Vitamins and Coenzymes Practice Questions

Q: What is the coenzyme for methylmalonyl-CoA isomerase?

Cobalamine. ### Explanation **Correct Answer: B. Cobalamine (Vitamin B12)** The enzyme **methylmalonyl-CoA isomerase** (also known as methylmalonyl-CoA mutase) is responsible for converting L-methylmalonyl-CoA into **succinyl-CoA**, which then enters the TCA cycle. This reaction is a critical step in the catabolism of odd-chain fatty acids and certain amino acids (Valine, Isoleucine, Methionine, Threonine). This enzyme requires **Adenosylcobalamin** (a derivative of Vitamin B12) as a mandatory coenzyme. **Why the other options are incorrect:** * **A. Biotin (B7):** Biotin acts as a coenzyme for **carboxylation** reactions. It is involved in the *previous* step of this pathway, where propionyl-CoA is converted to methylmalonyl-CoA by *propionyl-CoA carboxylase*. * **C. Thiamine (B1):** Thiamine (as TPP) is a cofactor for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase, Alpha-ketoglutarate Dehydrogenase) and transketolase reactions. * **D. Niacin (B3):** Niacin (as NAD/NADP) is involved in redox reactions (electron transfer), not isomerization or carbon skeleton rearrangement. **Clinical Pearls for NEET-PG:** 1. **Methylmalonic Aciduria:** A deficiency in either Vitamin B12 or the mutase enzyme leads to an accumulation of methylmalonic acid in the blood and urine, causing metabolic acidosis and developmental delay. 2. **B12 vs. Folate:** Both B12 and Folate deficiencies cause megaloblastic anemia. However, **only B12 deficiency** causes elevated methylmalonic acid levels and neurological symptoms (Subacute Combined Degeneration of the Spinal Cord) due to impaired myelin synthesis. 3. **Two B12 Reactions:** Remember that Vitamin B12 is a cofactor for only two human enzymes: **Methylmalonyl-CoA mutase** (Adenosyl-B12) and **Methionine synthase** (Methyl-B12).

Q: Hemorrhagic disease of the newborn presents with bleeding on the second day of birth. Which of the following clotting factors may be involved?

2, 7, 9, 10. **Explanation:** Hemorrhagic Disease of the Newborn (HDN), now commonly referred to as **Vitamin K Deficiency Bleeding (VKDB)**, occurs because neonates have low stores of Vitamin K at birth. Vitamin K is a vital co-factor for the post-translational carboxylation of specific clotting factors, rendering them functional. **Why Option A is Correct:** Vitamin K is essential for the activation of **Factors II (Prothrombin), VII, IX, and X**, as well as proteins C and S. In the absence of Vitamin K, these factors are synthesized in an inactive form (known as PIVKAs—Proteins Induced by Vitamin K Absence). Since the question describes bleeding on the second day (Classical VKDB), it directly relates to the deficiency of these specific factors. **Why Other Options are Incorrect:** * **Option B:** Factor 3 (Tissue Factor) is a cell surface glycoprotein and is not Vitamin K-dependent. * **Option C:** Factor 8 (Anti-hemophilic factor) is part of the intrinsic pathway but is not Vitamin K-dependent; its deficiency causes Hemophilia A. * **Option D:** Factor 5 (Proaccelerin) is a cofactor in the common pathway but its synthesis does not require Vitamin K. **NEET-PG High-Yield Pearls:** * **Classification of VKDB:** * **Early:** Within 24 hours (usually due to maternal drugs like Phenytoin or Warfarin). * **Classical:** Days 2–7 (due to low intake/sterile gut). * **Late:** Week 2 to 6 months (often presents with intracranial hemorrhage; associated with exclusive breastfeeding). * **Prophylaxis:** 1 mg of Vitamin K intramuscularly (IM) is administered to all newborns at birth to prevent this condition. * **Lab Findings:** Prolonged PT (earliest change) and aPTT; normal Platelet count and Fibrinogen.

Q: A chronic alcoholic has recently developed trouble with their ability to balance, becomes easily confused, and displays nystagmus. An assay of which of the following enzymes can determine a biochemical reason for these symptoms?

Transketolase. ### Explanation **1. Why Transketolase is the Correct Answer:** The patient is presenting with the classic triad of **Wernicke Encephalopathy** (Ataxia, Confusion, and Ophthalmoplegia/Nystagmus), which is caused by a deficiency of **Thiamine (Vitamin B1)**. Thiamine pyrophosphate (TPP) is a vital cofactor for several enzymes, including **Transketolase** (HMP Shunt), Pyruvate Dehydrogenase, and α-ketoglutarate dehydrogenase. In a clinical setting, the diagnosis is confirmed by measuring **Erythrocyte Transketolase Activity**. If the enzyme activity increases significantly upon the addition of TPP in vitro, it confirms a thiamine deficiency. This is the most reliable biochemical marker for B1 status. **2. Why the Other Options are Incorrect:** * **Isocitrate Dehydrogenase (A):** This is a rate-limiting enzyme of the TCA cycle. While it is essential for aerobic metabolism, it does not require Thiamine as a cofactor and is not used to assay B1 levels. * **Transaldolase (B):** Like transketolase, this enzyme is part of the non-oxidative phase of the HMP shunt. However, unlike transketolase, it does **not** require Thiamine (TPP) for its catalytic activity. * **Glyceraldehyde-3-phosphate dehydrogenase (C):** This is a key enzyme in glycolysis. It requires NAD+ as a cofactor, not Thiamine. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "ATP" Mnemonic:** Thiamine is a cofactor for **A**lpha-ketoglutarate dehydrogenase, **T**ransketolase, and **P**yruvate dehydrogenase. * **Wernicke-Korsakoff Syndrome:** If untreated, Wernicke’s progresses to Korsakoff psychosis (irreversible confabulations and anterograde amnesia) involving damage to the **mammillary bodies**. * **Glucose Administration:** Never give IV glucose to a suspected alcoholic before Thiamine. Glucose oxidation consumes remaining Thiamine stores, potentially precipitating acute Wernicke encephalopathy.

Q: The use of broad-spectrum antibiotics can produce a bleeding diathesis characterized by hematomas, hematuria, melena, and bleeding from the gums by decreasing the normal gut flora and inducing a deficiency of:

Vitamin K. **Explanation:** **Correct Option: D (Vitamin K)** Vitamin K is a fat-soluble vitamin essential for the post-translational modification (gamma-carboxylation of glutamate residues) of clotting factors **II, VII, IX, and X**, as well as proteins C and S. While we obtain Vitamin K1 (phylloquinone) from green leafy vegetables, a significant portion of our Vitamin K2 (menaquinone) requirement is synthesized by **commensal bacteria in the large intestine**. Broad-spectrum antibiotics (like cephalosporins or sulfonamides) decimate this normal gut flora. This leads to a Vitamin K deficiency, resulting in non-functional clotting factors. Clinically, this manifests as a **bleeding diathesis** (increased prothrombin time/PT), characterized by easy bruising, hematuria, and mucosal bleeding. **Why Incorrect Options are Wrong:** * **Vitamin A:** Deficiency primarily affects vision (night blindness) and epithelial integrity (xerophthalmia), not the coagulation cascade. * **Vitamin B1 (Thiamine):** Deficiency leads to Beriberi or Wernicke-Korsakoff syndrome, affecting energy metabolism and neurological function. * **Vitamin B6 (Pyridoxine):** Deficiency typically causes sideroblastic anemia, peripheral neuropathy, or convulsions, but does not cause acute bleeding. **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** It inhibits **Vitamin K Epoxide Reductase (VKOR)**, mimicking a deficiency state. * **Newborns:** They have a sterile gut and poor placental transfer of Vitamin K, necessitating a prophylactic IM injection at birth to prevent **Hemorrhagic Disease of the Newborn**. * **Lab Findings:** Vitamin K deficiency prolongs **PT/INR** (Factor VII has the shortest half-life) and eventually aPTT.

Q: Vitamin B6 deficiency causes increased excretion of which of the following substances?

Xanthurenic acid. **Explanation:** **1. Why Xanthurenic Acid is Correct:** Vitamin B6 (Pyridoxine) in its active form, **Pyridoxal Phosphate (PLP)**, is a vital cofactor in the **Kynurenine pathway**, which converts the amino acid Tryptophan to Niacin (Vitamin B3). A key enzyme in this pathway, **Kynureninase**, is highly dependent on PLP. In Vitamin B6 deficiency, Kynureninase activity is impaired. This leads to a metabolic "bottleneck," causing an accumulation of the intermediate metabolite **3-hydroxykynurenine**. This excess metabolite is diverted into an alternate pathway where it is transaminated into **Xanthurenic acid**, which is then excreted in large amounts in the urine. This is the basis of the "Tryptophan Load Test" used to diagnose B6 deficiency. **2. Why the Other Options are Incorrect:** * **A. Methylmalonyl CoA:** Increased levels are seen in **Vitamin B12 deficiency**. B12 is a cofactor for Methylmalonyl-CoA mutase; its absence leads to Methylmalonic aciduria. * **C. Branched chain keto acids:** These accumulate in **Maple Syrup Urine Disease (MSUD)** due to a deficiency in the Branched-chain alpha-keto acid dehydrogenase complex (which requires B1, B2, B3, B5, and Lipoic acid, but not B6). * **D. Ketone bodies:** These increase during starvation, uncontrolled Diabetes Mellitus, or chronic alcoholism, unrelated to B6 status. **3. NEET-PG High-Yield Pearls:** * **Tryptophan Load Test:** Clinical gold standard for detecting B6 deficiency; look for elevated urinary Xanthurenic acid. * **Isoniazid (INH) Connection:** This anti-TB drug inhibits Pyridoxine kinase, leading to B6 deficiency, peripheral neuropathy, and sideroblastic anemia. * **Sideroblastic Anemia:** B6 is a cofactor for **ALA Synthase** (the rate-limiting step of heme synthesis). Deficiency leads to microcytic hypochromic anemia with ringed sideroblasts.

Q: Which of the following may be seen in children with Vitamin C excess?

Oxaluria. **Explanation:** **Vitamin C (Ascorbic Acid)** is a water-soluble vitamin that acts as a potent antioxidant and a coenzyme for prolyl and lysyl hydroxylase (essential for collagen synthesis). While generally safe due to its water solubility, megadoses can lead to specific toxicities. **Why Oxaluria is the correct answer:** The primary metabolic pathway for the degradation of Vitamin C involves its conversion into **oxalate**. Excessive intake of Vitamin C increases the urinary excretion of oxalate (**oxaluria**). Since oxalate can bind with calcium in the renal tubules, chronic megadoses significantly increase the risk of **calcium oxalate nephrolithiasis** (kidney stones), especially in predisposed individuals and children. **Analysis of Incorrect Options:** * **A. Alopecia:** This is not associated with Vitamin C excess. It is more commonly linked to Vitamin A toxicity or deficiencies in Biotin and Zinc. * **C. Peripheral neuropathy:** This is a classic manifestation of **Vitamin B6 (Pyridoxine) toxicity** (sensory neuropathy) or Vitamin B12 deficiency. * **D. Benign intracranial hypertension (Pseudotumor cerebri):** This is a hallmark sign of **Vitamin A toxicity** (Hypervitaminosis A), along with papilledema and skin peeling. **NEET-PG High-Yield Pearls:** * **Vitamin C and Iron:** Vitamin C enhances the absorption of **non-heme iron** by keeping it in the ferrous ($Fe^{2+}$) state. Excess Vitamin C can lead to iron overload in patients with hemochromatosis. * **Scurvy:** Deficiency leads to "corkscrew hair," perifollicular hemorrhages, and easy bruising due to defective collagen cross-linking. * **False Negatives:** High doses of Vitamin C can interfere with glucose oxidase-based urine dipstick tests, leading to **false-negative glucosuria** results.

Q: Pyridoxine hypervitaminosis causes which of the following?

Polyneuropathy. **Explanation:** **Pyridoxine (Vitamin B6)** is unique among water-soluble vitamins because, unlike most others that are easily excreted in the urine, chronic high-dose intake can lead to significant toxicity. **Why Polyneuropathy is correct:** Hypervitaminosis B6 (typically occurring at doses >200 mg/day over long periods) leads to **sensory polyneuropathy**. The underlying mechanism involves the accumulation of pyridoxine in the dorsal root ganglia, leading to neuronal degeneration. Patients typically present with progressive sensory loss, paresthesia, and ataxia (sensory ataxia), while motor strength usually remains intact. This is a high-yield "paradox" because while B6 deficiency causes neuropathy, its excess does as well. **Why the other options are incorrect:** * **Hypercalcemia:** This is characteristic of **Vitamin D toxicity**, where excessive calcium absorption from the gut and bone resorption occur. * **Liver damage:** While associated with high doses of **Vitamin B3 (Niacin)**—specifically hepatotoxicity and flushing—it is not a primary feature of Pyridoxine toxicity. * **Renal failure:** This is not a direct complication of Pyridoxine excess. However, Vitamin C (Ascorbic acid) megadoses can lead to oxalate kidney stones, potentially impacting renal function. **High-Yield Clinical Pearls for NEET-PG:** * **The B6 Paradox:** Both deficiency and toxicity of Vitamin B6 cause peripheral neuropathy. * **Isoniazid (INH) Link:** INH therapy for TB induces B6 deficiency by inhibiting pyridoxine phosphokinase; always co-administer B6 to prevent "stocking and glove" neuropathy. * **Sideroblastic Anemia:** B6 is a cofactor for ALA synthase; its deficiency leads to microcytic hypochromic anemia with ringed sideroblasts. * **Therapeutic Use:** B6 is used to treat homocystinuria (cystathionine synthase deficiency) and PMS symptoms.

Question 1

What is the coenzyme for methylmalonyl-CoA isomerase?

Accepted Answer

Cobalamine

Answer

Biotin

Answer

Thiamine

Answer

Niacine

Question 2

Hemorrhagic disease of the newborn presents with bleeding on the second day of birth. Which of the following clotting factors may be involved?

Accepted Answer

2, 7, 9, 10

Answer

3, 7, 9, 10

Answer

2, 8, 9, 10

Answer

2, 5, 9, 10

Question 3

A chronic alcoholic has recently developed trouble with their ability to balance, becomes easily confused, and displays nystagmus. An assay of which of the following enzymes can determine a biochemical reason for these symptoms?

Accepted Answer

Transketolase

Answer

Isocitrate dehydrogenase

Answer

Transaldolase

Answer

Glyceraldehyde-3-phosphate dehydrogenase

Question 4

The use of broad-spectrum antibiotics can produce a bleeding diathesis characterized by hematomas, hematuria, melena, and bleeding from the gums by decreasing the normal gut flora and inducing a deficiency of:

Accepted Answer

Vitamin K

Answer

Vitamin A

Answer

Vitamin B1

Answer

Vitamin B6

Question 5

Vitamin B6 deficiency causes increased excretion of which of the following substances?

Accepted Answer

Xanthurenic acid

Answer

Methylmalonyl CoA

Answer

Branched chain keto acids

Answer

Ketone bodies

Question 6

In the treatment of undiagnosed megaloblastic anemia, vitamin B12 and folic acid should be given together because?

Accepted Answer

Folic acid alone causes improvement of anemic symptoms but neurological dysfunction continues.

Answer

Vitamin B12 acts as a cofactor for dihydrofolate reductase.

Answer

Vitamin B12 deficiency may result in methylfolate trap.

Answer

Folic acid is required for conversion of methylmalonyl-CoA to succinyl CoA.

Question 7

Which of the following may be seen in children with Vitamin C excess?

Accepted Answer

Oxaluria

Answer

Alopecia

Answer

Peripheral neuropathy

Answer

Benign intracranial hypertension

Question 8

Which vitamin deficiency is principally manifested by lesions of the mucocutaneous surfaces of the mouth and skin, corneal vascularization, anemia, and personality changes?

Accepted Answer

Riboflavin

Answer

Thiamine

Answer

Pyridoxine

Answer

Cyanocobalamin

Question 9

All of the following statements are true regarding Vitamin A deficiency except?

Accepted Answer

Anterior segment of the eye is initially involved

Answer

Growth retardation is common

Answer

Frequent infections can occur

Answer

Hydrocephalus is infrequent

Question 10

Pyridoxine hypervitaminosis causes which of the following?

Accepted Answer

Polyneuropathy

Answer

Hypercalcemia

Answer

Liver damage

Answer

Renal failure

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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