Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 51: Pyruvate utilization is decreased in which of the following conditions?

A. Pernicious anemia
B. Scurvy
C. Folate trap
D. Beri-beri (Correct Answer)

Explanation: **Explanation:** The correct answer is **Beri-beri**, which is caused by a deficiency of **Vitamin B1 (Thiamine)**. **1. Why Beri-beri is correct:** Thiamine, in its active form **Thiamine Pyrophosphate (TPP)**, is a mandatory coenzyme for the **Pyruvate Dehydrogenase (PDH) complex**. This enzyme complex catalyzes the oxidative decarboxylation of pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. In Beri-beri, the lack of TPP leads to a failure of the PDH complex, resulting in decreased pyruvate utilization and a subsequent rise in blood lactate and pyruvate levels. TPP is also a cofactor for α-ketoglutarate dehydrogenase and branched-chain α-ketoacid dehydrogenase. **2. Why the other options are incorrect:** * **Pernicious Anemia:** Caused by Vitamin B12 deficiency (due to lack of intrinsic factor). B12 is a cofactor for Methionine Synthase and Methylmalonyl-CoA Mutase, not pyruvate metabolism. * **Scurvy:** Caused by Vitamin C deficiency. Vitamin C is essential for the hydroxylation of proline and lysine in collagen synthesis. * **Folate Trap:** This occurs in Vitamin B12 deficiency where folate is "trapped" as N5-methyl THF, leading to a functional folate deficiency and megaloblastic anemia, but it does not directly impair pyruvate utilization. **High-Yield Clinical Pearls for NEET-PG:** * **PDH Complex Cofactors:** Remember the mnemonic **"The Lovely Co-enzymes For Nerds"** — **T**hiamine (B1), **L**ipoic acid, **C**oA (B5), **F**AD (B2), and **N**AD (B3). * **Wernicke-Korsakoff Syndrome:** A neuropsychiatric manifestation of Thiamine deficiency often seen in alcoholics. * **Diagnostic Test:** Thiamine deficiency is best diagnosed by measuring **Erythrocyte Transketolase activity** (another TPP-dependent enzyme). * **Clinical Warning:** Always administer Thiamine *before* Glucose in a malnourished patient to prevent precipitating Wernicke’s encephalopathy due to sudden demand on the PDH complex.

Question 52: Which of the following is the major circulating form of Vitamin D?

A. 25-(OH) D3 (Correct Answer)
B. 1, 25-(OH)2 D3
C. 24, 25-(OH)2 D3
D. 1, 24-(OH)2 D3

Explanation: **Explanation:** Vitamin D metabolism involves two sequential hydroxylations. Cholecalciferol (Vitamin D3) is first transported to the liver, where the enzyme **25-hydroxylase** converts it into **25-hydroxycholecalciferol [25-(OH) D3]**, also known as **Calcidiol**. **Why Option A is Correct:** 25-(OH) D3 is the **major circulating form** of Vitamin D. It has a long half-life (approximately 2–3 weeks) and its serum concentration directly reflects the body's total Vitamin D stores (from both skin synthesis and diet). Therefore, it is the standard clinical marker used to diagnose Vitamin D deficiency. **Why Other Options are Incorrect:** * **Option B: 1, 25-(OH)2 D3 (Calcitriol):** This is the **biologically active form**. It is produced in the kidney by 1-alpha-hydroxylase. While it is the most potent form, it has a very short half-life (hours) and circulates at much lower concentrations (picograms vs. nanograms), making it unsuitable as a measure of nutritional status. * **Option C: 24, 25-(OH)2 D3:** This is an **inactive metabolite** produced by the kidney when Vitamin D levels are sufficient, acting as a pathway to divert metabolism away from the active form. * **Option D: 1, 24-(OH)2 D3:** This is a minor catabolic product with negligible physiological activity. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The 1-alpha-hydroxylation in the kidney (stimulated by PTH and low phosphate). * **Storage:** Vitamin D is stored primarily in adipose tissue, but 25-(OH) D3 is the main circulating reservoir. * **Clinical Marker:** Always choose **25-(OH) D** to assess Vitamin D status, NOT Calcitriol.

Question 53: A 40-year-old chronically ill man presents with painful sores around his mouth and prominent fissures at the angles of his mouth. Cheilosis in this patient is most likely caused by a deficiency of which of the following vitamins?

A. Vitamin A
B. Vitamin B1 (thiamine)
C. Vitamin B2 (riboflavin) (Correct Answer)
D. Vitamin B12

Explanation: **Explanation:** The clinical presentation of **cheilosis** (inflammation and cracking of the lips) and **angular stomatitis** (fissures at the corners of the mouth) is a classic manifestation of **Vitamin B2 (Riboflavin) deficiency**, also known as **ariboflavinosis**. **Why Vitamin B2 is correct:** Riboflavin is a precursor for the coenzymes **FMN (Flavin Mononucleotide)** and **FAD (Flavin Adenine Dinucleotide)**, which are essential for oxidation-reduction reactions in the mitochondrial electron transport chain and the TCA cycle. Deficiency typically occurs in chronic alcoholics or chronically ill patients due to poor dietary intake. Key clinical features include the "3 Cs": **C**heilosis, **C**orneal vascularization, and **C**onjunctivitis/Glossitis (magenta tongue). **Why other options are incorrect:** * **Vitamin A:** Deficiency primarily affects the eyes (Xerophthalmia, Bitot’s spots, Night blindness) and causes follicular hyperkeratosis, not oral fissures. * **Vitamin B1 (Thiamine):** Deficiency leads to Beriberi (Dry/Wet) or Wernicke-Korsakoff syndrome, characterized by neurological and cardiovascular symptoms rather than mucocutaneous lesions. * **Vitamin B12 (Cobalamin):** Deficiency causes megaloblastic anemia and subacute combined degeneration of the spinal cord (SCD). While it can cause glossitis, it is not the primary cause of isolated cheilosis. **High-Yield NEET-PG Pearls:** * **Magenta Tongue:** Characteristic of B2 deficiency (vs. "Beefy red tongue" in B12/B3 deficiency). * **Erythrocyte Glutathione Reductase Activity:** This is the functional gold-standard test to diagnose B2 deficiency (activity increases upon adding FAD). * **Phototherapy:** Newborns treated for neonatal jaundice with blue light are at risk for B2 deficiency as riboflavin is light-sensitive.

Question 54: Which vitamin is NOT synthesized by the intestinal bacterial flora?

A. Vitamin K
B. Biotin
C. Niacin (Correct Answer)
D. Pantothenic acid

Explanation: **Explanation:** The intestinal microbiome plays a crucial role in human nutrition by synthesizing several essential vitamins. However, **Niacin (Vitamin B3)** is not significantly produced by intestinal flora; instead, it is primarily obtained through dietary intake or synthesized endogenously in the liver from the essential amino acid **Tryptophan** (60 mg of Tryptophan yields 1 mg of Niacin). **Analysis of Options:** * **Vitamin K (Option A):** This is the classic example of a vitamin synthesized by gut bacteria (specifically *E. coli* and *Bacteroides* in the form of Menaquinone/K2). This is why newborns, who have a sterile gut, require a Vitamin K injection at birth to prevent hemorrhagic disease. * **Biotin (Option B):** Intestinal bacteria produce significant amounts of Biotin (B7). Clinical deficiency is rare unless there is excessive consumption of raw egg whites (avidin) or prolonged antibiotic use that depletes gut flora. * **Pantothenic acid (Option D):** Also known as Vitamin B5, it is synthesized by various enteric bacteria. While we also get it from many food sources (hence the name *pantos*, meaning "everywhere"), the gut flora contributes to the body's pool. **NEET-PG High-Yield Pearls:** 1. **Vitamins synthesized by gut flora:** Vitamin K, Biotin (B7), Pantothenic acid (B5), Pyridoxine (B6), Folate (B9), and Vitamin B12. 2. **The B12 Paradox:** Although gut bacteria synthesize Vitamin B12, it is produced in the colon—distal to the site of absorption (terminal ileum). Therefore, humans still require dietary B12. 3. **Niacin Deficiency:** Leads to **Pellagra** (3 Ds: Dermatitis, Diarrhea, Dementia). It is often seen in populations reliant on maize (corn) because the niacin in maize is bound (niacytin) and it is low in tryptophan.

Question 55: Which vitamins are considered anticariogenic?

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

Explanation: **Explanation:** **Anticariogenic vitamins** are those that prevent or inhibit the formation of dental caries (tooth decay). **Vitamin D and Vitamin K** are the primary vitamins involved in this protective process. 1. **Why Vitamin D and K are correct:** * **Vitamin D:** It is essential for the absorption of calcium and phosphorus from the gut. It maintains optimal serum levels of these minerals, which are vital for the **remineralization** of tooth enamel. Vitamin D also stimulates the production of **cathelicidins and defensins**, antimicrobial peptides that fight oral bacteria. * **Vitamin K:** Specifically Vitamin K2 (menaquinone), it activates **osteocalcin**, a protein that binds calcium into the matrix of bones and teeth. It also works synergistically with Vitamin D to ensure calcium is deposited in the dental tissues rather than soft tissues (like arteries). Historically, Vitamin K was also known as "Factor X" for its role in preventing dental decay. 2. **Why other options are incorrect:** * **Vitamin A:** While essential for the development of epithelial cells and salivary gland function (preventing xerostomia), it is not primarily classified as anticariogenic in the same capacity as D and K. * **Vitamin E:** It is a potent antioxidant that protects cell membranes from oxidative stress, but it has no direct clinical role in preventing dental caries or mineralizing enamel. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin D Deficiency:** Leads to enamel hypoplasia and increased susceptibility to caries in children. * **Vitamin C:** Deficiency causes **Scurvy**, characterized by swollen, bleeding gums and loosened teeth due to defective collagen synthesis in the periodontal ligament. * **Fluoride:** While not a vitamin, it is the most potent anticariogenic agent (converts hydroxyapatite to **fluoroapatite**, which is resistant to acid). * **Vitamin K2:** Often tested in the context of the "Calcium Paradox"—without K2, calcium cannot be effectively utilized by teeth and bones.

Question 56: A beefy red and painful tongue is characteristic of which condition?

A. Vitamin A deficiency
B. Any periodontal disease
C. Acute nicotinic acid deficiency (Correct Answer)
D. Ascorbic acid deficiency

Explanation: **Explanation:** The correct answer is **Acute nicotinic acid deficiency (Pellagra)**. **1. Why it is correct:** Nicotinic acid (Vitamin B3 or Niacin) is essential for cellular oxidation-reduction reactions as a precursor to NAD and NADP. A deficiency leads to **Pellagra**, classically characterized by the "4 Ds": Dermatitis, Diarrhea, Dementia, and Death. In the acute phase, the oral mucosa is severely affected. The tongue becomes intensely inflamed, swollen, and loses its papillae, resulting in a characteristic **"beefy red,"** smooth, and painful appearance (glossitis). This occurs due to the rapid turnover of epithelial cells, which are highly sensitive to the impaired energy metabolism caused by niacin deficiency. **2. Why other options are incorrect:** * **Vitamin A deficiency:** Primarily affects the eyes (Xerophthalmia, Bitot’s spots) and skin (Follicular hyperkeratosis), but does not cause a beefy red tongue. * **Periodontal disease:** Typically involves the gingiva (gums) and supporting structures of the teeth, presenting with bleeding gums or pocket formation, rather than generalized glossitis. * **Ascorbic acid deficiency (Scurvy):** Characterized by **"swollen, spongy, and bleeding gums"** and perifollicular hemorrhages. While it affects the mouth, it does not produce the classic beefy red tongue seen in Pellagra. **3. NEET-PG High-Yield Pearls:** * **Magenta Tongue:** Characteristic of Riboflavin (B2) deficiency. * **Strawberry Tongue:** Seen in Scarlet fever and Kawasaki disease. * **Casal’s Necklace:** The specific hyperpigmented rash around the neck seen in Pellagra. * **Hartnup Disease:** An autosomal recessive disorder of tryptophan transport that can lead to Pellagra-like symptoms (since Tryptophan is a precursor for Niacin).

Question 57: Which of the following soluble vitamins is synthesized in the body?

A. Thiamine
B. Vitamin C
C. Niacin (Correct Answer)
D. Pyridoxine

Explanation: **Explanation:** The correct answer is **Niacin (Vitamin B3)**. While most water-soluble vitamins must be obtained entirely from the diet, Niacin is unique because it can be synthesized endogenously in the liver from the essential amino acid **Tryptophan**. **Why Niacin is Correct:** The synthesis follows the **Kynurenine pathway**. A key high-yield fact for NEET-PG is the conversion ratio: **60 mg of Tryptophan yields 1 mg of Niacin**. This process requires Vitamin B6 (Pyridoxine), Vitamin B2 (Riboflavin), and Iron as cofactors. Therefore, Niacin is not strictly a vitamin in the classical sense if protein intake is sufficient. **Why Other Options are Incorrect:** * **Thiamine (B1):** An essential vitamin that must be ingested. Humans lack the enzymes to synthesize the pyrimidine and thiazole rings required for its structure. * **Vitamin C (Ascorbic Acid):** While most mammals synthesize Vitamin C from glucose, humans cannot because we lack the enzyme **L-gulonolactone oxidase**. * **Pyridoxine (B6):** This is an essential nutrient obtained from dietary sources like grains, meat, and vegetables. Humans cannot synthesize the pyridine ring de novo. **High-Yield Clinical Pearls for NEET-PG:** * **Hartnup Disease:** A defect in the transport of neutral amino acids (Tryptophan) leading to Niacin deficiency. * **Carcinoid Syndrome:** Can lead to Niacin deficiency (Pellagra) because Tryptophan is diverted toward the massive synthesis of Serotonin. * **Pellagra Symptoms:** Characterized by the **4 Ds**: Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Key Enzyme:** Tryptophan oxygenase is the rate-limiting enzyme in the synthesis of Niacin.

Question 58: Which of the following enzyme activities can be used to help diagnose riboflavin deficiency?

A. RBC transketolase
B. RBC glutathione reductase (Correct Answer)
C. Branched chain ketoacid dehydrogenase
D. RBC transaminase

Explanation: **Explanation:** Riboflavin (Vitamin B2) is the precursor for the coenzymes **FMN (Flavin Mononucleotide)** and **FAD (Flavin Adenine Dinucleotide)**. These coenzymes are essential for redox reactions throughout the body. **Why RBC Glutathione Reductase is the correct answer:** The gold standard for biochemical assessment of riboflavin status is the **Erythrocyte Glutathione Reductase Activity Coefficient (EGRAC) assay**. Glutathione reductase is an FAD-dependent enzyme that maintains reduced glutathione levels to protect RBCs from oxidative damage. In riboflavin deficiency, the basal activity of this enzyme is low. When FAD is added *in vitro*, a significant increase in enzyme activity (an activation coefficient >1.2 or 20% increase) confirms a deficiency. **Analysis of Incorrect Options:** * **A. RBC Transketolase:** This is a **Thiamine (Vitamin B1)** dependent enzyme. Its activity coefficient is used to diagnose Beriberi or Wernicke-Korsakoff syndrome. * **C. Branched-chain ketoacid dehydrogenase (BCKDH):** While BCKDH requires FAD as one of its five cofactors (along with TPP, Lipoate, NAD+, and CoA), it is not used as a diagnostic marker for riboflavin deficiency in clinical practice. * **D. RBC Transaminase:** Aminotransferases (like ALT and AST) require **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. RBC transaminase activity is used to assess B6 status. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Features of B2 Deficiency:** Cheilosis, angular stomatitis, glossitis (magenta tongue), and corneal neovascularization. * **The "Three Ds" of B2:** **D**ermatitis (seborrheic), **D**estruction of RBCs (anemia), and **D**ark tongue (magenta). * **Metabolic Role:** FAD is a prosthetic group for Succinate Dehydrogenase (Complex II) in the TCA cycle and Fatty Acyl CoA Dehydrogenase in Beta-oxidation.

Question 59: Man cannot synthesize ascorbic acid due to the deficiency of which enzyme?

A. L-gulonolactone oxidase (Correct Answer)
B. Glucocerebrosidase
C. Peroxidase
D. Lysyl oxidase

Explanation: **Explanation:** The synthesis of Vitamin C (Ascorbic acid) occurs via the **Uronic Acid Pathway**. In most mammals, glucose is converted into glucuronic acid, which is then reduced to L-gulonate and subsequently converted to L-gulonolactone. The final step involves the conversion of L-gulonolactone to 2-keto-L-gulonolactone by the enzyme **L-gulonolactone oxidase**. Humans, other primates, and guinea pigs lack this specific enzyme due to a mutation in the GULO gene. Consequently, Vitamin C becomes an essential dietary requirement (vitamin) for humans, as we cannot synthesize it endogenously. **Analysis of Incorrect Options:** * **Glucocerebrosidase:** This is a lysosomal enzyme responsible for breaking down glucosylceramide. Its deficiency leads to **Gaucher’s disease**, the most common lysosomal storage disorder. * **Peroxidase:** These are enzymes (like Glutathione peroxidase) that catalyze the reduction of hydrogen peroxide to water, protecting cells from oxidative damage. * **Lysyl oxidase:** This is a copper-dependent enzyme required for the cross-linking of collagen and elastin. While Vitamin C is a cofactor for *lysyl hydroxylase*, it is not related to *lysyl oxidase*. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to defective collagen synthesis because it is a co-factor for the **hydroxylation of proline and lysine** residues. * **Key Symptoms:** Perifollicular hemorrhage, corkscrew hair, bleeding gums, and impaired wound healing. * **Iron Absorption:** Vitamin C enhances the absorption of **non-heme iron** from the gut by maintaining it in the ferrous ($Fe^{2+}$) state.

Question 60: In Beri-Beri, which enzyme activity is measured?

A. Transketolase (Correct Answer)
B. Tranaminase
C. Decarboxylase
D. Deaminase

Explanation: **Explanation:** **Why Transketolase is the Correct Answer:** Beri-beri is caused by a deficiency of **Vitamin B1 (Thiamine)**. Thiamine, in its active form Thiamine Pyrophosphate (TPP), serves as a vital coenzyme for several enzymes, including **Erythrocyte Transketolase**. In clinical practice, the most reliable functional test to diagnose thiamine deficiency is measuring the activity of transketolase in Red Blood Cells (RBCs). If the enzyme activity increases significantly (usually >15-25%) after adding exogenous TPP in vitro, it confirms a thiamine deficiency. This is known as the **TPP effect**. **Analysis of Incorrect Options:** * **B. Transaminase:** These enzymes (like ALT and AST) require **Vitamin B6 (Pyridoxine)** as a cofactor, not B1. They are markers of liver or muscle injury. * **C. Decarboxylase:** While TPP is a cofactor for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase), "Decarboxylase" is a broad category. Transketolase is the specific diagnostic marker used in laboratory assays for Beri-beri. * **D. Deaminase:** These enzymes (e.g., Adenosine deaminase) are involved in amino acid or nucleotide metabolism and do not require Thiamine. **High-Yield Clinical Pearls for NEET-PG:** * **Key TPP-dependent enzymes:** Pyruvate Dehydrogenase (PDH), $\alpha$-Ketoglutarate Dehydrogenase, and Transketolase (HMP Shunt). * **Dry Beri-beri:** Characterized by peripheral neuropathy and muscle wasting. * **Wet Beri-beri:** Characterized by high-output heart failure and edema. * **Wernicke-Korsakoff Syndrome:** Often seen in alcoholics; presents with the triad of Ataxia, Ophthalmoplegia, and Confusion. * **Gold Standard Diagnosis:** Erythrocyte Transketolase Activity (ETKA) assay.

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

Practice Questions

Vitamin B6 and Transamination

Practice Questions

Folate and Vitamin B12 in One-Carbon Metabolism

Practice Questions

Vitamin C and Collagen Synthesis

Practice Questions

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