Vitamins and Coenzymes — MCQs
On this page
Which amino acid can be converted into a vitamin?
A 30-year-old woman presents with progressive lethargy, constipation, muscle weakness, and increased thirst and urination. She admits taking large doses of vitamin and mineral supplements daily. What is the most likely metabolic abnormality causing these symptoms, and in which clinical condition is it also seen?
Which of the following is a component of Coenzyme A in the TCA cycle?
Which vitamin facilitates iron absorption?
Which of the following metabolic reactions require vitamin B12 but not folate?
Which of the following has the highest amount of vitamin C?
Vitamers represent:
Which vitamin deficiency results in poor wound healing?
Vitamin E deficiency is similar to which of the following conditions?
The gene responsible for folic acid transport is situated on which chromosome?
Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs
Question 611: Which amino acid can be converted into a vitamin?
- A. Glycine
- B. Tryptophan (Correct Answer)
- C. Phenylalanine
- D. Lysine
Explanation: **Explanation:** The correct answer is **Tryptophan**, which serves as a precursor for the synthesis of **Niacin (Vitamin B3)**. **1. Why Tryptophan is Correct:** Tryptophan is an essential amino acid that undergoes the **Kynurenine pathway** to produce Nicotinic acid mononucleotide, which is eventually converted into NAD+ and NADP+. In humans, approximately **60 mg of Tryptophan is required to synthesize 1 mg of Niacin**. This conversion requires Vitamin B6 (Pyridoxine) as a cofactor for the enzyme kynureninase. **2. Why Other Options are Incorrect:** * **Glycine:** While glycine is a precursor for Heme, Creatine, and Glutathione, it does not synthesize any vitamin. * **Phenylalanine:** This is a precursor for Tyrosine, which further leads to the synthesis of Catecholamines (Dopamine, Epinephrine), Melanin, and Thyroid hormones, but not vitamins. * **Lysine:** Lysine is an essential amino acid primarily involved in protein synthesis and the production of Carnitine (a vitamin-like compound, but not a vitamin itself). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Pellagra Connection:** A deficiency in Tryptophan or a block in its conversion can lead to Pellagra (characterized by the 4 Ds: Dermatitis, Diarrhea, Dementia, and Death). * **Hartnup Disease:** An autosomal recessive disorder involving defective transport of neutral amino acids (including Tryptophan) in the gut and kidneys, leading to Pellagra-like symptoms. * **Carcinoid Syndrome:** In this condition, Tryptophan is diverted toward the synthesis of Serotonin, leading to a secondary Niacin deficiency. * **Cofactor Requirement:** Iron, Riboflavin (B2), and Pyridoxine (B6) are essential for the conversion of Tryptophan to Niacin.
Question 612: A 30-year-old woman presents with progressive lethargy, constipation, muscle weakness, and increased thirst and urination. She admits taking large doses of vitamin and mineral supplements daily. What is the most likely metabolic abnormality causing these symptoms, and in which clinical condition is it also seen?
- A. Hypercalcemia (Correct Answer)
- B. Hypocalcemia
- C. Hyperkalemia
- D. None of the above
Explanation: **Explanation:** The patient is presenting with the classic "moans, stones, groans, and psychiatric overtones" of **Hypercalcemia**, specifically caused by **Vitamin D toxicity**. **1. Why Hypercalcemia is Correct:** Vitamin D (Calcitriol) increases intestinal calcium absorption and bone resorption. Excessive intake leads to hypercalcemia, which manifests as: * **Renal:** Polyuria and polydipsia (due to nephrogenic diabetes insipidus). * **Gastrointestinal:** Constipation and nausea. * **Neuromuscular:** Muscle weakness and lethargy. * **Clinical Correlation:** This presentation is also seen in **Sarcoidosis**, where macrophages produce 1-alpha-hydroxylase, leading to endogenous overproduction of active Vitamin D. **2. Why Incorrect Options are Wrong:** * **Hypocalcemia:** This would present with increased neuromuscular excitability, such as tetany, Chvostek’s sign, Trousseau’s sign, and seizures, rather than constipation and polyuria. * **Hyperkalemia:** Typically presents with cardiac arrhythmias (tall peaked T-waves) and muscle paralysis, but does not cause the polyuria-polydipsia-constipation triad associated with vitamin overdose. **3. NEET-PG High-Yield Pearls:** * **Vitamin D Toxicity:** The most common cause of hypervitaminosis. It leads to metastatic calcification (calcium deposits in soft tissues like kidneys and arteries). * **Diagnosis:** Look for elevated 25-hydroxyvitamin D levels and hypercalciuria. * **Treatment:** Immediate cessation of Vitamin D/Calcium supplements, aggressive hydration with normal saline, and potentially corticosteroids or bisphosphonates. * **Key Association:** Always link Vitamin D-mediated hypercalcemia to **Granulomatous diseases** (Sarcoidosis, Tuberculosis).
Question 613: Which of the following is a component of Coenzyme A in the TCA cycle?
- A. Thiamine
- B. Riboflavin
- C. Pantothenic acid (Correct Answer)
- D. Nicotinic acid
Explanation: **Explanation:** **1. Why Pantothenic Acid is Correct:** Pantothenic acid (Vitamin B5) is the essential precursor for the synthesis of **Coenzyme A (CoA)**. Structurally, CoA is composed of adenosine 3',5'-bisphosphate, **pantothenic acid**, and beta-mercaptoethylamine. In the TCA cycle, CoA is vital for the formation of **Acetyl-CoA** and **Succinyl-CoA**. Its primary functional role is to act as a carrier of acyl groups (via a high-energy thioester bond), facilitating the entry of carbon units into the cycle and the oxidative decarboxylation of alpha-keto acids. **2. Analysis of Incorrect Options:** * **Thiamine (B1):** Functions as Thiamine Pyrophosphate (TPP). It is a cofactor for alpha-ketoglutarate dehydrogenase in the TCA cycle, but it is not a component of CoA itself. * **Riboflavin (B2):** Forms FAD and FMN. In the TCA cycle, FAD is the prosthetic group for Succinate Dehydrogenase. * **Nicotinic Acid (B3):** Forms NAD and NADP. NAD acts as an electron acceptor for various dehydrogenases (Isocitrate, alpha-ketoglutarate, and Malate dehydrogenase) in the TCA cycle. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Big Five" Cofactors:** The oxidative decarboxylation complexes (Pyruvate Dehydrogenase and $\alpha$-Ketoglutarate Dehydrogenase) require five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid. (Mnemonic: **T**ender **R**omance **N**ever **P**erishes **L**ately). * **Acyl Carrier Protein (ACP):** Pantothenic acid is also a component of the ACP domain of the Fatty Acid Synthase multienzyme complex, making it crucial for lipid synthesis. * **Burning Foot Syndrome:** This is the specific clinical manifestation of Vitamin B5 deficiency.
Question 614: Which vitamin facilitates iron absorption?
- A. Folic acid
- B. Ascorbic acid (Correct Answer)
- C. Biotin
- D. Para amino benzoic acid
Explanation: **Explanation:** **Correct Answer: B. Ascorbic acid (Vitamin C)** **Mechanism of Action:** Iron absorption occurs primarily in the duodenum and proximal jejunum. Dietary iron exists in two forms: Heme and Non-heme. Non-heme iron is mostly in the **Ferric (Fe³⁺)** state, which is insoluble and cannot be absorbed. Ascorbic acid facilitates iron absorption through two primary mechanisms: 1. **Reduction:** It acts as a reducing agent, converting Ferric (Fe³⁺) iron to the **Ferrous (Fe²⁺)** state, which is the only form compatible with the Divalent Metal Transporter-1 (DMT-1) for mucosal uptake. 2. **Chelation:** It forms a soluble iron-ascorbate complex in the acidic environment of the stomach, preventing the precipitation of iron by phytates or phosphates. **Why other options are incorrect:** * **A. Folic Acid:** Essential for DNA synthesis and one-carbon metabolism. While its deficiency causes macrocytic anemia, it does not directly influence the intestinal transport of iron. * **C. Biotin:** Acts as a coenzyme for carboxylation reactions (e.g., Pyruvate carboxylase). It has no role in mineral absorption. * **D. Para-amino benzoic acid (PABA):** An intermediate in the bacterial synthesis of folate. It is not an essential nutrient for humans and does not affect iron kinetics. **High-Yield Clinical Pearls for NEET-PG:** * **Enhancers of Iron Absorption:** Vitamin C, Gastric Acid (HCl), and Citrate. * **Inhibitors of Iron Absorption:** Phytates (cereals), Oxalates (spinach), Tannins (tea), and Calcium. * **Clinical Application:** Patients on oral iron supplements are often advised to take them with orange juice (rich in Vitamin C) to maximize bioavailability. * **Scurvy Connection:** Anemia is a common finding in Scurvy not just due to bleeding, but also due to impaired iron absorption.
Question 615: Which of the following metabolic reactions require vitamin B12 but not folate?
- A. Conversion of malonic acid to succinic acid (Correct Answer)
- B. Conversion of homocysteine to methionine
- C. Conversion of serine to glycine
- D. Thymidylate synthesis
Explanation: ### Explanation In human metabolism, **Vitamin B12 (Cobalamin)** acts as a coenzyme for only two specific reactions. Understanding the distinction between these two is critical for NEET-PG. **1. Why Option A is Correct:** The conversion of **Methylmalonyl-CoA to Succinyl-CoA** (often simplified in questions as malonic acid derivatives to succinic acid) is catalyzed by the enzyme **Methylmalonyl-CoA mutase**. This reaction requires **Adenosylcobalamin** (a form of B12) as a cofactor. Crucially, this pathway does **not** involve folate. A deficiency in B12 leads to an accumulation of Methylmalonic Acid (MMA), which is a specific diagnostic marker used to differentiate B12 deficiency from folate deficiency. **2. Why the Other Options are Incorrect:** * **Option B (Homocysteine to Methionine):** This is the second B12-dependent reaction (using Methylcobalamin), but it **also requires Folate** (as N5-methyl THF). This is known as the "Methyl Folate Trap" link. * **Options C & D (Serine to Glycine / Thymidylate synthesis):** These reactions are part of the one-carbon metabolism pool and are strictly **Folate-dependent** (specifically requiring THF and N5,N10-methylene THF). They do not require Vitamin B12. ### High-Yield Clinical Pearls for NEET-PG: * **The "Two-Reaction" Rule:** Remember that B12 is only needed for Methylmalonyl-CoA mutase and Methionine synthase. * **Diagnostic Marker:** If a patient has macrocytic anemia with **elevated Methylmalonic Acid (MMA)**, the diagnosis is Vitamin B12 deficiency. If MMA is normal but homocysteine is high, it is Folate deficiency. * **Neurological Symptoms:** The accumulation of Methylmalonyl-CoA (due to B12 deficiency) leads to the synthesis of abnormal fatty acids that incorporate into neuronal lipids, causing **Subacute Combined Degeneration (SCD)** of the spinal cord. Folate deficiency does not cause these neurological deficits.
Question 616: Which of the following has the highest amount of vitamin C?
- A. Orange
- B. Amla (Correct Answer)
- C. Lime
- D. Tomato
Explanation: **Explanation:** **Vitamin C (Ascorbic Acid)** is a potent water-soluble antioxidant and a vital coenzyme for the hydroxylation of proline and lysine during collagen synthesis. While all the options provided are sources of Vitamin C, **Amla (Indian Gooseberry)** is the richest natural source among them. * **Amla (Correct):** It contains approximately **600–700 mg of Vitamin C per 100g**, which is significantly higher than most other fruits. It is a classic "high-yield" fact in biochemistry that Amla is the premier source of this vitamin. * **Orange & Lime (Incorrect):** These are citrus fruits and excellent sources of Vitamin C, but they typically contain only **40–50 mg per 100g**, which is nearly 12–15 times less than Amla. * **Tomato (Incorrect):** While tomatoes contribute to dietary intake, they contain only about **10–15 mg per 100g**, making them the poorest source among the given options. **High-Yield NEET-PG Clinical Pearls:** 1. **Richest Source Overall:** While Amla is the answer here, **Barbados cherry (Acerola)** actually contains the highest concentration of Vitamin C (up to 1600 mg/100g) if listed in options. 2. **Biological Function:** Vitamin C is essential for the activity of **prolyl hydroxylase** and **lysyl hydroxylase** (requires $Fe^{2+}$ as a cofactor). Deficiency leads to **Scurvy**, characterized by "corkscrew hair," petechiae, and bleeding gums. 3. **Iron Absorption:** Vitamin C enhances the absorption of **non-heme iron** from the intestine by reducing it from the ferric ($Fe^{3+}$) to the ferrous ($Fe^{2+}$) state. 4. **Heat Sensitivity:** It is the most heat-labile vitamin; cooking or boiling significantly reduces its content in food.
Question 617: Vitamers represent:
- A. Chemically similar substances that possess qualitatively similar vitamin activity (Correct Answer)
- B. Chemically dissimilar substances that possess qualitatively similar vitamin activity
- C. Chemically similar substances that possess qualitatively less vitamin activity
- D. Chemically similar substances that possess qualitatively more vitamin activity
Explanation: ### Explanation **Vitamers** are defined as multiple forms of a specific vitamin that are **chemically similar** (structural analogs) and possess **qualitatively similar** biological activity. They are converted into the same active coenzyme in the body to perform specific metabolic functions. #### Why Option A is Correct: The definition of a vitamer hinges on two factors: structural similarity and functional equivalence. For example, **Vitamin B6** exists as three vitamers: **Pyridoxine, Pyridoxal, and Pyridoxamine**. All three are chemically related substituted pyridine derivatives and all three can be converted into the active coenzyme, Pyridoxal Phosphate (PLP). #### Why Other Options are Incorrect: * **Option B:** Substances that are chemically dissimilar but have similar activities are not vitamers. For instance, Carotene is a *provitamin* (precursor) to Vitamin A, but it is not a vitamer because its chemical structure is significantly different from Retinol. * **Options C & D:** Vitamers must exhibit the *same type* of biological activity (qualitatively similar). While their *quantitative* potency (IU per mg) might vary slightly, the core medical definition focuses on the qualitative similarity of their physiological role. --- ### High-Yield Clinical Pearls for NEET-PG: * **Vitamin A Vitamers:** Retinol (alcohol), Retinal (aldehyde), and Retinoic acid (acid). * **Vitamin D Vitamers:** Ergocalciferol (D2) and Cholecalciferol (D3). * **Vitamin E Vitamers:** There are eight vitamers (alpha, beta, gamma, delta-tocopherols and tocotrienols); **Alpha-tocopherol** is the most active. * **Vitamin K Vitamers:** Phylloquinone (K1), Menaquinone (K2), and Menadione (K3 - synthetic). * **Note:** Most water-soluble vitamins (except B6 and B12) do not have multiple vitamer forms commonly discussed in clinical biochemistry.
Question 618: Which vitamin deficiency results in poor wound healing?
- A. Vitamin K
- B. Vitamin C (Correct Answer)
- C. Vitamin D
- D. Vitamin E
Explanation: **Explanation:** **Vitamin C (Ascorbic Acid)** is the correct answer because it is an essential cofactor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes are responsible for the hydroxylation of proline and lysine residues during collagen synthesis. Hydroxylation is critical for the cross-linking of collagen fibers, which provides structural integrity and tensile strength to connective tissues. In deficiency (Scurvy), defective collagen synthesis leads to fragile capillaries and **poor wound healing**. **Analysis of Incorrect Options:** * **Vitamin K:** Primarily functions as a cofactor for γ-glutamyl carboxylase, essential for the activation of clotting factors (II, VII, IX, X). Deficiency leads to bleeding diathesis, not impaired wound healing. * **Vitamin D:** Regulates calcium and phosphorus homeostasis and bone mineralization. Deficiency causes Rickets (children) or Osteomalacia (adults). * **Vitamin E:** Acts as a potent lipid-soluble antioxidant, protecting cell membranes from free radical damage. While it supports skin health, its deficiency is more commonly associated with hemolytic anemia and neurological deficits (spinocerebellar ataxia). **NEET-PG High-Yield Pearls:** * **Scurvy Mnemonic (4 H’s):** **H**emorrhage (petechiae, ecchymosis), **H**yperkeratosis, **H**ypochondriasis, and **H**ematologic abnormalities (anemia). * **Corkscrew hair** and **swollen, bleeding gums** are classic clinical signs of Vitamin C deficiency. * Vitamin C also enhances **non-heme iron absorption** in the GI tract by maintaining iron in the ferrous ($Fe^{2+}$) state. * It is the most heat-labile vitamin; cooking significantly reduces its potency.
Question 619: Vitamin E deficiency is similar to which of the following conditions?
- A. Spinocerebellar ataxia (Correct Answer)
- B. Multiple sclerosis
- C. Chronic muscle weakness
- D. Peripheral neuropathy
Explanation: **Explanation:** **1. Why Spinocerebellar Ataxia is the Correct Answer:** Vitamin E (Alpha-tocopherol) acts as a potent lipid-soluble antioxidant, protecting cell membranes—especially those in the nervous system—from oxidative damage. A deficiency leads to the degeneration of the **posterior columns** (loss of vibration/proprioception) and the **spinocerebellar tracts** (ataxia). This clinical presentation is almost indistinguishable from **Friedreich’s Ataxia** or **Ataxia with Vitamin E Deficiency (AVED)**. Patients present with progressive limb ataxia, dysarthria, and loss of deep tendon reflexes. **2. Analysis of Incorrect Options:** * **Multiple Sclerosis:** This is an autoimmune demyelinating disease of the Central Nervous System. While it can cause ataxia, its pathophysiology involves inflammatory plaques rather than the oxidative nutritional degeneration seen in Vitamin E deficiency. * **Chronic Muscle Weakness:** While Vitamin E deficiency can cause mild myopathy, it is primarily a **neurological** syndrome. Chronic muscle weakness is more characteristic of Vitamin D deficiency (osteomalacic myopathy) or primary muscular dystrophies. * **Peripheral Neuropathy:** Although Vitamin E deficiency involves the peripheral nerves (loss of reflexes), the term "Peripheral Neuropathy" is too non-specific. It is more classically associated with Vitamin B1 (Beriberi), B6, or B12 deficiencies. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "B12 Mimic":** Vitamin E deficiency presents similarly to Subacute Combined Degeneration (SCD) of the spinal cord (B12 deficiency) but **without** the megaloblastic anemia and hypersegmented neutrophils. * **Hemolysis:** In neonates, Vitamin E deficiency causes **hemolytic anemia** due to increased oxidative stress on RBC membranes. * **Absorption:** Deficiency is usually secondary to fat malabsorption syndromes (e.g., Cystic Fibrosis, Abetalipoproteinemia, or Chronic Cholestasis). * **Key Triad:** Posterior column loss + Spinocerebellar ataxia + Hemolytic anemia = Vitamin E deficiency.
Question 620: The gene responsible for folic acid transport is situated on which chromosome?
- A. Chromosome 10
- B. Chromosome 5
- C. Chromosome X
- D. Chromosome 21 (Correct Answer)
Explanation: **Explanation:** The correct answer is **Chromosome 21**. This is a high-yield fact in biochemistry and genetics, particularly concerning the metabolism of folic acid (Vitamin B9). **1. Why Chromosome 21 is Correct:** The gene encoding the **Reduced Folate Carrier (RFC-1)**, also known as SLC19A1, is located on the long arm of **Chromosome 21 (21q22.3)**. This carrier is the primary protein responsible for the transport of 5-methyltetrahydrofolate (the active form of folate) into the cells. **2. Clinical Correlation (The "Why it Matters"):** This location is clinically significant in **Down Syndrome (Trisomy 21)**. Individuals with Down Syndrome have three copies of this gene, leading to increased expression of folate transporters. This altered folate metabolism is linked to an increased sensitivity to methotrexate (a folate antagonist) and may contribute to the specific leukemia profiles seen in these patients. **3. Analysis of Incorrect Options:** * **Chromosome 10:** While many metabolic genes are here, it does not harbor the primary folate transporter. * **Chromosome 5:** Associated with the *MTRR* gene (Methionine synthase reductase), which is involved in folate metabolism, but not the primary transport gene. * **Chromosome X:** Not associated with the primary RFC-1 transporter. **NEET-PG High-Yield Pearls:** * **RFC-1 (SLC19A1):** Located on Chromosome 21; mediates folate uptake. * **Folate Deficiency:** Leads to Megaloblastic Anemia and Neural Tube Defects (NTDs). * **FIGLU Test:** Formiminoglutamate (FIGLU) excretion in urine is a sensitive indicator of folate deficiency. * **DHFR:** Dihydrofolate reductase is the enzyme inhibited by Methotrexate.
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
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free