Vitamins and Coenzymes Practice Questions

Q: Maize is pellagragenic due to excess content of which amino acid?

Leucine. **Explanation:** Pellagra is a clinical deficiency of **Niacin (Vitamin B3)**. While maize (corn) is naturally low in Tryptophan (the precursor for niacin synthesis) and contains niacin in a bound, unabsorbable form (niacytin), its **pellagragenic** nature is primarily attributed to a high concentration of **Leucine**. **Why Leucine is the correct answer:** High dietary intake of Leucine interferes with the metabolic pathway that converts Tryptophan to Niacin (the Kynurenine pathway). Specifically, excess Leucine inhibits the enzyme **Quinolinate Phosphoribosyl Transferase (QPRT)**. This inhibition prevents the formation of Nicotinic Acid Mononucleotide, thereby reducing the endogenous synthesis of NAD/NADP and precipitating pellagra symptoms even if some tryptophan is present. **Analysis of Incorrect Options:** * **Lysine & Threonine:** Maize is actually **deficient** in these essential amino acids. Their absence contributes to protein-energy malnutrition but does not directly cause the biochemical block leading to pellagra. * **Tryptophan:** Maize is deficient in Tryptophan. While this deficiency contributes to the development of pellagra, the question specifically asks which amino acid is present in **excess**, making Leucine the correct choice. **Clinical Pearls for NEET-PG:** * **The 4 D’s of Pellagra:** Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **The 60:1 Rule:** 60 mg of dietary Tryptophan yields 1 mg of Niacin. * **Hartnup Disease:** A genetic cause of pellagra due to impaired intestinal and renal transport of neutral amino acids (including Tryptophan). * **Carcinoid Syndrome:** Can lead to secondary pellagra because Tryptophan is diverted toward the overproduction of Serotonin.

Q: A 50-year-old chronic alcoholic presented with acute confusion, delirium, ataxia, and mild memory loss. Ophthalmoplegia and hypotension were noted on ophthalmoscopic examination. Serum electrolyte levels were normal, but lab findings revealed increased RBC transketolase activity along with reduced blood lactate levels. A CT scan of the brain showed no findings. Which of the following enzymes uses the vitamin deficient in this patient as a coenzyme?

Pyruvate dehydrogenase. **Explanation:** The patient presents with the classic triad of **Wernicke’s Encephalopathy** (confusion, ataxia, and ophthalmoplegia), a condition caused by a deficiency of **Vitamin B1 (Thiamine)**, commonly seen in chronic alcoholics. **Why Pyruvate Dehydrogenase is Correct:** Thiamine is converted into its active form, **Thiamine Pyrophosphate (TPP)**. TPP serves as a vital coenzyme for several key oxidative decarboxylation reactions: 1. **Pyruvate Dehydrogenase (PDH):** Converts pyruvate to Acetyl-CoA (linking glycolysis to the TCA cycle). 2. **$\alpha$-Ketoglutarate Dehydrogenase:** A key enzyme in the TCA cycle. 3. **Branched-chain $\alpha$-ketoacid dehydrogenase:** Involved in the metabolism of Leucine, Isoleucine, and Valine. 4. **Transketolase:** An enzyme in the Hexose Monophosphate (HMP) shunt. (Note: The question mentions increased transketolase activity *after* TPP addition in lab tests, confirming deficiency). **Analysis of Incorrect Options:** * **B. Transaminase:** These enzymes (e.g., ALT, AST) require **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * **C. Xanthine dehydrogenase:** This enzyme, involved in purine catabolism, requires **Molybdenum**, FAD, and Iron. * **D. Ornithine decarboxylase:** This is the rate-limiting enzyme in polyamine synthesis and requires **Vitamin B6**. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Wernicke’s is a clinical diagnosis. The "gold standard" lab test is the **Erythrocyte Transketolase Activation Test** (activity increases >15% upon adding TPP). * **Management:** Always administer **Thiamine before Glucose** in alcoholic patients. Giving glucose first can precipitate Wernicke’s by consuming the remaining thiamine stores during glycolysis. * **Progression:** If untreated, it progresses to **Korsakoff Psychosis**, characterized by irreversible confabulation and anterograde amnesia.

Q: Which of the following is NOT seen with vitamin C toxicity?

Thrombosis. **Explanation:** Vitamin C (Ascorbic acid) is a water-soluble vitamin with a wide therapeutic index; however, megadoses (typically >2g/day) can lead to specific toxic effects. **Why Thrombosis is the correct answer:** Thrombosis is **not** a recognized complication of Vitamin C toxicity. In fact, Vitamin C is essential for collagen synthesis (prolyl and lysyl hydroxylase cofactor); while deficiency leads to impaired vessel wall integrity and bleeding (Scurvy), excess intake does not pathologically trigger the coagulation cascade or platelet aggregation to cause thrombosis. **Analysis of Incorrect Options:** * **Abdominal pain:** High doses of Vitamin C act as an osmotic laxative in the gastrointestinal tract, leading to abdominal cramps, bloating, and diarrhea. * **Kidney stones:** Ascorbic acid is metabolically converted into **oxalate**. Excessive intake increases urinary oxalate levels (hyperoxaluria), which promotes the formation of **calcium oxalate stones**, especially in predisposed individuals. * **Hemolysis:** In patients with **G6PD deficiency**, high doses of Vitamin C (a reducing agent) can paradoxically induce oxidative stress or alter the redox balance, leading to acute hemolysis. **High-Yield Clinical Pearls for NEET-PG:** * **Biological Function:** Acts as a co-factor for **hydroxylation** of proline and lysine residues in collagen. * **Iron Absorption:** Vitamin C enhances the absorption of **non-heme iron** by maintaining it in the ferrous ($Fe^{2+}$) state. * **Scurvy:** Characterized by "corkscrew hair," perifollicular hemorrhages, and swollen, bleeding gums. * **Rebound Scurvy:** Can occur in neonates if the mother took megadoses during pregnancy, due to the induction of degradative enzymes.

Q: A 38-year-old female patient on isoniazid therapy for tuberculosis developed rashes on sun-exposed areas of her body and disorientation. Family members also report diarrhea. What is the most likely diagnosis?

Niacin deficiency. **Explanation:** The patient is presenting with the classic triad of **Pellagra**: Dermatitis (sun-exposed rashes), Diarrhea, and Dementia (disorientation). This condition is caused by a deficiency of **Niacin (Vitamin B3)**. **Why Niacin deficiency is the correct answer:** Isoniazid (INH) is a structural analog of **Pyridoxine (Vitamin B6)**. It interferes with B6 metabolism in two ways: it increases its excretion and inhibits the enzyme *pyridoxal kinase*. Vitamin B6 is a crucial cofactor for the enzyme **Kynureninase**, which converts the amino acid Tryptophan into Niacin. Therefore, prolonged INH therapy leads to a secondary Niacin deficiency, manifesting as Pellagra. **Analysis of Incorrect Options:** * **A. Tuberculosis skin lesions:** While TB can cause skin manifestations (like Lupus Vulgaris), they do not typically present as photosensitive rashes associated with systemic symptoms like diarrhea and disorientation. * **C. Isoniazid-induced neuropathy:** While INH commonly causes peripheral neuropathy due to B6 deficiency, this presents as numbness and paresthesia in a "glove and stocking" distribution, not as dermatitis or diarrhea. * **D. Adverse drug reaction:** While many drugs cause rashes, the specific combination of the "3 Ds" (Dermatitis, Diarrhea, Dementia) in the context of INH therapy specifically points to the metabolic pathway of Niacin. **High-Yield NEET-PG Pearls:** * **The 4 Ds of Pellagra:** Dermatitis, Diarrhea, Dementia, and Death (if untreated). * **Casal’s Necklace:** A characteristic broad-collar rash around the neck seen in Pellagra. * **Hartnup Disease:** A genetic defect in neutral amino acid (Tryptophan) transporters that also leads to Pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause Niacin deficiency because Tryptophan is diverted to produce excessive Serotonin.

Q: Tryptophan loading test is done to detect deficiency of:

Pyridoxal phosphate (PLP). ### Explanation **1. Why Pyridoxal Phosphate (PLP) is Correct:** The Tryptophan loading test is a functional biochemical assay used to detect **Vitamin B6 (Pyridoxine)** deficiency. Tryptophan is normally metabolized via the **Kynurenine pathway** to produce Niacin (Vitamin B3). A key enzyme in this pathway, **Kynureninase**, is strictly dependent on **Pyridoxal Phosphate (PLP)** as a coenzyme. In B6 deficiency, Kynureninase activity is impaired. Consequently, the intermediate metabolite **3-hydroxykynurenine** cannot be converted further down the primary pathway. Instead, it is shunted toward an alternative route, leading to the excessive production and urinary excretion of **Xanthurenic acid**. In this test, a loading dose of Tryptophan is given; if high levels of Xanthurenic acid are found in the urine, it confirms a functional B6 deficiency. **2. Why Other Options are Incorrect:** * **Thiamine (TPP):** Deficiency is typically assessed via the **Erythrocyte Transketolase activation test** or by measuring blood lactate/pyruvate levels. * **Biotin:** Deficiency is rare and usually monitored through urinary excretion of **3-hydroxyisovaleric acid**, not tryptophan metabolites. * **Folate:** Deficiency is detected using the **FIGLU (Formiminoglutamic acid) excretion test** following a Histidine load, or by measuring serum/RBC folate levels. **3. Clinical Pearls for NEET-PG:** * **The "Rule of 60":** 60 mg of Tryptophan yields 1 mg of Niacin. * **Drug-Induced Deficiency:** **Isoniazid (INH)**, used in TB treatment, inhibits Pyridoxine, leading to peripheral neuropathy. Always co-administer B6 with INH. * **Xanthurenic Aciduria:** This is the hallmark biochemical finding in the Tryptophan load test for B6 deficiency. * **Other PLP-dependent enzymes:** Transaminases (ALT/AST), ALA synthase (heme synthesis), and DOPA decarboxylase.

Q: Diagnosis of folic acid deficiency is done by all the following methods except?

Transketolase activity. **Explanation:** The diagnosis of folic acid deficiency involves assessing biochemical markers related to one-carbon metabolism. **Why Transketolase activity is the correct answer:** **Transketolase** is a thiamine (Vitamin B1) dependent enzyme in the Pentose Phosphate Pathway. Measuring erythrocyte transketolase activity (ETKA) is the gold standard for diagnosing **Thiamine deficiency**, not folic acid deficiency. An increase in enzyme activity after adding Thiamine Pyrophosphate (TPP) confirms a B1 deficiency. **Analysis of other options (Methods used for Folic Acid deficiency):** * **FIGLU estimation:** Formiminoglutamate (FIGLU) is an intermediate in histidine catabolism. Its conversion to glutamate requires Tetrahydrofolate (THF). In folate deficiency, FIGLU cannot be converted and is excreted in excess in the urine, especially after a histidine load test. * **Peripheral blood smears:** Folate deficiency leads to **Megaloblastic anemia**. A blood smear typically shows macro-ovalocytes and **hypersegmented neutrophils** (more than 5 lobes), which is a classic diagnostic hallmark. * **AICAR estimation:** Aminoimidazole carboxamide ribotide (AICAR) is an intermediate in purine synthesis. Its conversion requires folate. In deficiency, AICAR accumulates and is excreted in the urine (AICAR-uria). **NEET-PG High-Yield Pearls:** * **Most sensitive indicator:** Serum folate reflects recent intake; **RBC folate** is a better indicator of long-term body stores. * **The Folate Trap:** Vitamin B12 deficiency leads to functional folate deficiency by trapping folate as N5-methyl-THF. * **Homocysteine:** Levels are elevated in both B12 and Folate deficiency, but **Methylmalonic acid (MMA)** is elevated *only* in B12 deficiency.

Q: An alcoholic patient with chronic thiamine deficiency shows central nervous system manifestations, including memory loss and confabulatory psychosis. What condition is this patient likely suffering from?

Wernicke-Korsakoff syndrome. **Explanation:** The correct answer is **Wernicke-Korsakoff syndrome (WKS)**. This condition represents a spectrum of neurological disorders caused by a severe deficiency of **Vitamin B1 (Thiamine)**, commonly seen in chronic alcoholics due to poor dietary intake and impaired absorption. Thiamine is a critical cofactor for enzymes in glucose metabolism, specifically **Pyruvate Dehydrogenase**, **$\alpha$-Ketoglutarate Dehydrogenase**, and **Transketolase**. In its absence, ATP production in the brain drops, leading to neuronal damage. WKS consists of two phases: 1. **Wernicke’s Encephalopathy:** An acute, reversible stage characterized by the triad of ophthalmoplegia (eye muscle paralysis), ataxia (unsteady gait), and confusion. 2. **Korsakoff Psychosis:** A chronic, irreversible stage characterized by **anterograde amnesia** (memory loss) and **confabulation** (making up stories to fill memory gaps). The presence of these psychiatric symptoms in this patient confirms the progression to Wernicke-Korsakoff syndrome. **Why other options are incorrect:** * **Option A:** Wernicke's encephalopathy is the acute phase. Since the patient already exhibits memory loss and confabulation, the condition has progressed to the full syndrome. * **Option C:** Menke’s syndrome is a genetic disorder of **Copper absorption** (ATP7A mutation) leading to "kinky hair" and growth failure. * **Option D:** Wilson’s disease is a disorder of **Copper excretion** (ATP7B mutation) leading to hepatolenticular degeneration and Kayser-Fleischer rings. **High-Yield NEET-PG Pearls:** * **Diagnosis:** Erythrocyte **Transketolase activity** assay (activity increases upon adding thiamine). * **Imaging:** MRI may show atrophy of the **mammillary bodies**. * **Treatment Rule:** Always administer **Thiamine before Glucose** in alcoholic patients to prevent precipitating acute Wernicke’s encephalopathy.

Q: Which of the following vitamins is involved in the electron transport chain?

Riboflavin. **Explanation:** The Electron Transport Chain (ETC) relies on specific coenzymes to transfer electrons through various complexes. **Riboflavin (Vitamin B2)** is the precursor for **FMN (Flavin Mononucleotide)** and **FAD (Flavin Adenine Dinucleotide)**. In the ETC, FMN is a crucial component of **Complex I** (NADH dehydrogenase), while FAD is an integral part of **Complex II** (Succinate dehydrogenase). These flavoproteins act as prosthetic groups that undergo reversible redox reactions to facilitate electron flow toward Oxygen. **Analysis of Options:** * **Riboflavin (B2):** Correct. It forms FMN and FAD, which are essential electron carriers in Complexes I and II. * **Thiamine (B1):** Incorrect. Its active form, TPP, is a cofactor for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase) but does not participate directly in the ETC. * **Nicotinic acid (B3):** While NAD+ (derived from B3) carries electrons *to* the ETC, it is technically considered a substrate that dissociates from enzymes, whereas the question specifically targets the structural/functional vitamins integrated *within* the chain's complexes (flavoproteins). *Note: In some contexts, B3 is also involved, but B2 is the classic answer for "integral" ETC components.* * **Vitamin B12:** Incorrect. It is involved in DNA synthesis and the conversion of propionyl-CoA to succinyl-CoA, but not the ETC. **High-Yield NEET-PG Pearls:** * **Complex II** is the only enzyme of the TCA cycle (Succinate dehydrogenase) that is also a component of the ETC. * **Iron-Sulfur (Fe-S) clusters** are present in Complexes I, II, and III and are essential for single-electron transfers. * **Inhibitors:** Remember Rotenone (Complex I), Antimycin A (Complex III), and Cyanide/CO (Complex IV) for related MCQ patterns.

Q: What is the role of Vitamin K in the activation of clotting factors?

Carboxylation. **Explanation:** **Why Carboxylation is correct:** Vitamin K acts as a vital co-factor for the enzyme **$\gamma$-glutamyl carboxylase**. This enzyme catalyzes the **post-translational modification** of specific glutamic acid residues into **$\gamma$-carboxyglutamic acid (Gla)** on clotting factors **II, VII, IX, and X** (as well as proteins C and S). This addition of a second carboxyl group gives the proteins a negative charge, allowing them to bind to positively charged **Calcium ($Ca^{2+}$) ions**. This calcium bridge enables the clotting factors to anchor onto the phospholipid surfaces of platelets, which is essential for the coagulation cascade to proceed. **Why the other options are incorrect:** * **B. Hydroxylation:** This is primarily associated with **Vitamin C**, which acts as a co-factor for prolyl and lysyl hydroxylase in collagen synthesis. * **C. Transamination:** This process involves the transfer of an amino group, requiring **Pyridoxal Phosphate (Vitamin B6)** as a co-enzyme. * **D. Deamination:** This is the removal of an amino group (e.g., oxidative deamination by glutamate dehydrogenase) and does not involve Vitamin K. **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the regeneration of active Vitamin K (hydroquinone), thereby inhibiting carboxylation. * **Newborns:** They are deficient in Vitamin K due to sterile guts and poor placental transfer; hence, a prophylactic IM dose is given at birth to prevent **Hemorrhagic Disease of the Newborn**. * **Lab Marker:** Vitamin K deficiency or Warfarin use is monitored using **Prothrombin Time (PT/INR)**.

Q: Excessive intake of which vitamin can cause metabolic bone disease?

Vitamin A. **Explanation:** **Vitamin A (Retinol)** is the correct answer because hypervitaminosis A has a direct stimulatory effect on osteoclasts and an inhibitory effect on osteoblasts. Excessive intake leads to increased bone resorption, cortical thinning, and a higher risk of fractures. Clinically, this manifests as bone pain, hypercalcemia, and premature epiphyseal closure in children. Chronic toxicity also suppresses Vitamin D-dependent intestinal calcium absorption, further exacerbating metabolic bone disease. **Why other options are incorrect:** * **Vitamin B:** These are water-soluble vitamins. Excess intake is generally excreted in the urine and does not cause bone pathology. * **Vitamin C:** Deficiency (Scurvy) causes bone issues due to defective collagen synthesis; however, toxicity is rare and typically results in renal stones (oxalate) rather than metabolic bone disease. * **Vitamin D:** While Vitamin D toxicity causes hypercalcemia and metastatic calcification (soft tissue calcification), it is primarily associated with bone *resorption* only at extreme levels. However, in the context of standard medical examinations, **Vitamin A** is the classic answer specifically linked to the clinical syndrome of "metabolic bone disease" and cortical thickening/fractures. **High-Yield Clinical Pearls for NEET-PG:** * **Acute Vit A Toxicity:** Presents with **Pseudotumor cerebri** (idiopathic intracranial hypertension), headache, and vomiting. * **Chronic Vit A Toxicity:** Presents with alopecia, hepatosplenomegaly, skin desquamation, and bone pain. * **Teratogenicity:** Isotretinoin (a Vit A derivative) is highly teratogenic; a negative pregnancy test is mandatory before prescription. * **Golden Rule:** While Vitamin D *deficiency* causes Rickets/Osteomalacia, Vitamin A *excess* causes bone fragility.

Question 1

Maize is pellagragenic due to excess content of which amino acid?

Accepted Answer

Leucine

Answer

Lysine

Answer

Threonine

Answer

Tryptophan

Question 2

A 50-year-old chronic alcoholic presented with acute confusion, delirium, ataxia, and mild memory loss. Ophthalmoplegia and hypotension were noted on ophthalmoscopic examination. Serum electrolyte levels were normal, but lab findings revealed increased RBC transketolase activity along with reduced blood lactate levels. A CT scan of the brain showed no findings. Which of the following enzymes uses the vitamin deficient in this patient as a coenzyme?

Accepted Answer

Pyruvate dehydrogenase

Answer

Transaminase

Answer

Xanthine dehydrogenase

Answer

Ornithine decarboxylase

Question 3

Which of the following is NOT seen with vitamin C toxicity?

Accepted Answer

Thrombosis

Answer

Abdominal pain

Answer

Kidney stones

Answer

Hemolysis

Question 4

A 38-year-old female patient on isoniazid therapy for tuberculosis developed rashes on sun-exposed areas of her body and disorientation. Family members also report diarrhea. What is the most likely diagnosis?

Accepted Answer

Niacin deficiency

Answer

Tuberculosis skin lesions

Answer

Isoniazid-induced neuropathy

Answer

Adverse drug reaction to another medication

Question 5

Tryptophan loading test is done to detect deficiency of:

Accepted Answer

Pyridoxal phosphate (PLP)

Answer

Thiamine pyrophosphate (TPP)

Answer

Biotin

Answer

Folate

Question 6

Diagnosis of folic acid deficiency is done by all the following methods except?

Accepted Answer

Transketolase activity

Answer

FIGLU estimation

Answer

Peripheral blood smears

Answer

AICAR estimation

Question 7

An alcoholic patient with chronic thiamine deficiency shows central nervous system manifestations, including memory loss and confabulatory psychosis. What condition is this patient likely suffering from?

Accepted Answer

Wernicke-Korsakoff syndrome

Answer

Wernicke's encephalopathy

Answer

Menke's syndrome

Answer

Wilson's disease

Question 8

Which of the following vitamins is involved in the electron transport chain?

Accepted Answer

Riboflavin

Answer

Thiamine

Answer

Nicotinic acid

Answer

Vitamin B12

Question 9

What is the role of Vitamin K in the activation of clotting factors?

Accepted Answer

Carboxylation

Answer

Hydroxylation

Answer

Transamination

Answer

Deamination

Question 10

Excessive intake of which vitamin can cause metabolic bone disease?

Accepted Answer

Vitamin A

Answer

Vitamin B

Answer

Vitamin C

Answer

Vitamin D

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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