Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 631: Thiamine is a coenzyme for all the following enzymes except?

A. alpha-ketoglutarate dehydrogenase
B. Pyruvate dehydrogenase
C. Succinate dehydrogenase (Correct Answer)
D. Transketolase

Explanation: **Explanation:** The correct answer is **Succinate dehydrogenase** because it is a member of the TCA cycle that utilizes **FAD (Vitamin B2)** as its primary coenzyme, not Thiamine. **Understanding the Concept:** Thiamine (Vitamin B1) is converted into its active form, **Thiamine Pyrophosphate (TPP)**. TPP acts as a crucial coenzyme for enzymes involved in oxidative decarboxylation and the Pentose Phosphate Pathway. A simple mnemonic to remember TPP-dependent enzymes is **"ATP"**: **A**lpha-ketoglutarate dehydrogenase, **T**ransketolase, and **P**yruvate dehydrogenase (plus Branched-chain alpha-ketoacid dehydrogenase). **Analysis of Options:** * **A. Alpha-ketoglutarate dehydrogenase:** This TCA cycle enzyme requires five cofactors: TPP, CoA, NAD, FAD, and Lipoic acid. * **B. Pyruvate dehydrogenase:** This multi-enzyme complex converts Pyruvate to Acetyl-CoA and requires the same five cofactors as alpha-ketoglutarate dehydrogenase, including TPP. * **D. Transketolase:** This is a key enzyme in the HMP Shunt (Pentose Phosphate Pathway). It requires TPP to transfer two-carbon units. Measuring erythrocyte transketolase activity is the gold standard for diagnosing Thiamine deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Wernicke-Korsakoff Syndrome:** Classic triad of ophthalmoplegia, ataxia, and confusion due to Thiamine deficiency, often seen in chronic alcoholics. * **Beriberi:** "Dry" (peripheral neuropathy) vs. "Wet" (high-output heart failure/edema). * **Maple Syrup Urine Disease (MSUD):** Defect in Branched-chain alpha-ketoacid dehydrogenase (TPP-dependent). Some variants respond to high-dose Thiamine supplementation. * **Biochemical Marker:** In Thiamine deficiency, pyruvate and lactate levels rise because the PDH complex cannot function.

Question 632: Deficiency of Vitamin D causes which of the following conditions?

A. Scurvy
B. Rickets (Correct Answer)
C. Phrynoderma
D. Beriberi

Explanation: **Explanation:** **Vitamin D (Calciferol)** plays a critical role in calcium and phosphorus homeostasis. It facilitates the intestinal absorption of calcium and maintains adequate serum concentrations to allow normal mineralization of the bone matrix. **Why Rickets is Correct:** In children, a deficiency of Vitamin D leads to **Rickets**. This condition is characterized by a failure of osteoid mineralization at the growth plates. Clinical features include "bow-legs" (genu varum), "knock-knees" (genu valgum), rachitic rosary (prominent costochondral junctions), and delayed closure of fontanelles. In adults, the same deficiency causes **Osteomalacia**, where newly formed bone matrix fails to mineralize. **Analysis of Incorrect Options:** * **A. Scurvy:** Caused by **Vitamin C (Ascorbic acid)** deficiency. It leads to defective collagen synthesis, resulting in bleeding gums, petechiae, and impaired wound healing. * **C. Phrynoderma:** Also known as "toad skin," this follicular hyperkeratosis is primarily associated with **Vitamin A** deficiency (and sometimes essential fatty acids). * **D. Beriberi:** Caused by **Vitamin B1 (Thiamine)** deficiency. It presents as Dry Beriberi (polyneuropathy) or Wet Beriberi (high-output heart failure). **High-Yield Clinical Pearls for NEET-PG:** * **Active Form:** 1,25-dihydroxycholecalciferol (Calcitriol). * **Rate-limiting Enzyme:** 1-alpha-hydroxylase (located in the proximal convoluted tubule of the kidney). * **Biochemical Markers:** Deficiency typically shows **low serum Calcium**, **low Phosphorus**, and **elevated Alkaline Phosphatase (ALP)** due to increased osteoblastic activity. * **Radiological Sign:** "Cupping and splaying" of the metaphyses is a classic sign of Rickets.

Question 633: Which vitamin acts on intranuclear receptors?

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

Explanation: **Explanation:** **Vitamin D** is the correct answer because it functions more like a steroid hormone than a traditional cofactor. The active form of Vitamin D, **1,25-dihydroxycholecalciferol (Calcitriol)**, is lipophilic, allowing it to cross the cell membrane and enter the nucleus. Once inside, it binds to the **Vitamin D Receptor (VDR)**. This ligand-receptor complex then heterodimerizes with the Retinoid X Receptor (RXR) and binds to specific DNA sequences known as Vitamin D Response Elements (VDREs), thereby regulating the transcription of genes involved in calcium and phosphate homeostasis (e.g., Calbindin). **Analysis of Incorrect Options:** * **Vitamin K:** Acts as a coenzyme for **gamma-glutamyl carboxylase**, an enzyme located in the endoplasmic reticulum responsible for the post-translational modification of clotting factors II, VII, IX, and X. * **Vitamin E:** Functions primarily as a lipid-soluble **antioxidant** within cell membranes, protecting them from lipid peroxidation by scavenging free radicals. * **Vitamin B1 (Thiamine):** Acts as a water-soluble coenzyme (Thiamine Pyrophosphate - TPP) for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase) in the mitochondria and cytosol. **High-Yield Clinical Pearls for NEET-PG:** * **Intranuclear Receptors:** Remember the mnemonic **"VET-S"** for vitamins/hormones acting on nuclear receptors: **V**itamin D, **E**strogen, **T**hyroid hormone (T3/T4), and **S**teroids (Glucocorticoids/Retinoic Acid). * **Vitamin A** (Retinoic acid) also acts via nuclear receptors (RAR/RXR). * Vitamin D deficiency leads to **Rickets** in children (defective mineralization of osteoid) and **Osteomalacia** in adults (demineralization of previously formed bone).

Question 634: Cobalt is a constituent of which vitamin?

A. Retinol
B. Pyridoxine
C. Cyanocobalamin (Correct Answer)
D. Ascorbic acid

Explanation: **Explanation:** **Cyanocobalamin (Vitamin B12)** is the correct answer because it is the only vitamin that contains a metal ion as an integral part of its structure. The name itself is derived from its components: **"Cobal"** refers to the central **Cobalt** atom, which is held within a tetrapyrrole **Corrin ring** (similar to the porphyrin ring in hemoglobin, but with a cobalt ion instead of iron). **Analysis of Incorrect Options:** * **Retinol (Vitamin A):** A fat-soluble vitamin derived from beta-carotene. It consists of a beta-ionone ring with an isoprenoid side chain; it contains no metal ions. * **Pyridoxine (Vitamin B6):** A water-soluble vitamin that acts as a precursor to Pyridoxal Phosphate (PLP). Its structure is based on a pyridine ring. * **Ascorbic acid (Vitamin C):** A six-carbon sugar derivative (lactone) that functions as a powerful antioxidant and reducing agent; it does not contain cobalt. **High-Yield Clinical Pearls for NEET-PG:** * **Absorption:** Vitamin B12 requires **Intrinsic Factor (IF)** secreted by gastric parietal cells for absorption in the **terminal ileum**. * **Storage:** Unlike other water-soluble vitamins, B12 is stored in the **liver** for 3–5 years. * **Deficiency:** Leads to **Megaloblastic Anemia** and neurological symptoms (Subacute Combined Degeneration of the spinal cord) due to defective myelin synthesis. * **Biochemical Role:** It is a coenzyme for two critical reactions: 1. **Methionine Synthase:** Conversion of Homocysteine to Methionine (links B12 to Folate metabolism). 2. **Methylmalonyl-CoA Mutase:** Conversion of Methylmalonyl-CoA to Succinyl-CoA.

Question 635: Which derivative of a fat-soluble vitamin functions like a hormone?

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

Explanation: **Explanation:** **Vitamin D** is the correct answer because its active form, **Calcitriol (1,25-dihydroxycholecalciferol)**, functions as a steroid hormone rather than a traditional enzyme cofactor. Like other steroid hormones, calcitriol is synthesized in one organ (kidneys), transported via the blood, and acts on distant target tissues (intestine, bone, kidneys). It binds to specific intracellular **Vitamin D Receptors (VDR)**, which then act as transcription factors in the nucleus to regulate gene expression (e.g., inducing the synthesis of Calbindin for calcium absorption). **Analysis of Incorrect Options:** * **Vitamin A:** While its derivative, Retinoic Acid, also acts via nuclear receptors to influence gene expression (differentiation), Vitamin D is the classic textbook example of a "hormone" because it strictly regulates systemic mineral homeostasis (Calcium/Phosphate) via a feedback loop involving the Parathyroid Hormone (PTH). * **Vitamin E (Tocopherol):** Functions primarily as a potent lipid-soluble **antioxidant**, protecting cell membranes from lipid peroxidation. It does not have hormonal signaling properties. * **Vitamin K:** Acts as a coenzyme for the **gamma-carboxylation** of glutamate residues in clotting factors (II, VII, IX, X) and bone proteins (Osteocalcin). **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The conversion of 25-OH-D3 to 1,25-(OH)2-D3 by the enzyme **1-alpha-hydroxylase** in the proximal renal tubules. * **Activation:** This enzyme is stimulated by **PTH** and low serum phosphate, and inhibited by high calcitriol (negative feedback). * **Target:** The primary action is increasing intestinal calcium and phosphorus absorption.

Question 636: Biotin may be used for the treatment of which of the following conditions?

A. Maple syrup urine disease
B. Methylmalonic acidemia
C. Phenylketonuria
D. Multiple carboxylase deficiency (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Multiple carboxylase deficiency** **Mechanism:** Biotin (Vitamin B7) serves as a vital coenzyme for four key carboxylase enzymes in humans: **Pyruvate carboxylase**, **Acetyl-CoA carboxylase**, **Propionyl-CoA carboxylase**, and **3-Methylcrotonyl-CoA carboxylase**. Multiple carboxylase deficiency (MCD) occurs due to defects in **holocarboxylase synthetase** (which attaches biotin to these enzymes) or **biotinidase** (which recycles biotin). This leads to a systemic failure of all four enzymes, resulting in metabolic acidosis, skin rashes, and neurological symptoms. High-dose biotin supplementation is the definitive treatment as it bypasses the recycling defect or compensates for reduced enzyme affinity. **Why other options are incorrect:** * **A. Maple Syrup Urine Disease (MSUD):** Caused by a deficiency of the Branched-chain alpha-keto acid dehydrogenase complex. The required cofactor is **Thiamine (B1)**, not Biotin. * **B. Methylmalonic acidemia:** Most commonly results from a deficiency of methylmalonyl-CoA mutase. The essential cofactor for this enzyme is **Adenosylcobalamin (Vitamin B12)**. * **C. Phenylketonuria (PKU):** Caused by a deficiency of phenylalanine hydroxylase. The required cofactor is **Tetrahydrobiopterin (BH4)**. **High-Yield Clinical Pearls for NEET-PG:** * **Biotin "ABC" Rule:** Biotin is required for enzymes involved in **A**TP, **B**iotin, and **C**O2 (Carboxylation) reactions. * **Egg White Injury:** Consumption of raw egg whites can lead to biotin deficiency because the protein **avidin** binds biotin with high affinity, preventing its absorption. * **Mnemonic for Biotin-dependent enzymes:** "**P**lease **A**ccept **P**ropionate **M**ethylcrotonyl" (Pyruvate, Acetyl-CoA, Propionyl-CoA, and 3-Methylcrotonyl-CoA carboxylases).

Question 637: Which food source is richest in vitamin C?

A. Guava (Correct Answer)
B. Lime
C. Orange
D. Tomato

Explanation: **Explanation:** Vitamin C (Ascorbic acid) is a water-soluble vitamin essential for collagen synthesis and antioxidant defense. While all the options listed are sources of Vitamin C, **Guava** is the richest source among them. **1. Why Guava is correct:** In nutritional biochemistry, Guava (*Psidium guajava*) is considered a "superfood" for Vitamin C. It contains approximately **222 mg per 100g**, which is nearly four times higher than the concentration found in citrus fruits like oranges. **2. Why the other options are incorrect:** * **Orange:** Often mistakenly thought to be the highest source, oranges contain about **53 mg per 100g**. While a great source, it is significantly lower than guava. * **Lime:** Contains approximately **30 mg per 100g**. It is less potent than both oranges and guavas. * **Tomato:** Contains roughly **14 mg per 100g**. While it contributes to daily intake, it is a relatively poor source compared to tropical fruits. **High-Yield NEET-PG Clinical Pearls:** * **Richest overall source:** While Guava is the richest among these options, **Amla (Indian Gooseberry)** is the richest natural source overall (~600-700 mg/100g), followed by Barbados cherry. * **Biochemical Function:** Vitamin C is a co-factor for **prolyl and lysyl hydroxylase**, essential for the hydroxylation of proline and lysine residues during collagen synthesis. * **Clinical Deficiency:** Deficiency leads to **Scurvy**, characterized by "corkscrew hair," petechiae, and bleeding gums due to defective collagen in capillary walls. * **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.

Question 638: All are features of Vitamin D intoxication, except?

A. Nausea and vomiting
B. Muscular weakness
C. Anorexia
D. Oliguria (Correct Answer)

Explanation: **Explanation:** Vitamin D intoxication (Hypervitaminosis D) is primarily characterized by **Hypercalcemia**. When serum calcium levels rise excessively, it leads to multisystemic manifestations. **Why Oliguria is the correct answer (The "Except"):** In Vitamin D toxicity, the kidneys attempt to excrete the excess calcium load. Hypercalcemia impairs the concentrating ability of the distal nephron by antagonizing the action of ADH (Antidiuretic Hormone), leading to **Nephrogenic Diabetes Insipidus**. This results in **Polyuria** (increased urine output) and compensatory polydipsia, rather than Oliguria (decreased urine output). Chronic toxicity can eventually lead to nephrocalcinosis and renal failure, but the hallmark initial renal sign is polyuria. **Analysis of Incorrect Options:** * **A, B, and C (Nausea, Vomiting, Muscular Weakness, Anorexia):** These are classic symptoms of hypercalcemia. High calcium levels decrease neuromuscular excitability (leading to weakness and hypotonia) and affect the gastrointestinal smooth muscle and CNS, causing loss of appetite (anorexia), nausea, and vomiting. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Vitamin D toxicity is usually due to excessive intake of Vitamin D3 supplements, leading to increased intestinal calcium absorption and bone resorption. * **Mnemonic for Hypercalcemia:** "Stones (renal calculi), Bones (pain), Groans (abdominal pain/vomiting), and Psychic Moans (depression/confusion)." * **Diagnosis:** Elevated 25-hydroxyvitamin D [25(OH)D] levels >150 ng/mL. * **Treatment:** Discontinuation of Vitamin D/Calcium, aggressive hydration with normal saline, and administration of loop diuretics (Furosemide) to enhance calcium excretion. Bisphosphonates or steroids may be used in severe cases.

Question 639: Which of the following acts as a coenzyme in carboxylation reactions?

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

Explanation: **Explanation:** **Biotin (Vitamin B7)** is the essential coenzyme for all **carboxylation reactions** (addition of $CO_2$). It acts as a carrier of activated carbon dioxide. The mechanism involves the covalent attachment of Biotin to the enzyme via a lysine residue (forming a **biocytin** residue). Biotin-dependent enzymes typically require ATP and $Mg^{2+}$. **Analysis of Options:** * **Biotin (Correct):** Key enzymes include **Pyruvate Carboxylase** (gluconeogenesis), **Acetyl-CoA Carboxylase** (fatty acid synthesis), and **Propionyl-CoA Carboxylase** (metabolism of odd-chain fatty acids). * **Riboflavin (B2):** Acts as a precursor for **FMN and FAD**, which are involved in oxidation-reduction (redox) reactions (e.g., Succinate dehydrogenase). * **Niacin (B3):** Acts as a precursor for **NAD and NADP**, primarily involved in electron transfer in redox reactions. * **Pantothenic acid (B5):** A structural component of **Coenzyme A (CoA)** and Acyl Carrier Protein (ACP), essential for acyl group transfer. **High-Yield Clinical Pearls for NEET-PG:** * **ABC Enzymes:** Remember that most carboxylases require **A**TP, **B**iotin, and **C**O2. * **Egg White Injury:** Consumption of raw egg whites can lead to biotin deficiency because **Avidin** (a protein in raw egg whites) binds biotin with high affinity, preventing its absorption. * **Holocarboxylase Synthetase Deficiency:** A rare genetic disorder where biotin cannot be attached to enzymes, leading to multiple carboxylase deficiency.

Question 640: Which vitamin deficiency in alcoholics inhibits pyruvate dehydrogenase?

A. Thiamin (Correct Answer)
B. Niacin
C. Riboflavin
D. Ascorbic acid

Explanation: **Explanation:** The correct answer is **Thiamin (Vitamin B1)**. **Why Thiamin is Correct:** The Pyruvate Dehydrogenase (PDH) complex is a multi-enzyme system that converts pyruvate to acetyl-CoA, linking glycolysis to the TCA cycle. It requires five cofactors: **Thiamin pyrophosphate (TPP)**, FAD, NAD+, CoA, and Lipoic acid. Thiamin (B1) is the precursor for TPP (the prosthetic group of the E1 subunit). In chronic alcoholics, thiamin deficiency is common due to poor dietary intake, impaired intestinal absorption, and decreased hepatic storage. Without TPP, the PDH complex is inhibited, leading to an accumulation of pyruvate, which is then shunted to lactate. This biochemical block is a primary driver of the neurological and cardiac manifestations seen in Wernicke-Korsakoff syndrome and Beriberi. **Why Incorrect Options are Wrong:** * **Niacin (B3):** While NAD+ (derived from Niacin) is a cofactor for PDH, its deficiency primarily causes Pellagra (Dermatitis, Diarrhea, Dementia, Death) and is not the classic "limiting" deficiency triggered by alcoholism in the context of PDH inhibition. * **Riboflavin (B2):** FAD is a cofactor for the E2 subunit of PDH, but Riboflavin deficiency (Ariboflavinosis) typically presents with cheilosis and glossitis rather than acute metabolic inhibition of PDH. * **Ascorbic acid (Vit C):** Vitamin C is involved in collagen synthesis and antioxidant defense; it plays no role as a cofactor in the PDH complex. **High-Yield Clinical Pearls for NEET-PG:** * **The "Big Four" TPP-dependent enzymes:** Pyruvate Dehydrogenase, $\alpha$-Ketoglutarate Dehydrogenase, Branched-chain $\alpha$-ketoacid Dehydrogenase, and Transketolase. * **Clinical Rule:** Always administer Thiamin *before* Glucose in an alcoholic patient. Giving glucose first can precipitate Wernicke encephalopathy by consuming the last remaining TPP stores during glycolysis/PDH activity. * **Diagnostic Marker:** Erythrocyte transketolase activity is used to assess thiamin status.

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

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Folate and Vitamin B12 in One-Carbon Metabolism

Practice Questions

Vitamin C and Collagen Synthesis

Practice Questions

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