Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 581: ADEK deficiency is seen in which of the following conditions?

A. Malabsorption syndrome
B. Obstructive jaundice
C. Deficiency of pancreatic lipase
D. All of the above (Correct Answer)

Explanation: **Explanation:** The absorption of fat-soluble vitamins (**A, D, E, and K**) is inextricably linked to the digestion and absorption of dietary lipids. Any clinical condition that disrupts the emulsification, enzymatic digestion, or mucosal absorption of fats will lead to a secondary deficiency of these vitamins. 1. **Malabsorption Syndrome (Option A):** Conditions like Celiac disease, Crohn’s disease, or Tropical Sprue damage the intestinal mucosa. This reduces the surface area available for the absorption of micelles (which carry ADEK), leading to steatorrhea and vitamin deficiency. 2. **Obstructive Jaundice (Option B):** Bile salts are essential for the emulsification of fats and the formation of micelles. In obstructive jaundice (e.g., gallstones or head of pancreas cancer), bile cannot reach the duodenum. Without bile, fat-soluble vitamins cannot be solubilized for absorption. 3. **Deficiency of Pancreatic Lipase (Option C):** Chronic pancreatitis or Cystic Fibrosis leads to exocrine pancreatic insufficiency. Without lipase, dietary triglycerides are not broken down into fatty acids and monoglycerides, preventing the incorporation of ADEK into absorbable forms. **Conclusion:** Since all three mechanisms (lack of bile, lack of enzymes, and mucosal damage) impair fat absorption, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Steatorrhea:** The presence of foul-smelling, bulky, oily stools is a hallmark sign that ADEK deficiency may occur. * **Vitamin K:** Usually the first fat-soluble vitamin to show deficiency signs (increased PT/INR and bleeding tendencies) because it has the smallest body stores. * **Mineral Oil/Orlistat:** Chronic use of these (laxatives/weight loss drugs) can also cause ADEK deficiency by interfering with lipid absorption.

Question 582: A 32-year-old vegan woman presents with weakness and pallor. Laboratory studies reveal a macrocytic, normochromic anemia (hemoglobin = 6.2 g/dL). Which of the following vitamin deficiencies is most likely responsible for her condition?

A. Vitamin A
B. Vitamin B12 (Correct Answer)
C. Vitamin D
D. Vitamin E

Explanation: **Explanation:** The patient presents with **macrocytic, normochromic anemia** (elevated Mean Corpuscular Volume) and a history of a **strict vegan diet**. This is a classic presentation of **Vitamin B12 (Cobalamin) deficiency**. **Why Vitamin B12 is correct:** Vitamin B12 is essential for DNA synthesis. It acts as a cofactor for *methionine synthase*, which converts homocysteine to methionine. During this process, methyl-tetrahydrofolate is converted to tetrahydrofolate (THF). A deficiency leads to the "folate trap," where folate is stuck in its methylated form, impairing DNA synthesis in rapidly dividing erythroblasts. This results in **megaloblastic erythropoiesis**, where the nucleus matures slower than the cytoplasm, leading to large (macrocytic) red blood cells. Since B12 is found almost exclusively in animal products, strict vegans are at high risk. **Why other options are incorrect:** * **Vitamin A:** Deficiency typically presents with ocular symptoms (night blindness, Xerophthalmia) and follicular hyperkeratosis, not macrocytic anemia. * **Vitamin D:** Deficiency leads to Rickets (children) or Osteomalacia (adults) due to impaired calcium and phosphate homeostasis. * **Vitamin E:** Deficiency causes hemolytic anemia (due to oxidative stress on RBC membranes) and neurological symptoms mimicking Friedreich's ataxia, but not macrocytic anemia. **High-Yield Clinical Pearls for NEET-PG:** * **The "Folate Trap":** B12 deficiency causes a functional folate deficiency. * **Neurological Symptoms:** Unlike Folate deficiency, B12 deficiency presents with **Subacute Combined Degeneration (SCD)** of the spinal cord (involving dorsal columns and lateral corticospinal tracts). * **Biochemical Markers:** In B12 deficiency, both **Homocysteine** and **Methylmalonic Acid (MMA)** levels are elevated. In Folate deficiency, only Homocysteine is elevated. * **Storage:** B12 is stored in the liver for 3–5 years; therefore, deficiency takes years to develop after stopping intake.

Question 583: Which vitamin is a constituent of the Warburg yellow enzyme?

A. Vitamin B1
B. Vitamin B2 (Correct Answer)
C. Niacin
D. Pantothenic acid

Explanation: **Explanation:** The **Warburg yellow enzyme** (Old Yellow Enzyme) was the first flavoprotein to be discovered (by Otto Warburg and Christian in 1932). It contains **Riboflavin (Vitamin B2)** in the form of its active coenzyme, **FMN (Flavin Mononucleotide)**. 1. **Why Vitamin B2 is correct:** Riboflavin is the precursor for two vital coenzymes: FMN and FAD (Flavin Adenine Dinucleotide). These coenzymes act as prosthetic groups for various oxidoreductases (flavoproteins). The Warburg yellow enzyme specifically utilizes FMN to catalyze the oxidation of NADPH. 2. **Why other options are incorrect:** * **Vitamin B1 (Thiamine):** Its active form is Thiamine Pyrophosphate (TPP), which acts as a coenzyme for decarboxylation reactions (e.g., Pyruvate Dehydrogenase). * **Niacin (Vitamin B3):** It forms NAD+ and NADP+. While also involved in redox reactions, it is not the constituent of the "yellow" enzyme; niacin-dependent enzymes are generally colorless. * **Pantothenic acid (Vitamin B5):** It is a constituent of Coenzyme A (CoA) and the Acyl Carrier Protein (ACP), primarily involved in acyl group transfer. **High-Yield Clinical Pearls for NEET-PG:** * **The "Yellow" Clue:** Riboflavin derives its name from "flavos" (Latin for yellow). Its presence gives the enzyme its characteristic yellow color. * **Deficiency Sign:** Cheilosis, glossitis (magenta tongue), and corneal neovascularization are classic signs of B2 deficiency. * **Redox Potential:** Unlike NAD+, which transfers 2 electrons at once, FMN/FAD can transfer electrons one at a time, making them essential for the Electron Transport Chain (Complex I and II).

Question 584: Which mineral has an action similar to Vitamin E?

A. Calcium
B. Iron
C. Selenium (Correct Answer)
D. Magnesium

Explanation: **Explanation:** The correct answer is **Selenium (C)**. This relationship is a classic high-yield concept in biochemistry known as the **synergistic antioxidant effect**. **Why Selenium is correct:** Vitamin E (Tocopherol) and Selenium work together to protect cells from oxidative damage caused by free radicals. 1. **Vitamin E** acts as a chain-breaking antioxidant in cell membranes, preventing the peroxidation of polyunsaturated fatty acids (PUFAs). 2. **Selenium** is an essential cofactor for the enzyme **Glutathione Peroxidase**. This enzyme converts potentially damaging hydrogen peroxide and lipid hydroperoxides into harmless water and alcohols. By reducing the concentration of peroxides, Selenium "spares" Vitamin E, allowing it to remain effective for longer. Thus, they have a complementary, "sparing" action. **Why other options are incorrect:** * **Calcium (A):** Primarily involved in bone mineralization, muscle contraction, and blood coagulation; it has no direct antioxidant synergy with Vitamin E. * **Iron (B):** Actually acts as a pro-oxidant in high amounts via the **Fenton reaction**, which generates hydroxyl radicals—the opposite of Vitamin E’s function. * **Magnesium (D):** Acts as a cofactor for over 300 enzymes (mostly kinases involving ATP), but does not share an antioxidant mechanism with Vitamin E. **High-Yield Clinical Pearls for NEET-PG:** * **Glutathione Peroxidase:** Contains Selenium in the form of **Selenocysteine** (the 21st amino acid). * **Deficiency Synergy:** A deficiency in both Vitamin E and Selenium can lead to severe oxidative stress, manifesting as **Keshan Disease** (cardiomyopathy) or **White Muscle Disease** in animals. * **Vitamin C connection:** While Selenium works with Vitamin E in the peroxide pathway, Vitamin C (Ascorbic acid) is responsible for *regenerating* reduced Vitamin E from its oxidized state.

Question 585: 1,25-dihydroxyvitamin D is formed in which organ?

A. Skin
B. Liver
C. Spleen
D. Kidney (Correct Answer)

Explanation: **Explanation:** The synthesis of the active form of Vitamin D (Calcitriol) is a multi-step process involving the skin, liver, and kidneys. **1. Why Kidney is the Correct Answer:** The final and rate-limiting step in Vitamin D synthesis occurs in the **proximal convoluted tubules of the kidney**. Here, the enzyme **1-α-hydroxylase** converts 25-hydroxyvitamin D [25(OH)D] into **1,25-dihydroxyvitamin D [1,25(OH)₂D]**, also known as **Calcitriol**. This is the biologically active form of the vitamin that regulates calcium and phosphate homeostasis. **2. Why Other Options are Incorrect:** * **Skin:** This is the site of initial synthesis. Under UV-B light, 7-dehydrocholesterol is converted into Cholecalciferol (Vitamin D3). * **Liver:** This is the site of the first hydroxylation. The enzyme 25-hydroxylase converts Vitamin D3 into 25-hydroxyvitamin D (Calcidiol), which is the major storage form. * **Spleen:** The spleen has no physiological role in the hydroxylation or activation of Vitamin D. **3. NEET-PG High-Yield Clinical Pearls:** * **Rate-limiting enzyme:** 1-α-hydroxylase is stimulated by **PTH** (Parathyroid Hormone) and low serum phosphate, and inhibited by FGF-23 and high calcium. * **Storage Form:** 25-hydroxyvitamin D is the form measured clinically to assess a patient's Vitamin D status due to its long half-life. * **Chronic Kidney Disease (CKD):** Patients with CKD develop secondary hyperparathyroidism and renal osteodystrophy because the damaged kidneys cannot produce 1,25-dihydroxyvitamin D. * **Sarcoidosis:** Macrophages in granulomas contain 1-α-hydroxylase, leading to extra-renal production of Calcitriol and subsequent hypercalcemia.

Question 586: Which of the following vitamins has been evaluated for the treatment of fish odor syndrome?

A. Riboflavin (Correct Answer)
B. Thiamine
C. Biotin
D. Niacin

Explanation: **Explanation:** **Fish Odor Syndrome (Trimethylaminuria)** is a metabolic disorder characterized by the body's inability to break down **trimethylamine (TMA)**, a compound derived from the diet (choline, lecithin, and carnitine) that has a pungent, rotting fish smell. 1. **Why Riboflavin is correct:** The conversion of odorous TMA into the odorless trimethylamine N-oxide (TMAO) is catalyzed by the enzyme **Flavin-containing monooxygenase 3 (FMO3)**. Riboflavin (Vitamin B2) is a precursor to **FAD (Flavin Adenine Dinucleotide)**, which acts as an essential prosthetic group/cofactor for the FMO3 enzyme. High-dose riboflavin supplementation is used to maximize the residual activity of the mutant FMO3 enzyme, thereby reducing the excretion of the foul-smelling compound. 2. **Why other options are incorrect:** * **Thiamine (B1):** Acts as a cofactor for oxidative decarboxylation (e.g., Pyruvate dehydrogenase) and transketolase; it has no role in TMA metabolism. * **Biotin (B7):** Serves as a cofactor for carboxylation reactions (e.g., Pyruvate carboxylase). * **Niacin (B3):** Precursor for NAD/NADP, involved in redox reactions, but not the specific cofactor for the FMO3 enzyme system. **Clinical Pearls for NEET-PG:** * **Enzyme Defect:** FMO3 deficiency. * **Diagnosis:** Elevated levels of trimethylamine in urine. * **Management:** Low-choline diet (avoiding eggs, legumes, and certain fish) and **Riboflavin** supplementation. * **High-Yield Fact:** Riboflavin is also a cofactor for **Glutathione Reductase** (used to clinically test for B2 deficiency) and **Succinate Dehydrogenase** (Complex II of ETC).

Question 587: Proper functioning of the Krebs cycle does not occur with a deficiency of which of the following?

A. Thiamine (Correct Answer)
B. Riboflavin
C. Vitamin E
D. Vitamin K

Explanation: The correct answer is **Thiamine (Vitamin B1)**. ### **Explanation** The Krebs cycle (TCA cycle) relies on several multi-enzyme complexes that require specific coenzymes to function. Thiamine, in its active form **Thiamine Pyrophosphate (TPP)**, is a critical cofactor for the **Alpha-ketoglutarate dehydrogenase complex**. This enzyme catalyzes the oxidative decarboxylation of $\alpha$-ketoglutarate to Succinyl-CoA. Without thiamine, this step is blocked, leading to an accumulation of $\alpha$-ketoglutarate and a failure of the cycle to generate ATP and reducing equivalents (NADH). ### **Analysis of Incorrect Options** * **Riboflavin (B2):** While Riboflavin is a precursor for FAD (a cofactor for Succinate Dehydrogenase), the question asks for the most direct link often tested in clinical scenarios. In the context of "failure to function," Thiamine deficiency is the classic bottleneck. * **Vitamin E:** This is a lipid-soluble antioxidant that protects cell membranes from lipid peroxidation. It has no direct role as a coenzyme in the Krebs cycle. * **Vitamin K:** This is essential for the post-translational gamma-carboxylation of clotting factors (II, VII, IX, X) and proteins C and S. It is not involved in energy metabolism. ### **High-Yield Clinical Pearls for NEET-PG** * **The "Tender Loving Care For Nancy" Mnemonic:** Five cofactors are required by both **Pyruvate Dehydrogenase (PDH)** and **$\alpha$-Ketoglutarate Dehydrogenase**: **T**hiamine (B1), **L**ipoic acid, **C**oenzyme A (B5), **F**AD (B2), and **N**AD (B3). * **Clinical Correlation:** In chronic alcoholism (Wernicke-Korsakoff syndrome), thiamine deficiency impairs PDH and the Krebs cycle. Administering glucose before thiamine can worsen the condition because glucose metabolism consumes the remaining TPP, leading to severe lactic acidosis. * **Other TPP-dependent enzymes:** Transketolase (HMP Shunt) and Branched-chain $\alpha$-ketoacid dehydrogenase (deficient in Maple Syrup Urine Disease).

Question 588: Besides the 3 D's (Diarrhoea, Dermatitis & Dementia) of Niacin deficiency, what does the 4th D indicate?

A. Death (Correct Answer)
B. Destruction
C. Debilitating
D. Disability

Explanation: **Explanation:** Niacin (Vitamin B3) deficiency leads to a clinical condition known as **Pellagra**. Classically, Pellagra is characterized by a triad of symptoms known as the "3 Ds": **Diarrhoea, Dermatitis, and Dementia**. However, if the condition remains untreated, it progresses to the **4th D: Death**. 1. **Why "Death" is correct:** Pellagra is a systemic disease resulting from the lack of NAD and NADP, which are essential cofactors for redox reactions and DNA repair. Severe deficiency leads to multi-organ failure. The dermatitis is typically photosensitive (Casal’s necklace), the diarrhoea results from atrophy of the intestinal villi, and dementia arises from neuronal degeneration. Without niacin supplementation, these metabolic disruptions are fatal. 2. **Why other options are incorrect:** While Pellagra is certainly *debilitating* and can cause *disability* or tissue *destruction* (especially of the skin and mucosa), these are descriptive terms and not part of the formal medical mnemonic used to describe the clinical progression of the disease. **High-Yield Clinical Pearls for NEET-PG:** * **Amino Acid Precursor:** Niacin can be synthesized in the body from the amino acid **Tryptophan** (60 mg Tryptophan = 1 mg Niacin). * **Hartnup Disease:** A genetic disorder involving defective transport of neutral amino acids (Tryptophan), leading to pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause niacin deficiency because tryptophan is diverted to produce excessive Serotonin. * **Corn/Maize Staples:** Diets based primarily on corn lead to Pellagra because the niacin in corn is bound (**Niacytin**) and unavailable for absorption.

Question 589: 1,25-dihydroxyvitamin D is formed in which organ?

A. Skin
B. Liver
C. Spleen
D. Kidney (Correct Answer)

Explanation: ### Explanation The synthesis of the active form of Vitamin D (Calcitriol) is a multi-step process involving several organs. The correct answer is **Kidney** because it is the site of the final and rate-limiting hydroxylation step. **1. Why Kidney is Correct:** The precursor 25-hydroxycholecalciferol (Calcidiol), produced in the liver, travels to the **proximal convoluted tubules** of the kidney. Here, the enzyme **1-alpha-hydroxylase** adds a hydroxyl group to the 1st carbon position, converting it into **1,25-dihydroxycholecalciferol (Calcitriol)**. This is the most potent, biologically active form of Vitamin D. **2. Why Other Options are Incorrect:** * **Skin (Option A):** This is the site of **initial synthesis**. Under UV-B light, 7-dehydrocholesterol is converted into Cholecalciferol (Vitamin D3). * **Liver (Option B):** The liver performs the **first hydroxylation**. The enzyme 25-hydroxylase converts Vitamin D3 into 25-hydroxycholecalciferol (the major storage form). * **Spleen (Option C):** The spleen has no physiological role in the metabolic activation of Vitamin D. **3. NEET-PG High-Yield Pearls:** * **Rate-limiting Step:** The renal 1-alpha-hydroxylase step is the primary regulatory point, stimulated by **Parathyroid Hormone (PTH)** and low serum phosphate. * **Storage Form:** 25-hydroxycholecalciferol is the form measured clinically to assess a patient's Vitamin D status due to its long half-life. * **Chronic Kidney Disease (CKD):** Patients with CKD often develop secondary hyperparathyroidism and renal osteodystrophy because they cannot perform this final hydroxylation, leading to Calcitriol deficiency. * **Sarcoidosis:** In granulomatous diseases, macrophages can express 1-alpha-hydroxylase independently, leading to hypercalcemia.

Question 590: Thiamine is a co-enzyme for which of the following enzymes?

A. Transketolase (Correct Answer)
B. Alanine transferase
C. ALA synthase
D. Cystathionine synthase

Explanation: **Explanation:** **Thiamine (Vitamin B1)**, in its active form **Thiamine Pyrophosphate (TPP)**, serves as a vital co-enzyme for enzymes involved in oxidative decarboxylation and the Pentose Phosphate Pathway (PPP). 1. **Why Transketolase is Correct:** Transketolase is a key enzyme in the non-oxidative phase of the PPP, responsible for the interconversion of sugars. It requires TPP as a cofactor to transfer two-carbon units. Measuring **Erythrocyte Transketolase Activity (ETKA)** is the most sensitive laboratory method to diagnose thiamine deficiency. 2. **Why Other Options are Incorrect:** * **Alanine transferase (ALT):** This is a transaminase that requires **Pyridoxal Phosphate (PLP/Vitamin B6)** as a cofactor, not thiamine. * **ALA synthase:** The rate-limiting enzyme of heme synthesis requires **PLP (Vitamin B6)**. Deficiency leads to sideroblastic anemia. * **Cystathionine synthase:** Involved in the transsulfuration pathway (converting homocysteine to cystathionine), this enzyme also requires **PLP (Vitamin B6)**. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for TPP-dependent enzymes:** **"ATP"** * **A:** **A**lpha-ketoglutarate dehydrogenase (TCA cycle) * **T:** **T**ransketolase (HMP Shunt) * **P:** **P**yruvate dehydrogenase (Link reaction) * *Also: Branched-chain ketoacid dehydrogenase (deficient in Maple Syrup Urine Disease).* * **Clinical Deficiency:** Thiamine deficiency leads to **Beriberi** (Dry: polyneuritis; Wet: high-output heart failure) and **Wernicke-Korsakoff Syndrome** (triad of ataxia, ophthalmoplegia, and confusion), commonly seen in chronic alcoholics. * **Golden Rule:** Always administer thiamine *before* glucose in malnourished patients to prevent precipitating Wernicke encephalopathy.

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

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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

Practice Questions

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