Vitamins and Coenzymes Practice Questions

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: Pellagra-like dermatitis occurs in which condition?

Carcinoid syndrome. **Explanation:** Pellagra is caused by a deficiency of **Niacin (Vitamin B3)** or its precursor, the amino acid **Tryptophan**. Under normal physiological conditions, approximately 1% of dietary tryptophan is converted into niacin. In **Carcinoid Syndrome**, enterochromaffin cell tumors (usually in the small intestine) divert up to 60% of the body’s tryptophan to synthesize massive amounts of **Serotonin (5-HT)**. This "tryptophan steal" results in a profound secondary deficiency of niacin, leading to the classic clinical triad of Pellagra: Dermatitis (photosensitive Casal’s necklace), Diarrhea, and Dementia. **Analysis of Incorrect Options:** * **Dermatomyositis:** Presents with photosensitive rashes like the Heliotrope rash and Gottron’s papules, but these are autoimmune inflammatory processes, not nutritional deficiencies. * **Systemic Lupus Erythematosus (SLE):** Characterized by a malar (butterfly) rash and photosensitivity. While it may mimic the distribution of pellagra, the pathophysiology involves immune complex deposition (Type III hypersensitivity). * **Lymphoma:** While some cutaneous lymphomas (like Mycosis Fungoides) cause erythroderma, they do not present with the specific niacin-deficiency pattern seen in pellagra. **NEET-PG High-Yield Pearls:** * **The 4 D’s of Pellagra:** Dermatitis, Diarrhea, Dementia, and Death. * **Hartnup Disease:** An autosomal recessive disorder of neutral amino acid transport that also causes pellagra-like symptoms due to impaired tryptophan absorption. * **Drug-induced Pellagra:** Isoniazid (INH) is a common culprit as it inhibits the enzyme pyridoxine phosphokinase; Vitamin B6 is a necessary cofactor for converting tryptophan to niacin. * **Dietary Cause:** Diets primarily consisting of **Maize** (corn) or **Jowar** (sorghum) are pellagragenic because niacin is in a bound form (niacytin) and they are low in tryptophan.

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: All of the following are biotin-independent carboxylation reactions, EXCEPT?

Acetyl-CoA carboxylase. **Explanation:** The question asks to identify the reaction that **requires** biotin (the exception to the biotin-independent list). Biotin (Vitamin B7) acts as a coenzyme for most **carboxylation reactions** where CO2 is added to a substrate. These enzymes typically follow a "ABC" rule: they require **A**TP, **B**iotin, and **C**O2. **1. Why Acetyl-CoA Carboxylase (ACC) is the correct answer:** Acetyl-CoA carboxylase is a classic **biotin-dependent** enzyme. It catalyzes the rate-limiting step of fatty acid synthesis: the conversion of Acetyl-CoA to Malonyl-CoA. It requires biotin as a carrier to transfer the carboxyl group to the substrate. **2. Why the other options are incorrect (Biotin-Independent):** * **Addition of CO2 to form C6 in purine ring:** This step (catalyzed by AIR carboxylase) is unique because it does not require biotin or ATP; the CO2 is added directly to the imidazole ring. * **Malic Enzyme:** This enzyme converts pyruvate to malate (or vice versa) using NADPH. It is a decarboxylation/carboxylation reaction that does **not** utilize biotin. * **Carbamoyl Phosphate Synthetase (CPS I & II):** These enzymes incorporate CO2 (as bicarbonate) into carbamoyl phosphate for the Urea cycle and Pyrimidine synthesis. They require ATP but are notably **biotin-independent**. **High-Yield Clinical Pearls for NEET-PG:** * **The "Big Four" Biotin-Dependent Enzymes:** 1. **Pyruvate Carboxylase** (Gluconeogenesis) 2. **Acetyl-CoA Carboxylase** (Fatty acid synthesis) 3. **Propionyl-CoA Carboxylase** (Metabolism of VOMIT: Valine, Odd-chain FAs, Methionine, Isoleucine, Threonine) 4. **3-Methylcrotonyl-CoA Carboxylase** (Leucine catabolism) * **Avidin**, a protein in raw egg whites, binds biotin tightly and can lead to deficiency. * Biotin deficiency presents with dermatitis, alopecia, and enteritis.

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.

Q: Which of the following is the most appropriate screening test for vitamin D deficiency?

25 hydroxy vitamin D. **Explanation:** The most appropriate screening test for Vitamin D status is the measurement of **25-hydroxy vitamin D [25(OH)D]**, also known as Calcidiol. **Why 25-hydroxy vitamin D is the correct answer:** 1. **Longer Half-life:** It has a circulating half-life of approximately 2–3 weeks, reflecting long-term stores, whereas the active form lasts only hours. 2. **Reflects Total Intake:** It accounts for Vitamin D obtained from both cutaneous synthesis (UV light) and dietary intake. 3. **Substrate Availability:** Its concentration is not strictly regulated by PTH or calcium levels, making it a reliable indicator of overall vitamin D status. **Why the other options are incorrect:** * **7-dehydrocholesterol:** This is the precursor found in the skin. While it is converted to Cholecalciferol (D3) by UV light, its levels do not reflect the nutritional or functional status of the vitamin. * **1,25 dihydroxy vitamin D (Calcitriol):** Although this is the biologically active form, it is **not** a good screening tool. In deficiency, compensatory increases in Parathyroid Hormone (PTH) can stimulate the kidneys to over-convert 25(OH)D into 1,25(OH)₂D, often resulting in "normal" levels despite severe underlying deficiency. * **Serum calcium levels:** Calcium is tightly regulated by PTH and bone resorption. Serum calcium usually remains normal until vitamin D deficiency is very advanced; therefore, it lacks the sensitivity required for screening. **High-Yield Clinical Pearls for NEET-PG:** * **Storage form:** 25-hydroxy vitamin D (Calcidiol). * **Active form:** 1,25-dihydroxy vitamin D (Calcitriol). * **Rate-limiting enzyme:** 1-alpha-hydroxylase (found in the kidney, stimulated by PTH). * **Deficiency Cut-off:** Levels <20 ng/mL are generally considered deficient; 21–29 ng/mL are considered insufficient.

Q: The gene responsible for folic acid transport is situated on which chromosome?

Chromosome 21. **Explanation:** The correct answer is **Chromosome 21**. The gene responsible for the primary transport of folic acid into cells is the **SLC19A1 gene** (Solute Carrier Family 19 Member 1), which encodes the **Reduced Folate Carrier 1 (RFC1)**. This gene is located on the long arm of chromosome 21 (21q22.3). **Why Chromosome 21 is correct:** The RFC1 protein is the major transporter for 5-methyltetrahydrofolate (the predominant form of folate in plasma) into systemic cells. This is clinically significant in **Down Syndrome (Trisomy 21)**; individuals with this condition have three copies of the SLC19A1 gene, leading to altered folate metabolism and increased sensitivity to methotrexate (a folate antagonist). **Analysis of Incorrect Options:** * **Chromosome 10:** While many metabolic genes are located here, it does not harbor the primary RFC1 transporter. * **Chromosome 5:** This chromosome contains the gene for the *Folate Receptor Alpha (FOLR1)*, which is involved in folate uptake in specific tissues (like the choroid plexus), but the primary "folate transport gene" referred to in standard biochemistry texts is SLC19A1 on Chromosome 21. * **X Chromosome:** No major systemic folate transport genes are mapped to the X chromosome. **High-Yield Clinical Pearls for NEET-PG:** * **Hereditary Folate Malabsorption:** Caused by mutations in the **SLC46A1** gene (Proton-coupled folate transporter - PCFT) located on **Chromosome 17**. * **Methotrexate Toxicity:** Because the SLC19A1 gene is on Chromosome 21, children with Down Syndrome are at a higher risk of toxicity when treated for Leukemia due to increased intracellular drug accumulation. * **Folate vs. B12:** Remember that folate deficiency leads to increased homocysteine but **normal** methylmalonic acid (MMA) levels, distinguishing it from B12 deficiency.

Q: Which vitamin is absent in eggs?

Vitamin C. **Explanation:** The correct answer is **Vitamin C (Ascorbic Acid)**. Eggs are often described as a "complete food" because they contain almost every essential vitamin and mineral required for life, with one notable exception: Vitamin C. **Why Vitamin C is absent:** Vitamin C is primarily synthesized by plants and is found in citrus fruits and leafy vegetables. While most animals can synthesize Vitamin C endogenously from glucose, the avian embryo does not require an external source of Vitamin C within the egg for its initial development. Consequently, a standard chicken egg contains zero or only negligible traces of Ascorbic Acid. **Analysis of Incorrect Options:** * **Vitamin B1 (Thiamine):** Present in the egg yolk. It is essential for carbohydrate metabolism in the developing embryo. * **Vitamin B6 (Pyridoxine):** Found in eggs; it serves as a vital cofactor for amino acid metabolism (transamination reactions). * **Vitamin E (Tocopherol):** Eggs are a significant source of Vitamin E, which acts as a lipid-soluble antioxidant protecting the high fat content of the yolk from oxidative damage. **High-Yield NEET-PG Pearls:** * **The "Two Absences":** Remember that eggs lack two major nutritional components: **Vitamin C** and **Dietary Fiber**. * **Milk vs. Eggs:** Like eggs, **milk** is also notoriously deficient in Vitamin C and Iron. * **Egg White vs. Yolk:** The egg white (albumin) contains **Avidin**, which binds to **Biotin (B7)** and prevents its absorption, leading to "egg white injury" if consumed raw in large quantities. * **Reference Protein:** Egg protein is considered the "Gold Standard" or Reference Protein (Biological Value = 100) against which other proteins are compared.

Question 1

Harup disease causes deficiency of?

Accepted Answer

Nicotinamide

Answer

Thiamine

Answer

Riboflavin

Answer

Biotin

Question 2

Pellagra-like dermatitis occurs in which condition?

Accepted Answer

Carcinoid syndrome

Answer

Dermatomyositis

Answer

Systemic Lupus Erythematosus (SLE)

Answer

Lymphoma

Question 3

A man who had undergone total gastrectomy remains well for five years. Then he develops anaemia. What is the most likely cause of this anaemia?

Accepted Answer

Vitamin B12 deficiency

Answer

Protein deficiency

Answer

Zinc deficiency

Answer

Folic acid deficiency

Question 4

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 5

All of the following are biotin-independent carboxylation reactions, EXCEPT?

Accepted Answer

Acetyl-CoA carboxylase

Answer

Addition of CO2 to form C6 in purine ring

Answer

Conversion of pyruvate to malate by malic enzyme

Answer

Carbamoyl phosphate synthetase

Question 6

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 7

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

Accepted Answer

Niacin

Answer

Riboflavin

Answer

Thiamine

Answer

Biotin

Question 8

Which of the following is the most appropriate screening test for vitamin D deficiency?

Accepted Answer

25 hydroxy vitamin D

Answer

7-dehydrocholestrol

Answer

1,25 dihydroxy vitamin D

Answer

Serum calcium levels

Question 9

The gene responsible for folic acid transport is situated on which chromosome?

Accepted Answer

Chromosome 21

Answer

Chromosome 10

Answer

Chromosome 5

Answer

X chromosome

Question 10

Which vitamin is absent in eggs?

Accepted Answer

Vitamin C

Answer

Vitamin B1

Answer

Vitamin B6

Answer

Vitamin E

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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