Vitamins and Coenzymes — MCQs

A 30-year-old female patient complains of weakness and fatigue. The patient has a history of consuming antiepileptic drugs and is also a lactating mother. Laboratory investigations reveal Hb of 8 gm% and the presence of macrocytic RBCs on peripheral blood smear. This patient may be suffering from a deficiency of?

Q527Easy

Peripheral neuropathy is seen with the deficiency of which vitamin?

Q528Easy

Which of the following is an expected biochemical change in Vitamin B12 deficiency?

Q529Easy

Which of the following forms of Vitamin A has maximum effects on DNA?

Q530Medium

Which of the following can cause dysphonia and hoarseness of voice?

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 521: Which vitamin possesses antioxidant properties?

A. Vitamin C (Correct Answer)
B. Vitamin D
C. Vitamin B3
D. Vitamin B2

Explanation: ### Explanation **Correct Answer: A. Vitamin C (Ascorbic Acid)** Vitamin C is a potent water-soluble **antioxidant** that protects cells from oxidative stress by scavenging free radicals (reactive oxygen species). It acts as a reducing agent, donating electrons to neutralize radicals, thereby preventing lipid peroxidation and DNA damage. Furthermore, it plays a crucial role in "recycling" Vitamin E (a lipid-soluble antioxidant) back into its active form. **Why the other options are incorrect:** * **Vitamin D (Cholecalciferol):** Primarily functions as a hormone involved in calcium and phosphorus homeostasis and bone mineralization. It does not possess direct antioxidant properties. * **Vitamin B3 (Niacin):** Functions as a precursor to coenzymes **NAD and NADP**, which are essential for redox reactions in glycolysis and the TCA cycle. While NADPH is involved in maintaining glutathione in a reduced state, Niacin itself is not classified as a direct antioxidant. * **Vitamin B2 (Riboflavin):** Functions as a precursor for **FMN and FAD**, which serve as prosthetic groups for enzymes like succinate dehydrogenase. Like B3, it facilitates redox reactions but is not a direct scavenger of free radicals. **High-Yield Clinical Pearls for NEET-PG:** * **The "ACE" Antioxidants:** Remember Vitamins **A** (Beta-carotene), **C**, and **E** are the primary antioxidant vitamins. * **Enzymatic Cofactor:** Vitamin C is a mandatory cofactor for **prolyl and lysyl hydroxylase**, essential for collagen synthesis (deficiency leads to Scurvy). * **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 in the stomach. * **Glutathione Link:** Vitamin C works in synergy with Glutathione to maintain the redox balance of the cell.

Question 522: What is the first product of tryptophan catabolism?

A. kynurenine (Correct Answer)
B. Bradykinin
C. PAF
D. Xantheurenate

Explanation: ### Explanation **Correct Answer: A. Kynurenine** Tryptophan is an essential amino acid that follows two major metabolic pathways: the **Kynurenine pathway** (90-95%) and the Serotonin pathway. The first step of the kynurenine pathway involves the oxidative cleavage of the indole ring of tryptophan. This reaction is catalyzed by the enzyme **Tryptophan 2,3-dioxygenase (TDO)** in the liver or **Indoleamine 2,3-dioxygenase (IDO)** in extrahepatic tissues. The immediate product formed is *N-formylkynurenine*, which is rapidly converted to **Kynurenine** by the enzyme formamidase. In the context of medical exams, Kynurenine is recognized as the first stable major product of this catabolic route. **Analysis of Incorrect Options:** * **B. Bradykinin:** This is a potent vasodilator peptide belonging to the kinin system, derived from high-molecular-weight kininogen (HMWK), not from amino acid catabolism. * **C. PAF (Platelet Activating Factor):** This is a phospholipid mediator of inflammation and platelet aggregation; it is not a product of tryptophan metabolism. * **D. Xanthurenate:** This is a downstream metabolite of the kynurenine pathway. It is clinically significant because it is excreted in excess during **Vitamin B6 deficiency**, as the enzyme kynureninase requires Pyridoxal Phosphate (PLP). **High-Yield Clinical Pearls for NEET-PG:** * **Niacin Synthesis:** The kynurenine pathway is the source of **NAD+/NADP+**. Approximately 60 mg of Tryptophan yields 1 mg of Niacin. * **Hartnup Disease:** A defect in the transport of neutral amino acids (including tryptophan) leads to pellagra-like symptoms due to niacin deficiency. * **Vitamin B6 Dependency:** In B6 deficiency, the conversion of 3-hydroxykynurenine to anthranilic acid is blocked, leading to the side-tracking of metabolites into **Xanthurenic acid** (a diagnostic marker in urine).

Question 523: PLP is the coenzyme form of which vitamin?

A. Vitamin B1
B. Vitamin B3
C. Vitamin B6 (Correct Answer)
D. Vitamin B12

Explanation: **Explanation:** **Vitamin B6 (Pyridoxine)** is the correct answer. Its active coenzyme form is **Pyridoxal Phosphate (PLP)**. PLP is essential for numerous metabolic reactions, primarily involving amino acid metabolism. It acts as a cofactor for enzymes in **transamination** (e.g., ALT, AST), **decarboxylation** (e.g., synthesis of GABA, Histamine, Serotonin), and **deamination**. It is also a crucial cofactor for **Cystathionine synthase** (homocysteine metabolism) and **ALA synthase** (the rate-limiting step in Heme synthesis). **Analysis of Incorrect Options:** * **Vitamin B1 (Thiamine):** Its active form is **Thiamine Pyrophosphate (TPP)**, involved in oxidative decarboxylation (e.g., Pyruvate dehydrogenase) and the HMP shunt (Transketolase). * **Vitamin B3 (Niacin):** Its active forms are **NAD+ and NADP+**, which function as electron carriers in redox reactions. * **Vitamin B12 (Cobalamin):** Its active forms are **Methylcobalamin** (methionine synthesis) and **Deoxyadenosylcobalamin** (conversion of Methylmalonyl CoA to Succinyl CoA). **High-Yield Clinical Pearls for NEET-PG:** * **Isoniazid (INH) Therapy:** This anti-tubercular drug can induce Vitamin B6 deficiency by forming an inactive complex with PLP, leading to **peripheral neuropathy**. Always co-administer B6 with INH. * **Sideroblastic Anemia:** Deficiency of B6 leads to microcytic anemia because PLP is required for the first step of heme synthesis (ALA synthase). * **Xanthurenic Acid:** Increased urinary excretion of xanthurenic acid is a sensitive indicator of Vitamin B6 deficiency.

Question 524: Which of the following enzymic activities would you expect to be decreased in thiamine deficiency?

A. Pyruvate carboxylase
B. Isocitrate dehydrogenase
C. Fumarase
D. a-Ketoglutarate dehydrogenase (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. α-Ketoglutarate dehydrogenase.** **Underlying Concept:** Thiamine (Vitamin B1) is the precursor for **Thiamine Pyrophosphate (TPP)**, an essential coenzyme for enzymes involved in oxidative decarboxylation and the pentose phosphate pathway. Specifically, TPP is required by four major enzyme complexes: 1. **Pyruvate Dehydrogenase (PDH):** Converts pyruvate to Acetyl-CoA. 2. **α-Ketoglutarate Dehydrogenase (α-KGDH):** A key rate-limiting enzyme in the TCA cycle. 3. **Branched-chain α-ketoacid dehydrogenase (BCKDH):** Involved in the metabolism of Leucine, Isoleucine, and Valine. 4. **Transketolase:** Involved in the HMP Shunt. In thiamine deficiency, the activity of these enzymes is significantly impaired, leading to a failure in ATP production and the accumulation of ketoacids. **Analysis of Incorrect Options:** * **A. Pyruvate carboxylase:** This enzyme requires **Biotin (B7)**, not thiamine, to convert pyruvate into oxaloacetate. * **B. Isocitrate dehydrogenase:** This is a NAD+-dependent enzyme in the TCA cycle; it does not require TPP. * **C. Fumarase:** This enzyme catalyzes the hydration of fumarate to malate and does not require any vitamin cofactors. **NEET-PG High-Yield Pearls:** * **Clinical Presentation:** Thiamine deficiency manifests as **Wernicke-Korsakoff syndrome** (ataxia, ophthalmoplegia, confusion) or **Beriberi** (Dry: peripheral neuropathy; Wet: high-output heart failure). * **Diagnostic Test:** The most reliable biochemical marker for thiamine deficiency is an **increase in erythrocyte transketolase activity** after the addition of TPP. * **Management Rule:** Always administer thiamine **before** glucose in malnourished/alcoholic patients to prevent precipitating Wernicke encephalopathy (as glucose metabolism consumes remaining TPP via the PDH reaction).

Question 525: Which of the following statements about folic acid is FALSE?

A. It is present in all green leafy vegetables.
B. It is proven to decrease the occurrence of neural tube defects when taken pre-natally.
C. Wheat flour in India is fortified with folate, similar to the USA. (Correct Answer)
D. The methyl folate trap is caused by a defect in methionine synthase.

Explanation: **Explanation** The correct answer is **C**. While the USA and several other countries have mandatory folic acid fortification of wheat flour to prevent neural tube defects (NTDs), **India does not have a mandatory national policy for folate fortification of wheat flour.** Although India has a "Food Fortification Resource Centre" (FFRC) and voluntary fortification exists, it is not a universal mandate similar to the US FDA regulations. **Analysis of other options:** * **Option A:** Folic acid (Vitamin B9) derives its name from "folium" (leaf). It is abundantly present in **green leafy vegetables** (e.g., spinach), as well as legumes and liver. * **Option B:** Periconceptional supplementation of folic acid (400 mcg/day) is clinically proven to reduce the incidence of **Neural Tube Defects (NTDs)** like spina bifida and anencephaly. * **Option D:** The **Methyl Folate Trap** occurs in Vitamin B12 deficiency. B12 is a cofactor for **methionine synthase**, which converts N5-methyl-THF back to THF. Without B12, folate is "trapped" in the N5-methyl-THF form, leading to a functional folate deficiency and megaloblastic anemia. **High-Yield Clinical Pearls for NEET-PG:** * **Storage:** Unlike other B-vitamins, the liver stores enough folate for only **3–4 months** (deficiency develops rapidly). * **Absorption:** Occurs primarily in the **jejunum**. * **Antagonists:** Methotrexate, Trimethoprim, and Pyrimethamine inhibit **Dihydrofolate Reductase (DHFR)**. * **Diagnostic Test:** The **FIGLU excretion test** (Formiminoglutamic acid) is used to detect folate deficiency.

Question 526: A 30-year-old female patient complains of weakness and fatigue. The patient has a history of consuming antiepileptic drugs and is also a lactating mother. Laboratory investigations reveal Hb of 8 gm% and the presence of macrocytic RBCs on peripheral blood smear. This patient may be suffering from a deficiency of?

A. Vitamin C
B. Vitamin B1 (Thiamine)
C. Vitamin B2 (Riboflavin)
D. Vitamin B9 (Folate) (Correct Answer)

Explanation: ### Explanation The correct answer is **Vitamin B9 (Folate)**. **1. Why Vitamin B9 (Folate) is correct:** The patient presents with **Macrocytic Anemia** (Hb 8 gm%, macrocytic RBCs). Folate (Vitamin B9) is essential for one-carbon metabolism and DNA synthesis. Its deficiency impairs erythropoiesis, leading to the production of large, immature RBCs (megaloblasts). * **Antiepileptic Drugs (AEDs):** Drugs like Phenytoin, Phenobarbital, and Primidone interfere with folate absorption or metabolism, making users highly susceptible to deficiency. * **Lactation:** This is a high-demand physiological state where folate requirements increase significantly to support the infant and the mother's metabolic needs. The combination of AEDs and lactation creates a "perfect storm" for folate depletion. **2. Why other options are incorrect:** * **Vitamin C:** Deficiency leads to Scurvy (bleeding gums, petechiae). While it aids iron absorption, its deficiency typically causes microcytic or normocytic anemia, not macrocytic. * **Vitamin B1 (Thiamine):** Deficiency causes Beriberi (Wet/Dry) or Wernicke-Korsakoff syndrome. It does not cause macrocytic anemia. * **Vitamin B2 (Riboflavin):** Deficiency presents with cheilosis, glossitis, and seborrheic dermatitis, but not macrocytic anemia. **3. NEET-PG High-Yield Pearls:** * **Drug-Induced Folate Deficiency:** Always look for **Phenytoin, Methotrexate, or Trimethoprim** in the history. * **Folate vs. B12:** Both cause macrocytic anemia. However, Folate deficiency **does not** present with neurological symptoms (subacute combined degeneration), whereas B12 deficiency does. * **Storage:** Folate stores in the liver last only **3–4 months**, whereas Vitamin B12 stores last **3–5 years**. * **FIGLU Test:** Histidine load test (FIGLU excretion in urine) is a specific biochemical marker for Folate deficiency.

Question 527: Peripheral neuropathy is seen with the deficiency of which vitamin?

A. Pyridoxine (Correct Answer)
B. Vitamin E
C. Vitamin A
D. Pantothenic acid

Explanation: **Explanation:** **Pyridoxine (Vitamin B6)** is the correct answer because it is essential for the synthesis of neurotransmitters (such as GABA, dopamine, and serotonin) and the formation of sphingolipids required for myelin sheath integrity. Its deficiency leads to axonal degeneration, manifesting as **peripheral neuropathy**. A high-yield clinical association for NEET-PG is **Isoniazid (INH)** therapy for Tuberculosis. INH inhibits the enzyme pyridoxine phosphokinase and increases the renal excretion of B6, leading to induced deficiency. Therefore, B6 is always co-prescribed with INH to prevent neuropathy. **Analysis of Incorrect Options:** * **Vitamin E:** While deficiency can cause neurological symptoms (ataxia, loss of proprioception, and hemolytic anemia), it primarily presents as a **spinocerebellar degeneration** rather than isolated peripheral neuropathy. * **Vitamin A:** Deficiency primarily affects vision (**Nyctalopia**, Xerophthalmia) and epithelial integrity (Bitot’s spots, follicular hyperkeratosis), not the peripheral nerves. * **Pantothenic acid (B5):** Deficiency is extremely rare but is classically associated with **"Burning Foot Syndrome"** (Gopalan’s syndrome). While this involves the feet, Pyridoxine is the more standard answer for generalized peripheral neuropathy in a clinical/biochemical context. **Clinical Pearls for NEET-PG:** 1. **Sideroblastic Anemia:** B6 deficiency also causes microcytic anemia because it is a cofactor for **ALA synthase**, the rate-limiting step in heme synthesis. 2. **Homocysteinemia:** B6 is a cofactor for **Cystathionine beta-synthase**; deficiency leads to elevated homocysteine levels (a risk factor for thrombosis). 3. **Dependency States:** Infantile convulsions can occur in B6 dependency states due to decreased GABA levels.

Question 528: Which of the following is an expected biochemical change in Vitamin B12 deficiency?

A. Increased homocysteine in urine
B. Increased methylmalonic acid in urine (Correct Answer)
C. FIGLU in urine
D. Phenylalanine in urine

Explanation: **Explanation:** Vitamin B12 (Cobalamin) acts as a vital coenzyme for two major enzymatic reactions in the human body. The correct answer is based on the **Methylmalonyl-CoA Mutase** reaction. 1. **Why Option B is Correct:** In the catabolism of odd-chain fatty acids and certain amino acids (Valine, Isoleucine, Threonine, Methionine), **Propionyl-CoA** is converted to **Methylmalonyl-CoA**. The enzyme **Methylmalonyl-CoA Mutase** requires Vitamin B12 (as deoxyadenosylcobalamin) to convert it into **Succinyl-CoA**. In B12 deficiency, this reaction is blocked, leading to an accumulation of Methylmalonic Acid (MMA) in the blood and its subsequent excretion in the urine. This is a **highly specific marker** for B12 deficiency. 2. **Why Other Options are Incorrect:** * **Option A:** While B12 deficiency *does* cause high homocysteine levels (due to failure of the Methionine Synthase reaction), homocysteine is typically measured in the **blood** (hyperhomocysteinemia). Increased homocysteine in **urine** (homocystinuria) is more characteristic of genetic defects in Cystathionine $\beta$-synthase. * **Option C:** **FIGLU (Formiminoglutamic acid)** excretion in urine is a specific marker for **Folic Acid deficiency**, not B12. It occurs due to the failure of THF to accept the formimino group during histidine metabolism. * **Option D:** Phenylalanine in urine is the hallmark of **Phenylketonuria (PKU)**, caused by a deficiency of Phenylalanine Hydroxylase or BH4. **High-Yield Clinical Pearls for NEET-PG:** * **The "Folate Trap":** B12 deficiency leads to functional folate deficiency because folate remains trapped as N5-methyl-THF. * **Differentiating B12 vs. Folate:** Both cause megaloblastic anemia and raised homocysteine, but **only B12 deficiency** causes raised MMA and neurological symptoms (Subacute Combined Degeneration of the Spinal Cord). * **Schilling Test:** Historically used to determine the cause of B12 malabsorption (e.g., Pernicious Anemia).

Question 529: Which of the following forms of Vitamin A has maximum effects on DNA?

A. Isoretinal
B. Isotretinoin (Correct Answer)
C. Retinaldehyde
D. Retinol

Explanation: **Explanation:** The biological effects of Vitamin A on gene expression are mediated primarily by its acidic forms, known as **Retinoic Acids**. **1. Why Isotretinoin is correct:** Vitamin A acts as a hormone-like signal. Retinol is oxidized into Retinaldehyde and then into Retinoic Acid (RA). **Isotretinoin (13-cis-retinoic acid)** and its isomer, Tretinoin (all-trans-retinoic acid), are the active metabolites that enter the nucleus. They bind to specific nuclear receptors: **RAR (Retinoic Acid Receptor)** and **RXR (Retinoid X Receptor)**. These receptors act as ligand-activated transcription factors that bind to **RARE (Retinoic Acid Response Elements)** on the DNA, directly regulating the transcription of genes responsible for cell growth and differentiation. **2. Why the other options are incorrect:** * **Retinol:** This is the transport form of Vitamin A (bound to Retinol Binding Protein). It must be converted into retinoic acid to exert genomic effects. * **Retinaldehyde (Retinal):** This is the aldehyde form essential for the **visual cycle** (forming Rhodopsin). While vital for vision, it does not directly modulate DNA transcription. * **Isoretinal:** This is not a standard physiological form involved in gene regulation; it is likely a distractor term. **High-Yield Clinical Pearls for NEET-PG:** * **Teratogenicity:** Because retinoic acid (Isotretinoin) directly alters DNA expression, it is highly teratogenic. A negative pregnancy test and contraception are mandatory before prescribing it for acne. * **Therapeutic Use:** All-trans-retinoic acid (ATRA) is the first-line treatment for **Acute Promyelocytic Leukemia (M3)** as it induces differentiation of leukemic cells. * **Storage:** Vitamin A is stored in the liver in **Ito cells** (Stellate cells) as Retinyl palmitate.

Question 530: Which of the following can cause dysphonia and hoarseness of voice?

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

Explanation: **Explanation:** **Thiamine (Vitamin B1) Deficiency** is the correct answer because it can lead to **Infantile Beriberi**, specifically the **aphonic form**. In infants (usually 2–6 months old) born to thiamine-deficient mothers, the deficiency manifests as a characteristic "silent cry." This occurs due to **laryngeal nerve paralysis** or edema of the larynx, resulting in dysphonia, hoarseness, or a complete loss of voice (aphonia). In adults, severe chronic deficiency (Dry Beriberi) can also cause peripheral neuropathy involving cranial nerves, potentially affecting vocal cord function. **Incorrect Options:** * **Pyridoxine (B6):** Deficiency typically presents with microcytic anemia, seborrheic dermatitis, and neurological symptoms like convulsions (due to decreased GABA) or peripheral neuropathy, but not specific laryngeal nerve involvement. * **Vitamin B12 (Cobalamin):** Deficiency leads to Megaloblastic anemia and Subacute Combined Degeneration of the Spinal Cord (SCDSC). While it causes neurological deficits, it primarily affects the posterior and lateral columns, not the recurrent laryngeal nerve. * **Folic Acid (B9):** Deficiency causes Megaloblastic anemia and neural tube defects in fetuses. It does not have a direct association with vocal cord paralysis or dysphonia. **High-Yield Clinical Pearls for NEET-PG:** * **Wet Beriberi:** High-output cardiac failure + Edema. * **Dry Beriberi:** Symmetrical peripheral neuropathy (wasting and weakness). * **Wernicke-Korsakoff Syndrome:** Triad of Confusion, Ataxia, and Ophthalmoplegia (often seen in alcoholics). * **Key Enzyme:** Thiamine pyrophosphate (TPP) is a cofactor for **Pyruvate Dehydrogenase** and **Alpha-ketoglutarate dehydrogenase**. Its activity is measured via the **Erythrocyte Transketolase** assay.

Practice by Chapter

Fat-Soluble Vitamins: A, D, E, K

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Vitamin A and Vision

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Vitamin D and Calcium Metabolism

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Vitamin E and Antioxidant Functions

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Vitamin K and Blood Coagulation

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Water-Soluble Vitamins: B Complex and C

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Thiamine (B1) and Pyruvate Dehydrogenase

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Riboflavin (B2) and Flavin Coenzymes

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Niacin and NAD/NADP

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Vitamin B6 and Transamination

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Folate and Vitamin B12 in One-Carbon Metabolism

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Vitamin C and Collagen Synthesis

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