine toxicity. * **Renal failure:** This is not a direct complication of Pyridoxine excess. However, Vitamin C (Ascorbic acid) megadoses can lead to oxalate kidney stones, potentially impacting renal function. **High-Yield Clinical Pearls for NEET-PG:** * **The B6 Paradox:** Both deficiency and toxicity of Vitamin B6 cause peripheral neuropathy. * **Isoniazid (INH) Link:** INH therapy for TB induces B6 deficiency by inhibiting pyridoxine phosphokinase; always co-administer B6 to prevent "stocking and glove" neuropathy. * **Sideroblastic Anemia:** B6 is a cofactor for ALA synthase; its deficiency leads to microcytic hypochromic anemia with ringed sideroblasts. * **Therapeutic Use:** B6 is used to treat homocystinuria (cystathionine synthase deficiency) and PMS symptoms.

Question 444: Vitamin A is stored mainly as retinol esters in which organ?

A. Kidney
B. Muscle
C. Liver (Correct Answer)
D. Retina

Explanation: **Explanation:** **1. Why Liver is the Correct Answer:** The liver is the primary storage site for Vitamin A, containing approximately 80–90% of the body's total reserves. Dietary Vitamin A (retinol) is absorbed in the intestine, packaged into chylomicrons, and transported to the liver. Within the liver, it is esterified with long-chain fatty acids (primarily palmitic acid) to form **retinyl esters**. These esters are stored specifically in the **Perisinusoidal Stellate Cells (Ito cells)**. When the body requires Vitamin A, these esters are hydrolyzed back to retinol and released into the blood bound to **Retinol Binding Protein (RBP)**. **2. Why Other Options are Incorrect:** * **Kidney:** While the kidney is involved in the excretion of some vitamin metabolites and the synthesis of RBP, it does not serve as a significant storage reservoir for retinyl esters. * **Muscle:** Adipose tissue stores a small fraction of Vitamin A, but skeletal muscle is not a specialized storage site for this fat-soluble vitamin. * **Retina:** Although the retina is the functional site where Vitamin A (as retinal) is used for the visual cycle (Rhodopsin), it does not store the vitamin in bulk. It receives a constant supply from the liver. **3. High-Yield Clinical Pearls for NEET-PG:** * **Storage Form:** Retinyl Palmitate (most common ester). * **Storage Cell:** Ito cells (Stellate cells) of the liver. * **Transport Protein:** Retinol Binding Protein (RBP) in plasma; Transthyretin (TTR) prevents RBP from being filtered by the kidney. * **Toxicity:** Excessive intake leads to Hypervitaminosis A, causing hepatotoxicity and increased intracranial pressure (Pseudotumor cerebri). * **Deficiency:** The earliest sign is Nyctalopia (Night blindness); the most specific sign is Bitot’s spots.

Question 445: Thiamine is essential for which enzyme?

A. Isocitrate dehydrogenase
B. Succinate dehydrogenase
C. Pyruvate dehydrogenase (Correct Answer)
D. Acetyl CoA synthetase

Explanation: **Explanation:** Thiamine (Vitamin B1) is converted into its active form, **Thiamine Pyrophosphate (TPP)**. TPP acts as a vital coenzyme for enzymes involved in oxidative decarboxylation and the pentose phosphate pathway. **Why Pyruvate Dehydrogenase (PDH) is correct:** The PDH complex facilitates the conversion of Pyruvate to Acetyl-CoA. It requires five cofactors: **T**hiamine (TPP), **R**iboflavin (FAD), **N**iacin (NAD+), **P**antothenic acid (CoA), and **L**ipoic acid (Mnemonic: **T**ender **R**oving **N**ights **P**lease **L**ove). TPP specifically mediates the decarboxylation step of pyruvate. **Analysis of Incorrect Options:** * **Isocitrate Dehydrogenase:** This is the rate-limiting enzyme of the TCA cycle. It requires **NAD+** as a cofactor, not thiamine. * **Succinate Dehydrogenase:** This enzyme converts succinate to fumarate in the TCA cycle and also acts as Complex II of the Electron Transport Chain. It requires **FAD** (derived from Vitamin B2). * **Acetyl CoA Synthetase:** This enzyme catalyzes the conversion of acetate and CoA to Acetyl-CoA (outside the TCA cycle). It requires **ATP** and **Magnesium**, but not thiamine. **High-Yield Clinical Pearls for NEET-PG:** * **Other TPP-dependent enzymes:** $\alpha$-ketoglutarate dehydrogenase (TCA cycle), Branched-chain $\alpha$-ketoacid dehydrogenase (BCAA metabolism), and Transketolase (HMP Shunt). * **Clinical Correlation:** Thiamine deficiency leads to **Beriberi** (Dry/Wet) and **Wernicke-Korsakoff syndrome**. * **Biochemical Marker:** Erythrocyte transketolase activity is used to diagnose thiamine deficiency. * **Warning:** Always administer thiamine *before* glucose in malnourished/alcoholic patients to prevent precipitating Wernicke encephalopathy, as glucose oxidation consumes the remaining thiamine stores via the PDH reaction.

Question 446: Deficiency of folic acid leads to which type of anemia?

A. Pernicious anemia
B. Megaloblastic anemia (Correct Answer)
C. Iron deficiency anemia
D. Thalassemia

Explanation: ### Explanation **Correct Answer: B. Megaloblastic anemia** **Mechanism:** Folic acid (Vitamin B9) is essential for the synthesis of **dTMP (deoxythymidine monophosphate)** from dUMP. This process is a rate-limiting step in DNA synthesis. When folate is deficient, there is a failure of DNA replication and nuclear maturation. However, RNA synthesis and cytoplasmic growth continue at a normal rate. This **nucleo-cytoplasmic asynchrony** leads to the formation of large, immature red blood cell precursors in the bone marrow called **megaloblasts**, resulting in megaloblastic anemia. **Analysis of Incorrect Options:** * **A. Pernicious anemia:** This is a specific type of megaloblastic anemia caused by an autoimmune destruction of gastric parietal cells, leading to a deficiency of **Intrinsic Factor**, which is required for **Vitamin B12** absorption. * **C. Iron deficiency anemia:** This results in **microcytic hypochromic anemia** due to impaired hemoglobin synthesis, not DNA synthesis. * **D. Thalassemia:** This is a genetic quantitative defect in the synthesis of globin chains, also resulting in **microcytic hypochromic anemia** with characteristic target cells. **High-Yield Clinical Pearls for NEET-PG:** * **FIGLU Excretion Test:** Formiminoglutamic acid (FIGLU) is an intermediate in histidine metabolism. In folate deficiency, FIGLU cannot be converted to glutamate and is excreted in the urine (a diagnostic marker). * **The Folate Trap:** A deficiency of Vitamin B12 can lead to a functional folate deficiency because folate becomes "trapped" as **N5-methyl THF**, which cannot be converted back to the active forms needed for DNA synthesis. * **Peripheral Smear:** Look for **hypersegmented neutrophils** (more than 5 lobes), which is one of the earliest signs of megaloblastic anemia. * **Neural Tube Defects (NTDs):** Maternal folate deficiency during the first trimester is a major risk factor for NTDs like spina bifida.

Question 447: Which of the following is TRUE regarding vitamin B3, except?

A. Tryptophan is a precursor
B. Milk is a rich source (Correct Answer)
C. Pellagra results due to deficiency
D. Not excreted in urine

Explanation: **Explanation:** Vitamin B3 (Niacin) exists in two forms: nicotinic acid and nicotinamide. The question asks for the **false** statement regarding B3. **1. Why "Milk is a rich source" is the correct (False) answer:** While milk contains small amounts of Niacin, it is considered a **poor source** of the preformed vitamin. However, milk is rich in **Tryptophan**, which can be converted into Niacin in the body. For NEET-PG, remember that the primary dietary sources of Niacin are liver, yeast, whole grains, and legumes. **2. Analysis of other options:** * **Tryptophan is a precursor:** This is **True**. Approximately **60 mg of Tryptophan** is required to synthesize **1 mg of Niacin**. This process requires Vitamin B6 (Pyridoxine) as a cofactor. * **Pellagra results due to deficiency:** This is **True**. Niacin deficiency leads to Pellagra, characterized by the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Not excreted in urine:** This is **True**. Unlike most water-soluble vitamins, Niacin is not excreted as the free vitamin. It is metabolized in the liver and excreted as metabolites like **N-methylnicotinamide**. **Clinical Pearls for NEET-PG:** * **Hartnup Disease:** A genetic disorder of tryptophan transport that leads to pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause Niacin deficiency because tryptophan is diverted to synthesize excessive Serotonin. * **Corn/Maize Diets:** Maize is deficient in Tryptophan and contains Niacin in a bound, unavailable form (**Niacytin**), leading to Pellagra in populations where corn is a staple. * **Therapeutic use:** High-dose Niacin is used to treat Type IIb hyperlipoproteinemia as it inhibits lipolysis.

Question 448: Administration of which of the following can prevent neural tube defects in a newborn?

A. Folic acid (Correct Answer)
B. Vitamin B12
C. Pyridoxine
D. Vitamin C

Explanation: **Explanation:** **Correct Answer: A. Folic Acid** Folic acid (Vitamin B9) is essential for the synthesis of DNA and RNA. It acts as a carrier of one-carbon units, which are crucial for the conversion of homocysteine to methionine and the production of purines and thymidyne. During the first 28 days of pregnancy, the neural tube closes. Rapid cell division during this period requires high levels of folate. A deficiency leads to impaired DNA synthesis and failure of neural tube closure, resulting in **Neural Tube Defects (NTDs)** like spina bifida and anencephaly. **Why other options are incorrect:** * **Vitamin B12 (Cobalamin):** While B12 is a co-factor for methionine synthase (working alongside folate), its primary clinical association is with megaloblastic anemia and subacute combined degeneration of the spinal cord, not specifically the prevention of NTDs. * **Pyridoxine (B6):** This is a co-enzyme for transamination and decarboxylation reactions. It is used clinically to treat morning sickness and prevent peripheral neuropathy in patients taking Isoniazid (INH). * **Vitamin C (Ascorbic Acid):** Essential for collagen synthesis (hydroxylation of proline/lysine). Deficiency leads to Scurvy, characterized by capillary fragility and poor wound healing. **NEET-PG High-Yield Pearls:** * **Prophylactic Dose:** 400 mcg (0.4 mg) daily for all women of childbearing age, starting at least 1 month before conception. * **High-Risk Dose:** 4 mg daily for women with a previous history of a child with NTD or those on anti-epileptic drugs (e.g., Valproate). * **Biochemical Marker:** Elevated **Alpha-fetoprotein (AFP)** in maternal serum or amniotic fluid is a screening marker for NTDs. * **Enzyme Link:** Folate deficiency causes an increase in **FIGLU** (Formiminoglutamic acid) excretion in urine.

Question 449: Which vitamins have anticarcinogenic properties?

A. Vitamin A, Vitamin E, Vitamin D (Correct Answer)
B. Vitamin K, Vitamin E, Vitamin D
C. Vitamin K, Vitamin E, Vitamin A
D. All of the above

Explanation: **Explanation:** The anticarcinogenic properties of vitamins are primarily attributed to their roles in **antioxidant defense, cell differentiation, and immune modulation.** 1. **Vitamin A (Retinoids & Carotenoids):** Retinoids act as transcription factors that regulate cell growth and differentiation. They prevent malignant transformation by maintaining epithelial integrity and inducing apoptosis in precancerous cells. Beta-carotene also acts as a potent antioxidant, neutralizing free radicals that cause DNA damage. 2. **Vitamin E (Tocopherols):** This is the most powerful lipid-soluble antioxidant. It protects cell membranes from lipid peroxidation and prevents the formation of nitrosamines (potent carcinogens) in the stomach. 3. **Vitamin D (Calcitriol):** Beyond calcium homeostasis, Vitamin D has significant "non-calcemic" effects. It inhibits angiogenesis (blood vessel formation in tumors), promotes cell differentiation, and reduces the proliferation of cancer cells (especially in colon, breast, and prostate cancers). **Analysis of Options:** * **Option A is correct** because it includes the three primary vitamins with proven anti-neoplastic mechanisms. * **Options B and C are incorrect** because they include **Vitamin K**. While Vitamin K is essential for coagulation (gamma-carboxylation of factors II, VII, IX, X), it does not have established systemic anticarcinogenic properties compared to A, D, and E. * **Option D is incorrect** as Vitamin K is not considered a primary anticarcinogenic vitamin. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin C** is also anticarcinogenic as it scavenges free radicals in the aqueous phase. * **Lycopene** (a carotenoid found in tomatoes) is specifically linked to a reduced risk of **Prostate Cancer**. * **Retinoic Acid** is used clinically in the treatment of **Acute Promyelocytic Leukemia (APML)** to induce cell differentiation.

Question 450: Which of the following statements regarding vitamin A is TRUE?

A. Vitamin A promotes maintenance of epithelial tissue. (Correct Answer)
B. Vitamin A is necessary for hearing but not for vision.
C. Vitamin A is synthesized in the skin.
D. All vitamin A derivatives are safe to use during pregnancy.

Explanation: **Explanation:** **1. Why Option A is Correct:** Vitamin A (specifically **Retinoic Acid**) acts as a hormone-like signal that regulates gene expression by binding to nuclear receptors (RAR and RXR). It is essential for the **differentiation and maintenance of epithelial tissues**. It promotes the synthesis of mucus-secreting goblet cells and prevents **squamous metaplasia**. In its absence, epithelium undergoes keratinization, leading to conditions like xerophthalmia (eyes) and follicular hyperkeratosis (skin). **2. Why the Other Options are Incorrect:** * **Option B:** This is factually reversed. Vitamin A (as **11-cis-retinal**) is the prosthetic group of rhodopsin, making it indispensable for **vision** (especially night vision). It has no primary role in the physiology of hearing. * **Option C:** Vitamin A is not synthesized in the skin; it is a fat-soluble vitamin obtained from the diet (as preformed Vitamin A from animal sources or provitamin carotenoids from plants). **Vitamin D** is the vitamin synthesized in the skin via UV light. * **Option D:** Vitamin A derivatives (Retinoids like Isotretinoin) are highly **teratogenic**. They can cause severe craniofacial, cardiac, and CNS defects in the fetus. They are strictly contraindicated in pregnancy (Category X). **High-Yield Clinical Pearls for NEET-PG:** * **Earliest Sign of Deficiency:** Nyctalopia (Night blindness). * **Earliest Clinical Sign:** Conjunctival xerosis. * **Bitot’s Spots:** Triangular, pearly-white foamy plaques on the bulbar conjunctiva (pathognomonic). * **Therapeutic Use:** All-trans retinoic acid (ATRA) is used in the treatment of **Acute Promyelocytic Leukemia (M3)**. * **Toxicity:** Chronic hypervitaminosis A 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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