Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 281: Deficiency of which nutrient, commonly found in maize as a staple food, causes Pellagra?

A. Xerophthalmia
B. Scurvy
C. Beri-beri
D. Pellagra (Correct Answer)

Explanation: **Explanation:** The correct answer is **Pellagra**, which is caused by a deficiency of **Niacin (Vitamin B3)** or its precursor amino acid, **Tryptophan**. **Medical Concept:** Maize (corn) is a staple food in many regions but is inherently low in bioavailable niacin and the essential amino acid tryptophan. Furthermore, the niacin present in maize is often bound as **niacytin**, making it unabsorbable unless treated with alkali (like lime). A diet predominantly based on maize leads to a systemic deficiency of NAD and NADP, coenzymes vital for oxidation-reduction reactions, resulting in the clinical syndrome of Pellagra. **Analysis of Options:** * **A. Xerophthalmia:** Caused by **Vitamin A** deficiency. It primarily affects the eyes, leading to night blindness and corneal scarring. * **B. Scurvy:** Caused by **Vitamin C (Ascorbic acid)** deficiency. It results in defective collagen synthesis, leading to bleeding gums and impaired wound healing. * **C. Beri-beri:** Caused by **Thiamine (Vitamin B1)** deficiency. It is commonly associated with a diet of polished rice, not maize. **High-Yield Clinical Pearls for NEET-PG:** * **The 4 D’s of Pellagra:** Dermatitis (Casal’s necklace distribution), Diarrhea, Dementia, and Death. * **Hartnup Disease:** A genetic disorder of tryptophan transport that can present with pellagra-like symptoms. * **Carcinoid Syndrome:** Can lead to secondary niacin deficiency because tryptophan is diverted to overproduce serotonin. * **Isoniazid (INH) Therapy:** This anti-tubercular drug is a structural analog of pyridoxine (B6); since B6 is a cofactor for converting tryptophan to niacin, INH use can trigger pellagra.

Question 282: Which mineral is primarily associated with insulin secretion?

A. Zinc (Correct Answer)
B. Copper
C. Manganese
D. Chromium

Explanation: **Explanation:** **Zinc (Zn)** is the correct answer because it plays a structural and functional role in the storage and secretion of insulin. Within the **beta cells of the Islets of Langerhans** in the pancreas, insulin is stored as a **hexamer** (six insulin molecules) coordinated around **two zinc ions**. This zinc-insulin complex forms the characteristic crystalline structure of insulin secretory granules. When blood glucose levels rise, these granules undergo exocytosis, releasing both insulin and zinc into the portal circulation. **Why other options are incorrect:** * **Chromium (Cr):** While often confused with insulin secretion, Chromium is actually associated with **insulin action**. It is a component of **Chromodulin** (Glucose Tolerance Factor), which enhances the binding of insulin to its receptor, thereby improving insulin sensitivity rather than its secretion. * **Copper (Cu):** Copper is a cofactor for enzymes like Cytochrome c oxidase and Superoxide dismutase. It is not directly involved in the insulin secretory pathway. * **Manganese (Mn):** This mineral acts as a cofactor for Pyruvate carboxylase and Mitochondrial SOD but does not play a primary role in insulin storage or release. **High-Yield NEET-PG Pearls:** * **Zinc Deficiency:** Can lead to impaired glucose tolerance, acrodermatitis enteropathica, poor wound healing, and hypogonadism. * **Insulin Storage:** Insulin is synthesized as preproinsulin, converted to proinsulin, and finally cleaved into insulin and **C-peptide** (an important marker of endogenous insulin production). * **Zinc & Enzymes:** Zinc is also a vital component of **Carbonic Anhydrase**, **Alcohol Dehydrogenase**, and **DNA/RNA Polymerases** (Zinc fingers).

Question 283: Red blood cell transketolase level is a marker for deficiency of which vitamin?

A. Vitamin B1 (Thiamine) (Correct Answer)
B. Vitamin B2 (Riboflavin)
C. Vitamin B6 (Pyridoxine)
D. Vitamin B12 (Cobalamin)

Explanation: **Explanation:** **1. Why Vitamin B1 (Thiamine) is Correct:** Thiamine, in its active form **Thiamine Pyrophosphate (TPP)**, serves as an essential cofactor for several key enzymes in carbohydrate metabolism. One of these is **Transketolase**, an enzyme in the non-oxidative phase of the Pentose Phosphate Pathway (HMP Shunt). Since mature red blood cells lack mitochondria, they rely heavily on the HMP shunt for NADPH production. Measuring the activity of **Erythrocyte Transketolase (ETKA)**—specifically the increase in activity after adding TPP in vitro—is the most sensitive functional test to diagnose Thiamine deficiency. **2. Why the Other Options are Incorrect:** * **Vitamin B2 (Riboflavin):** Its deficiency is assessed by measuring **Erythrocyte Glutathione Reductase** activity. * **Vitamin B6 (Pyridoxine):** Its deficiency is typically assessed via **Erythrocyte Transaminase** (AST/ALT) activity or the Xanthurenic acid excretion test. * **Vitamin B12 (Cobalamin):** Deficiency is diagnosed via serum B12 levels, Methylmalonic acid (MMA) levels, or the Schilling test (historically). **3. High-Yield Clinical Pearls for NEET-PG:** * **TPP-dependent enzymes:** Remember the mnemonic **"ATP"**: **A**lpha-ketoglutarate dehydrogenase, **T**ransketolase, and **P**yruvate dehydrogenase. (Also Branched-chain ketoacid dehydrogenase). * **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. * **Management Rule:** Always administer Thiamine *before* Glucose in a malnourished patient to prevent precipitating Wernicke encephalopathy.

Question 284: Angular cheilosis is frequently associated with deficiency of which vitamin?

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

Explanation: **Explanation:** **Riboflavin (Vitamin B2)** is the precursor for the coenzymes **FAD** (Flavin Adenine Dinucleotide) and **FMN** (Flavin Mononucleotide), which are essential for redox reactions in the electron transport chain and various metabolic pathways. Deficiency of Riboflavin typically manifests as **Ariboflavinosis**, characterized by a "2C" clinical triad: **C**heilosis (angular stomatitis/fissures at the corners of the mouth) and **C**orneal vascularization. Other features include glossitis (magenta tongue) and seborrheic dermatitis. **Analysis of Incorrect Options:** * **Thiamine (B1):** Deficiency primarily leads to **Beriberi** (Dry: peripheral neuropathy; Wet: high-output heart failure) and **Wernicke-Korsakoff syndrome**, typically seen in chronic alcoholism. * **Niacin (B3):** Deficiency causes **Pellagra**, characterized by the "4 Ds": Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Folic Acid (B9):** Deficiency results in **Megaloblastic anemia** and neural tube defects in fetuses. It does not typically present with isolated angular cheilosis. **High-Yield Clinical Pearls for NEET-PG:** * **Magenta Tongue:** Specific for Riboflavin deficiency (vs. "Beefy red tongue" in B12/Niacin deficiency). * **Glutathione Reductase Assay:** The activity of this enzyme in RBCs is the gold standard functional test to diagnose Riboflavin deficiency. * **Light Sensitivity:** Riboflavin is photolabile; hence, newborns undergoing phototherapy for jaundice are at risk of B2 deficiency and may require supplementation.

Question 285: Which of the following is NOT a biochemical role of Ascorbic acid?

A. Role as a coenzyme in the hydroxylation of proline and lysine
B. Deamination of hydroxyl group-containing amino acids (Correct Answer)
C. Involvement in steroid biosynthesis
D. Involvement in bile acid synthesis

Explanation: **Explanation:** **Vitamin C (Ascorbic Acid)** acts primarily as a potent water-soluble antioxidant and a reducing agent (co-factor). It maintains metal ions, such as iron ($Fe^{2+}$) and copper ($Cu^{2+}$), in their reduced states, which is essential for the activity of various hydroxylase enzymes. **Why Option B is the correct answer:** The **deamination** of hydroxyl group-containing amino acids (like Serine and Threonine) is catalyzed by **dehydratases** (e.g., Serine dehydratase), which require **Pyridoxal Phosphate (Vitamin B6)** as a coenzyme, not Vitamin C. Ascorbic acid does not play a role in deamination reactions. **Analysis of Incorrect Options:** * **Option A:** Vitamin C is a mandatory co-factor for **Prolyl and Lysyl hydroxylases**. These enzymes hydroxylate proline and lysine residues in collagen, allowing for cross-linking and triple helix stability. Deficiency leads to **Scurvy**. * **Option C:** It is involved in **Steroidogenesis**, particularly in the adrenal cortex, where high concentrations of Vitamin C are found. It acts as a cofactor for hydroxylations in the synthesis of adrenal steroids. * **Option D:** In the liver, Vitamin C is required for the **7-alpha-hydroxylase** enzyme, which is the rate-limiting step in converting cholesterol into **bile acids**. **High-Yield Clinical Pearls for NEET-PG:** * **Iron Absorption:** Vitamin C enhances the absorption of non-heme iron from the gut by reducing $Fe^{3+}$ (ferric) to $Fe^{2+}$ (ferrous). * **Dopamine Metabolism:** It is a cofactor for **Dopamine $\beta$-hydroxylase**, which converts dopamine to norepinephrine. * **Scurvy Signs:** Characterized by "corkscrew hair," perifollicular hemorrhages, easy bruising, and "woody edema" of the legs.

Question 286: Which is the main antioxidant vitamin?

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

Explanation: **Explanation:** **Vitamin E (Tocopherol)** is considered the primary antioxidant vitamin because it acts as the body's first line of defense against **lipid peroxidation**. It is a potent fat-soluble antioxidant that resides within cell membranes, where it protects polyunsaturated fatty acids (PUFAs) from free radical damage by scavenging reactive oxygen species (ROS). By donating a hydrogen atom to a free radical, it terminates the chain reaction of lipid destruction, maintaining membrane integrity. **Analysis of Incorrect Options:** * **Vitamin D (Calciferol):** Primarily functions as a hormone involved in calcium and phosphorus homeostasis and bone mineralization. It does not possess significant direct antioxidant properties. * **Vitamin B (Complex):** This group consists of water-soluble vitamins that mainly function as **coenzymes** in metabolic pathways (e.g., B1 for decarboxylation, B12 for DNA synthesis). While some (like B2) are involved in redox reactions, they are not classified as "main antioxidants." * **Vitamin K (Phylloquinone/Menaquinone):** Its primary physiological role is as a cofactor for the gamma-carboxylation of glutamate residues in **clotting factors** (II, VII, IX, and X). **High-Yield NEET-PG Pearls:** * **The Antioxidant Trio:** Remember the mnemonic **"ACE"** for antioxidant vitamins (Vitamin A/Beta-carotene, Vitamin C, and Vitamin E). * **Synergy:** Vitamin C (Ascorbic acid) is essential for regenerating the reduced (active) form of Vitamin E after it has neutralized a free radical. * **Clinical Correlation:** Vitamin E deficiency can lead to **hemolytic anemia** (due to fragile RBC membranes) and posterior column neurological defects, mimicking Friedreich's ataxia.

Question 287: Deficiency of which vitamin causes glossitis, dementia, roughened keratotic areas on skin, and gastrointestinal symptoms?

A. Riboflavin
B. Pyridoxine
C. Niacin (Correct Answer)
D. Pantothenic acid

Explanation: The clinical presentation described—**Glossitis, Dementia, Dermatitis (roughened keratotic skin), and Diarrhea (GI symptoms)**—represents the classic triad (or tetrad, including death) of **Pellagra**, which is caused by a deficiency of **Niacin (Vitamin B3)**. ### **1. Why Niacin is Correct** Niacin is the precursor for **NAD and NADP**, essential cofactors for redox reactions in energy metabolism and DNA repair. Deficiency leads to cellular dysfunction in high-turnover tissues: * **Dermatitis:** Characterized by a symmetrical, scaly rash in sun-exposed areas (e.g., **Casal’s necklace** around the neck). * **Dementia:** Results from neuronal degeneration, leading to confusion, memory loss, and psychosis. * **Diarrhea:** Caused by atrophy of the intestinal columnar epithelium. * **Glossitis:** Inflammation of the tongue (beefy red appearance). ### **2. Why Other Options are Incorrect** * **Riboflavin (B2):** Deficiency causes **Ariboflavinosis**, characterized by cheilosis, angular stomatitis, and corneal neovascularization, but lacks the "3 Ds" of Pellagra. * **Pyridoxine (B6):** Deficiency leads to peripheral neuropathy, sideroblastic anemia, and seizures. While it can cause glossitis, it does not present with the specific keratotic dermatitis of Niacin deficiency. * **Pantothenic acid (B5):** Deficiency is extremely rare but is classically associated with **"Burning Feet Syndrome."** ### **3. High-Yield Clinical Pearls for NEET-PG** * **The Tryptophan Connection:** Niacin can be synthesized from the amino acid Tryptophan (**60 mg Tryptophan = 1 mg Niacin**). * **Secondary Pellagra:** Can occur in **Hartnup disease** (impaired tryptophan absorption) or **Carcinoid syndrome** (tryptophan is diverted to serotonin synthesis). * **Corn/Maize Diets:** Endemic pellagra is seen in populations relying on corn, as its niacin is bound (**Niacytin**) and it is deficient in tryptophan.

Question 288: Which vitamin, in combination with Vitamin D, is beneficial in osteoporosis?

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

Explanation: **Explanation:** The correct answer is **Vitamin K**. **Why Vitamin K is correct:** Vitamin K (specifically Vitamin K2 or Menaquinone) plays a critical role in bone metabolism through the **carboxylation of Osteocalcin**. Osteocalcin is a protein secreted by osteoblasts that is essential for binding calcium to the bone matrix (hydroxyapatite). Vitamin K acts as a coenzyme for *gamma-glutamyl carboxylase*, which converts glutamate residues on osteocalcin into gamma-carboxyglutamate (Gla). This "Gla" form has a high affinity for calcium. While Vitamin D stimulates the synthesis of osteocalcin, Vitamin K is required to activate it. Therefore, the combination ensures both the production and the functional activation of bone-forming proteins, reducing fracture risk in osteoporosis. **Why the other options are incorrect:** * **Vitamin E:** Primarily an antioxidant that protects cell membranes from lipid peroxidation. It has no established synergistic role with Vitamin D in increasing bone mineral density. * **Vitamin A:** While necessary for bone growth, chronic excessive intake of Vitamin A (Hypervitaminosis A) is actually associated with *increased* osteoclast activity and a higher risk of hip fractures. * **Vitamin B:** While B12 and Folate deficiencies are linked to high homocysteine (a risk factor for osteoporosis), they do not work directly with Vitamin D in the mineralization process like Vitamin K does. **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Paradox:** Patients on long-term Warfarin (Vitamin K antagonist) often show increased vascular calcification and decreased bone density because osteocalcin remains uncarboxylated. * **Vitamin K2 (MK-7)** is considered more effective for bone health than K1 (Phylloquinone). * **Osteocalcin** is often used as a biochemical marker for bone formation.

Question 289: Which of the following is not a chain-breaking antioxidant?

A. Superoxide Dismutase
B. Uric Acid
C. Catalase (Correct Answer)
D. Vitamin E

Explanation: **Explanation:** Antioxidants are classified based on their mechanism of action into two main categories: **Preventive antioxidants** and **Chain-breaking antioxidants.** **1. Why Catalase is the Correct Answer:** Catalase is a **preventive antioxidant**. Its primary role is to intercept and degrade reactive precursors (like Hydrogen Peroxide) before they can react with metal ions to form highly reactive hydroxyl radicals. It catalyzes the reaction: $2H_2O_2 \rightarrow 2H_2O + O_2$. Since it prevents the initiation of the chain reaction rather than stopping an ongoing one, it is not classified as "chain-breaking." **2. Analysis of Incorrect Options:** * **Vitamin E (α-tocopherol):** This is the most potent lipid-soluble **chain-breaking antioxidant**. It protects polyunsaturated fatty acids in cell membranes by trapping peroxyl radicals ($ROO^\bullet$), thus terminating the lipid peroxidation chain reaction. * **Uric Acid:** It acts as a significant chain-breaking antioxidant in the plasma, scavenging hydroxyl radicals and preventing the oxidation of proteins and lipids. * **Superoxide Dismutase (SOD):** While often debated, SOD is traditionally classified as a **chain-breaking antioxidant** because it "breaks" the chain of reactions initiated by the superoxide radical ($O_2^{\bullet-}$) by converting it into less harmful $H_2O_2$. **Clinical Pearls for NEET-PG:** * **Chain-breaking antioxidants** include Vitamin E, Vitamin C, Uric acid, Bilirubin, and SOD. * **Preventive antioxidants** include Catalase, Glutathione Peroxidase, EDTA, and metal-binding proteins like Transferrin and Ceruloplasmin (which sequester iron/copper to prevent the Fenton reaction). * **Vitamin E** is unique because it is regenerated by **Vitamin C** (Ascorbate) after it has neutralized a radical.

Question 290: Cobalt is a component of which of the following vitamins?

A. Biotin
B. Vitamin B12 (Correct Answer)
C. Vitamin A
D. Tocopherol

Explanation: **Explanation:** **Vitamin B12 (Cobalamin)** is the correct answer because it is the only vitamin that contains a metal ion as an integral part of its structure. The central atom of the **corrin ring** in Vitamin B12 is **Cobalt (Co³⁺)**. This unique structure is why the chemical name for Vitamin B12 is **Cyanocobalamin** (or Hydroxocobalamin/Methylcobalamin depending on the ligand attached). **Analysis of Incorrect Options:** * **Biotin (Vitamin B7):** This is a sulfur-containing vitamin. It acts as a coenzyme for carboxylation reactions (e.g., Pyruvate carboxylase). It does not contain cobalt. * **Vitamin A (Retinol):** This is a fat-soluble vitamin derived from beta-carotenes. Its structure consists of a beta-ionone ring and an isoprenoid side chain; it contains no metal ions. * **Tocopherol (Vitamin E):** This is a fat-soluble antioxidant consisting of a chromane ring and a phytyl side chain. It does not contain cobalt. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** Vitamin B12 is synthesized exclusively by microorganisms; it is **not found in plant foods**, making strict vegetarians (vegans) prone to deficiency. * **Absorption:** Requires **Intrinsic Factor (IF)** secreted by gastric parietal cells. Absorption occurs in the **terminal ileum**. * **Storage:** Unlike other water-soluble vitamins, B12 is stored in the **liver** for 3–5 years. * **Deficiency:** Leads to **Megaloblastic Anemia** and neurological complications like **Subacute Combined Degeneration (SCD)** of the spinal cord due to impaired myelin synthesis (accumulation of Methylmalonyl CoA). * **Biochemical Role:** It is a cofactor for two essential enzymes: **Methionine Synthase** and **Methylmalonyl-CoA Mutase**.

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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