Vitamins and Coenzymes Practice Questions

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

Succinate dehydrogenase. **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.

Q: Deficiency of Vitamin D causes which of the following conditions?

Rickets. **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.

Q: Which vitamin acts on intranuclear receptors?

Vitamin D. **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).

Q: Cobalt is a constituent of which vitamin?

Cyanocobalamin. **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.

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

Vitamin D. **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.

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

Multiple carboxylase deficiency. **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).

Q: Which food source is richest in vitamin C?

Guava. **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.

Q: All are features of Vitamin D intoxication, except?

Oliguria. **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.

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

Biotin. **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 1

Thiamine is a coenzyme for all the following enzymes except?

Accepted Answer

Succinate dehydrogenase

Answer

alpha-ketoglutarate dehydrogenase

Answer

Pyruvate dehydrogenase

Answer

Transketolase

Question 2

Deficiency of Vitamin D causes which of the following conditions?

Accepted Answer

Rickets

Answer

Scurvy

Answer

Phrynoderma

Answer

Beriberi

Question 3

Which vitamin acts on intranuclear receptors?

Accepted Answer

Vitamin D

Answer

Vitamin K

Answer

Vitamin E

Answer

Vitamin B1

Question 4

Cobalt is a constituent of which vitamin?

Accepted Answer

Cyanocobalamin

Answer

Retinol

Answer

Pyridoxine

Answer

Ascorbic acid

Question 5

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

Accepted Answer

Vitamin D

Answer

Vitamin A

Answer

Vitamin E

Answer

Vitamin K

Question 6

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

Accepted Answer

Multiple carboxylase deficiency

Answer

Maple syrup urine disease

Answer

Methylmalonic acidemia

Answer

Phenylketonuria

Question 7

Which food source is richest in vitamin C?

Accepted Answer

Guava

Answer

Lime

Answer

Orange

Answer

Tomato

Question 8

All are features of Vitamin D intoxication, except?

Accepted Answer

Oliguria

Answer

Nausea and vomiting

Answer

Muscular weakness

Answer

Anorexia

Question 9

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

Accepted Answer

Biotin

Answer

Riboflavin

Answer

Niacin

Answer

Pantothenic acid

Question 10

Which vitamin deficiency in alcoholics inhibits pyruvate dehydrogenase?

Accepted Answer

Thiamin

Answer

Niacin

Answer

Riboflavin

Answer

Ascorbic acid

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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