Vitamins and Coenzymes Practice Questions

Q: What is the major function of Vitamin E in the body?

Protection of biological membranes from free radical damage. **Explanation:** **1. Why Option D is correct:** Vitamin E (specifically **α-tocopherol**) is the body’s most potent **lipid-soluble antioxidant**. Its primary role is to protect polyunsaturated fatty acids (PUFAs) in biological membranes and low-density lipoproteins (LDL) from **lipid peroxidation**. It acts as a "chain-breaker" by scavenging free radicals (like superoxide and hydroxyl radicals), donating a hydrogen atom to neutralize them before they can damage the cell membrane. This is particularly vital in red blood cell (RBC) membranes and neurological tissues. **2. Why other options are incorrect:** * **Option A (Energy metabolism):** This is the primary role of **Vitamin B complex** (e.g., Thiamine/B1, Riboflavin/B2, Niacin/B3), which act as coenzymes in the TCA cycle and Electron Transport Chain. * **Option B (Carboxylation):** This is the specific function of **Biotin (B7)** (e.g., Pyruvate carboxylase) and **Vitamin K** (gamma-carboxylation of glutamic acid). * **Option C (Blood clotting):** This is the classic function of **Vitamin K**, which is essential for the post-translational modification of clotting factors II, VII, IX, and X. **3. NEET-PG High-Yield Pearls:** * **Deficiency Manifestations:** Hemolytic anemia (due to RBC membrane fragility), posterior column signs (loss of position and vibration sense), and spinocerebellar ataxia. * **Regeneration:** Vitamin E is regenerated (reduced) back to its active form by **Vitamin C (Ascorbic acid)**. * **Toxicity:** High doses of Vitamin E can interfere with Vitamin K metabolism, leading to an increased risk of bleeding (prolonged PT/INR). * **Storage:** Unlike other fat-soluble vitamins stored in the liver, Vitamin E is primarily stored in **adipose tissue**.

Q: Which of the following demonstrates the functional assessment of Vitamin B1 deficiency?

RBC Transketolase. ### Explanation **Correct Option: A. RBC Transketolase** Vitamin B1 (Thiamine) acts as a vital coenzyme in the form of **Thiamine Pyrophosphate (TPP)**. To assess thiamine status functionally, we measure the activity of the TPP-dependent enzyme **Transketolase** within Red Blood Cells (RBCs). * **The Concept:** In a laboratory setting, RBC transketolase activity is measured before and after the addition of exogenous TPP. An increase in enzyme activity (known as the "TPP effect") of >15–25% indicates a functional deficiency of Vitamin B1. This is considered more reliable than static serum levels because it reflects intracellular metabolic status. **Analysis of Incorrect Options:** * **B. RBC Glutathione Reductase:** This is the functional marker for **Vitamin B2 (Riboflavin)** deficiency. This enzyme requires **FAD** (Flavin Adenine Dinucleotide) as a cofactor. * **C. Serum Thiamine Levels:** While these can be measured, they are "static" markers that reflect recent dietary intake rather than "functional" intracellular stores. They are less sensitive and specific than the transketolase assay. * **D. RBC Glutathione Peroxidase:** This is a functional marker used to assess **Selenium** status. It requires Selenium as a cofactor to protect cells from oxidative damage. **High-Yield Clinical Pearls for NEET-PG:** * **Key TPP-dependent enzymes:** Pyruvate Dehydrogenase (PDH), $\alpha$-Ketoglutarate Dehydrogenase, and Transketolase (HMP Shunt). * **Clinical Triad of Wernicke’s Encephalopathy:** Ophthalmoplegia, Ataxia, and Confusion. * **Korsakoff Psychosis:** Characterized by irreversible confabulation and amnesia (damage to mammillary bodies). * **Management Rule:** Always administer Thiamine **before** Glucose in a malnourished/alcoholic patient to prevent precipitating acute Wernicke’s (as glucose oxidation consumes remaining thiamine).

Q: Vitamin required for the activity of transaminases is:

Pyridoxine. **Explanation:** The correct answer is **Pyridoxine (Vitamin B6)**. **1. Why Pyridoxine is Correct:** Pyridoxine, in its active coenzyme form **Pyridoxal Phosphate (PLP)**, is essential for the metabolism of amino acids. Transamination is the process where an amino group is transferred from an amino acid to a keto-acid, catalyzed by enzymes called **transaminases** (e.g., ALT and AST). PLP acts as a temporary carrier of the amino group, shifting between its aldehyde form (PLP) and its aminated form (Pyridoxamine phosphate) during the reaction. **2. Why Other Options are Incorrect:** * **Folic Acid (B9):** Primarily involved in **one-carbon metabolism** (transfer of methyl, formyl, or methylene groups), essential for DNA synthesis and erythropoiesis. * **Nicotinic Acid (B3):** Functions as NAD/NADP, which are coenzymes for **redox (oxidation-reduction) reactions**, such as those in glycolysis and the TCA cycle. * **Thiamine (B1):** Its active form, TPP, is a cofactor for **oxidative decarboxylation** (e.g., Pyruvate Dehydrogenase) and the **transketolase** reaction in the HMP shunt. **3. High-Yield Clinical Pearls for NEET-PG:** * **PLP-Dependent Reactions:** Besides transamination, PLP is required for **decarboxylation** (e.g., synthesis of GABA, Histamine, Serotonin, Heme) and **cystathionine synthesis**. * **Isoniazid (INH) Link:** INH (anti-TB drug) inhibits pyridoxine kinase, leading to B6 deficiency and **peripheral neuropathy**. Always co-administer B6 with INH. * **Xanthurenic Acid:** Deficiency of B6 leads to increased urinary excretion of xanthurenic acid due to impaired tryptophan metabolism.

Q: Deficiency of which of the following vitamins does not affect tooth development?

Vitamin K. **Explanation:** The correct answer is **Vitamin K**. Tooth development (odontogenesis) relies on the coordinated action of vitamins that regulate epithelial differentiation, mineral homeostasis, and collagen synthesis. Vitamin K is primarily involved in the gamma-carboxylation of clotting factors (II, VII, IX, X) and bone proteins (osteocalcin). While it plays a role in bone mineralization, its deficiency does not have a recognized direct impact on the structural development of teeth. **Why the other options are incorrect:** * **Vitamin A:** Essential for the differentiation of epithelial cells. In the context of teeth, it is required for **ameloblasts** (enamel-forming cells). Deficiency leads to enamel hypoplasia and defective dentin formation. * **Vitamin D:** Crucial for calcium and phosphorus homeostasis. Deficiency during tooth development leads to **delayed eruption**, enamel hypoplasia, and a widened predentin layer (similar to Rickets in bone). * **Vitamin C:** Necessary for the hydroxylation of proline and lysine during **collagen synthesis**. Since dentin is approximately 90% collagen, deficiency (Scurvy) leads to defective dentin formation and atrophy of **odontoblasts**. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin A deficiency:** Causes keratinizing metaplasia of specialized epithelium. * **Vitamin C deficiency:** Leads to "scorbutic teeth," characterized by internal hemorrhages in the pulp and gingival bleeding. * **Vitamin D-resistant Rickets:** Often presents with spontaneous periapical abscesses due to large pulp chambers and thin dentin. * **Vitamin K:** Remember the mnemonic **"1972"** for the clotting factors it activates (Factors II, VII, IX, and X).

Q: Harup disease causes deficiency of?

Nicotinamide. **Explanation:** **Hartnup disease** is an autosomal recessive metabolic disorder characterized by a defect in the **SLC6A19 transporter**. This transporter is responsible for the absorption of neutral amino acids (especially **Tryptophan**) in the small intestine and their reabsorption in the renal proximal tubules. **Why Nicotinamide is the correct answer:** Tryptophan is a vital precursor for the endogenous synthesis of **Niacin (Vitamin B3)**; approximately 60 mg of Tryptophan yields 1 mg of Niacin. In Hartnup disease, the profound malabsorption and renal loss of Tryptophan lead to a secondary deficiency of Niacin/Nicotinamide. This results in **Pellagra-like symptoms**, as Niacin is essential for the formation of NAD+ and NADP+, which are critical coenzymes in redox reactions. **Why other options are incorrect:** * **Thiamine (B1):** Deficiency causes Beriberi or Wernicke-Korsakoff syndrome; its transport is unrelated to the SLC6A19 transporter. * **Riboflavin (B2):** Deficiency causes cheilosis and corneal vascularization; it is not derived from Tryptophan. * **Biotin (B7):** Deficiency is usually associated with raw egg white consumption (avidin) or carboxylase defects, not neutral amino acid transport. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Dermatitis (photosensitive rash/Casal’s necklace), Cerebellar Ataxia, and Aminoaciduria. * **Diagnostic Hallmark:** **Neutral Aminoaciduria** (detected via chromatography). Note that proline, hydroxyproline, and arginine are excreted normally (distinguishing it from Fanconi syndrome). * **Treatment:** High-protein diet and **Nicotinamide** supplementation. * **Blue Diaper Syndrome:** A related condition where bacterial degradation of unabsorbed tryptophan in the gut leads to indicanuria, turning diapers blue.

Q: Which vitamin also acts as a hormone?

Vitamin D. **Explanation:** **Vitamin D** is considered a hormone rather than just a vitamin because it is synthesized endogenously in the skin (via UV light), travels through the bloodstream to distant target organs, and exerts its effects via specific intracellular receptors (**VDR**). The active form, **1,25-dihydroxycholicalciferol (Calcitriol)**, functions like a steroid hormone by binding to nuclear receptors and regulating gene expression to maintain calcium and phosphorus homeostasis. **Analysis of Options:** * **Vitamin A (Retinol):** While its derivative, Retinoic Acid, acts via nuclear receptors to regulate cell differentiation, Vitamin D is the classic answer for this question as it strictly follows the endocrine definition (produced in one site, acting on distant sites like the gut and bone). * **Vitamin B1 (Thiamine):** This is a water-soluble vitamin that acts as a coenzyme (Thiamine Pyrophosphate - TPP) for decarboxylation reactions (e.g., Pyruvate Dehydrogenase). It has no hormonal activity. * **Vitamin C (Ascorbic Acid):** This acts as a water-soluble antioxidant and a co-factor for hydroxylation reactions (e.g., collagen synthesis). It does not function through hormonal signaling pathways. **High-Yield NEET-PG Pearls:** * **Rate-limiting enzyme:** 1-alpha-hydroxylase (located in the proximal convoluted tubule of the kidney). * **Storage form:** 25-hydroxyvitamin D [25(OH)D] is the major circulating form used to clinically assess Vitamin D status. * **Mechanism:** Calcitriol increases the synthesis of **Calbindin** in intestinal cells to facilitate calcium absorption. * **Clinical Correlation:** Deficiency leads to Rickets (children) and Osteomalacia (adults).

Q: In folic acid deficiency, which of the following is excreted in urine?

Formiminoglutamic acid (FIGLU). **Explanation:** The correct answer is **Formiminoglutamic acid (FIGLU)**. **1. Why FIGLU is the correct answer:** The metabolism of the amino acid **Histidine** involves its conversion to Glutamate. A key intermediate in this pathway is FIGLU. Under normal conditions, the enzyme *formiminotransferase* transfers the formimino group from FIGLU to **Tetrahydrofolate (THF)**, producing Glutamate and N5-formimino-THF. In **Folic acid deficiency**, THF is unavailable to accept the formimino group. Consequently, FIGLU accumulates in the body and is excreted in the urine. The **FIGLU Excretion Test** (often performed after a histidine load) is a sensitive functional indicator of folate status. **2. Why the other options are incorrect:** * **Kynurenine & Xanthurenic acid (Options A & B):** These are intermediates in the metabolism of **Tryptophan**. Their excretion in urine is a hallmark of **Vitamin B6 (Pyridoxine) deficiency**, as the enzyme kynureninase requires B6. * **Methylmalonate (Option D):** Methylmalonic acid (MMA) is excreted in the urine in **Vitamin B12 (Cobalamin) deficiency**. B12 is a cofactor for *methylmalonyl-CoA mutase*; its absence leads to MMA accumulation. This is a crucial lab finding to differentiate B12 deficiency from Folate deficiency. **3. Clinical Pearls for NEET-PG:** * **FIGLU Test:** Used for Folate deficiency. * **MMA Test:** Used for B12 deficiency (More specific than homocysteine). * **Homocysteine:** Elevated in **both** B12 and Folate deficiency. * **Schilling Test:** Historically used to determine the cause of B12 malabsorption (e.g., Pernicious anemia). * **Histidinemia:** A rare metabolic disorder where histidine levels are high due to histidase deficiency, but FIGLU is not elevated.

Q: Acyl carrier protein (ACP) occurs in a bound state to a derivative of which vitamin?

Pantothenate. **Explanation:** **Acyl Carrier Protein (ACP)** is a key component of the **Fatty Acid Synthase (FAS) multienzyme complex** involved in de novo lipogenesis. The correct answer is **Pantothenate (Vitamin B5)** because ACP contains a prosthetic group called **4'-phosphopantetheine**. 1. **Why Pantothenate is correct:** Pantothenic acid is the precursor for 4'-phosphopantetheine. In the FAS complex, this derivative is covalently linked to a serine residue of the ACP. It acts as a "swinging arm," carrying the growing fatty acyl chain between the different catalytic sites of the enzyme complex. This is analogous to how Coenzyme A (which also contains pantothenate) carries acyl groups in other metabolic pathways. 2. **Why other options are incorrect:** * **Biotin (B7):** Acts as a coenzyme for **carboxylation** reactions (e.g., Acetyl-CoA carboxylase, the rate-limiting step of fatty acid synthesis). It carries $CO_2$, not acyl groups. * **Folic acid (B9):** Functions in **one-carbon metabolism** (transfer of methyl, formyl groups), essential for DNA synthesis. * **Pyridoxine (B6):** Its active form, Pyridoxal Phosphate (PLP), is the coenzyme for **transamination**, decarboxylation, and heme synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **The "Swinging Arm" trio:** Three enzymes use flexible arms to move substrates: **ACP** (4'-phosphopantetheine), **Pyruvate Dehydrogenase** (Lipoamide), and **Acetyl-CoA Carboxylase** (Biotin). * **FAS Complex:** In humans, it is a homodimer where each monomer has 7 enzyme activities and an ACP domain. * **Vitamin B5 Deficiency:** Extremely rare, but classically associated with **"Burning Feet Syndrome"** (Gopalan’s syndrome).

Q: Why is nicotinamide generally preferred over niacin?

Niacin causes flushing.. **Explanation:** **1. Why Option A is Correct:** Nicotinic acid (Niacin) and Nicotinamide are both forms of Vitamin B3. However, in clinical practice, nicotinamide is preferred for treating deficiency (Pellagra) because **Niacin causes intense cutaneous flushing**, itching, and burning sensations. This occurs because nicotinic acid triggers the release of **Prostaglandins (PGD2 and PGE2)** in the skin, leading to vasodilation. Nicotinamide does not have this vasodilatory effect and is therefore better tolerated by patients. **2. Why Other Options are Incorrect:** * **Option B & C:** While high doses of niacin can occasionally cause GI distress or exacerbate asthma, these are not the primary or most common reasons for preferring nicotinamide. Flushing is the classic, dose-limiting side effect unique to the nicotinic acid form. * **Option D:** Both forms have excellent bioavailability and are readily absorbed from the gastrointestinal tract. The preference is based on the side-effect profile, not pharmacokinetics. **3. NEET-PG High-Yield Pearls:** * **The "Niacin Flush":** Can be minimized by pre-treating with **Aspirin** (which inhibits prostaglandin synthesis) or by taking the drug with meals. * **Lipid Profile:** Only **Niacin** (not nicotinamide) is used to treat hyperlipidemia as it inhibits lipolysis in adipose tissue and reduces VLDL synthesis. * **Pellagra Symptoms:** Remember the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Biochemical Precursor:** Niacin can be synthesized in the body from the amino acid **Tryptophan** (60 mg Tryptophan = 1 mg Niacin). This process requires Vitamin B6 (Pyridoxine) as a cofactor.

Q: Which vitamin is required for glucose-6-phosphate dehydrogenase activity?

Niacin. **Explanation:** The correct answer is **Niacin (Vitamin B3)**. Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting enzyme of the **Pentose Phosphate Pathway (Hexose Monophosphate Shunt)**. This enzyme catalyzes the conversion of glucose-6-phosphate to 6-phosphogluconolactone. During this process, it reduces **NADP+ to NADPH**. Niacin is the essential precursor for the synthesis of nicotinamide adenine dinucleotide phosphate (NADP+). Therefore, without niacin, the coenzyme required for G6PD activity would be unavailable. **Analysis of Incorrect Options:** * **Riboflavin (B2):** Serves as a precursor for FAD and FMN. While it is involved in redox reactions (e.g., Glutathione reductase), it is not the coenzyme for G6PD. * **Thiamine (B1):** Its active form, TPP, is a cofactor for Transketolase in the HMP shunt, but not for G6PD. It is also vital for pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. * **Biotin (B7):** Acts as a coenzyme for carboxylation reactions (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). **High-Yield Clinical Pearls for NEET-PG:** * **NADPH Utility:** The NADPH generated by G6PD is crucial for maintaining the pool of **reduced glutathione**, which protects red blood cells against oxidative stress. * **G6PD Deficiency:** This is the most common enzymopathy worldwide, leading to neonatal jaundice and drug-induced (e.g., Primaquine) or fava bean-induced hemolytic anemia. * **Diagnostic Hallmark:** Look for **Heinz bodies** (denatured hemoglobin) and **Bite cells** on a peripheral smear in G6PD deficiency cases. * **Niacin Deficiency:** Leads to **Pellagra**, characterized by the 3 Ds: Dermatitis, Diarrhea, and Dementia.

Question 1

What is the major function of Vitamin E in the body?

Accepted Answer

Protection of biological membranes from free radical damage

Answer

Regulation of energy metabolism

Answer

Carboxylation reaction

Answer

Blood clotting

Question 2

Which of the following demonstrates the functional assessment of Vitamin B1 deficiency?

Accepted Answer

RBC Transketolase

Answer

RBC Glutathione Reductase

Answer

Serum Thiamine Levels

Answer

RBC Glutathione Peroxidase

Question 3

Vitamin required for the activity of transaminases is:

Accepted Answer

Pyridoxine

Answer

Folic acid

Answer

Nicotinic acid

Answer

Thiamine

Question 4

Deficiency of which of the following vitamins does not affect tooth development?

Accepted Answer

Vitamin K

Answer

Vitamin A

Answer

Vitamin D

Answer

Vitamin C

Question 5

Harup disease causes deficiency of?

Accepted Answer

Nicotinamide

Answer

Thiamine

Answer

Riboflavin

Answer

Biotin

Question 6

Which vitamin also acts as a hormone?

Accepted Answer

Vitamin D

Answer

Vitamin A

Answer

Vitamin B1

Answer

Vitamin C

Question 7

In folic acid deficiency, which of the following is excreted in urine?

Accepted Answer

Formiminoglutamic acid (FIGLU)

Answer

Kynurenine

Answer

Xantheurenic acid

Answer

Methylmalonate

Question 8

Acyl carrier protein (ACP) occurs in a bound state to a derivative of which vitamin?

Accepted Answer

Pantothenate

Answer

Biotin

Answer

Folic acid

Answer

Pyridoxine

Question 9

Why is nicotinamide generally preferred over niacin?

Accepted Answer

Niacin causes flushing.

Answer

Niacin causes abdominal pain.

Answer

Niacin causes breathlessness.

Answer

Niacin has lower bioavailability.

Question 10

Which vitamin is required for glucose-6-phosphate dehydrogenase activity?

Accepted Answer

Niacin

Answer

Riboflavin

Answer

Thiamine

Answer

Biotin

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?