Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 481: Which of the following is the cofactor for dopamine hydroxylase?

A. Fe
B. Mg
C. Mn
D. Cu (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Cu (Copper)**. **Dopamine Beta-Hydroxylase (DBH)** is the enzyme responsible for converting Dopamine into Norepinephrine within the catecholamine synthesis pathway. This enzyme is a copper-containing oxygenase located primarily in the synaptic vesicles of postganglionic sympathetic neurons and the adrenal medulla. It requires **Copper (Cu²⁺)** as a specific metal cofactor and **Vitamin C (Ascorbic acid)** as a reducing agent to function. During the reaction, copper undergoes cyclic oxidation and reduction to facilitate the hydroxylation of the substrate. **Why other options are incorrect:** * **A. Fe (Iron):** Iron is a cofactor for **Tyrosine Hydroxylase** (the rate-limiting step of catecholamine synthesis) and Phenylalanine Hydroxylase, but not for Dopamine Hydroxylase. * **B. Mg (Magnesium):** Magnesium is typically a cofactor for enzymes involving ATP transfer, such as kinases (e.g., Hexokinase) or **COMT** (Catechol-O-methyltransferase) in catecholamine degradation. * **C. Mn (Manganese):** Manganese is a cofactor for enzymes like Pyruvate Carboxylase and Arginase in the urea cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Vitamin C Connection:** Vitamin C deficiency (Scurvy) can lead to impaired norepinephrine synthesis because it is the essential co-substrate for Dopamine Beta-Hydroxylase. * **Menkes Disease:** This is an X-linked recessive disorder of copper absorption. Low copper levels lead to decreased activity of copper-dependent enzymes, including DBH (causing neurological issues) and Lysyl Oxidase (causing connective tissue defects/kinky hair). * **Rate-Limiting Step:** Remember that Tyrosine Hydroxylase is the rate-limiting step in catecholamine synthesis, while DBH is the only enzyme in the pathway located inside vesicles.

Question 482: What is the active form of folic acid?

A. Dihydrofolate
B. Folinic acid
C. Tetrahydrofolate (Correct Answer)
D. Methylated folic acid

Explanation: **Explanation:** **1. Why Tetrahydrofolate (THF) is Correct:** Folic acid (Vitamin B9) is biologically inactive in its dietary or supplemental form. To become functional, it must undergo a two-step reduction process catalyzed by the enzyme **Dihydrofolate Reductase (DHFR)**, using NADPH as a donor. This process converts folic acid first to Dihydrofolate (DHF) and finally to **Tetrahydrofolate (THF)**. THF is the active coenzyme form that acts as a carrier of one-carbon units (such as methyl, methylene, and formyl groups), which are essential for the synthesis of purines and thymidylate (dTMP), and thus, DNA synthesis. **2. Analysis of Incorrect Options:** * **A. Dihydrofolate (DHF):** This is an intermediate metabolic product. While it is a reduced form, it does not possess the full coenzymatic activity required for one-carbon metabolism until it is further reduced to THF. * **B. Folinic Acid:** Also known as Leucovorin (5-formyl-THF), this is a synthetic derivative of THF. While it is used clinically to "bypass" DHFR inhibition, it is a stable derivative rather than the primary endogenous active form. * **D. Methylated folic acid:** Specifically N5-methyl-THF, this is the primary form of folate found in the circulation. However, it is considered a "trap" (the Folate Trap) if Vitamin B12 is deficient, as it must be demethylated to regenerate active THF. **3. NEET-PG High-Yield Pearls:** * **The Folate Trap:** B12 deficiency leads to a functional folate deficiency because folate remains trapped as N5-methyl-THF. * **Drug Targets:** Methotrexate, Trimethoprim, and Pyrimethamine are potent inhibitors of **Dihydrofolate Reductase**, preventing the formation of THF. * **Clinical Use:** Folinic acid (Leucovorin) is used in "Leucovorin Rescue" to protect healthy cells during high-dose Methotrexate therapy. * **Deficiency:** Leads to **Megaloblastic Anemia** and is the most common cause of **Neural Tube Defects (NTDs)** in pregnancy.

Question 483: Niacin toxicity causes all except?

A. Hepatitis
B. Diabetes
C. Hypertriglyceridemia (Correct Answer)
D. Macular edema

Explanation: **Explanation:** Niacin (Vitamin B3), when used in pharmacological doses to treat dyslipidemia, is known for its lipid-lowering effects rather than causing lipid elevation. **Why Hypertriglyceridemia is the correct answer:** Niacin actually **decreases** triglyceride levels. It inhibits the enzyme **diacylglycerol acyltransferase-2 (DGAT2)** in the liver and reduces the mobilization of free fatty acids from adipose tissue. This leads to decreased VLDL synthesis and, consequently, lower triglyceride levels. Therefore, hypertriglyceridemia is a therapeutic effect/indication, not a toxic manifestation. **Analysis of Incorrect Options (Side Effects of Niacin):** * **Hepatitis:** Niacin, especially in sustained-release formulations, can be hepatotoxic. It may cause elevated liver enzymes and, in severe cases, fulminant hepatic failure. * **Diabetes (Hyperglycemia):** Niacin induces insulin resistance and decreases glucose tolerance. It should be used with caution in diabetic patients as it can worsen glycemic control. * **Macular Edema:** A rare but specific ocular side effect of high-dose niacin is **cystoid macular edema (CME)**, which typically presents with blurred vision and is reversible upon discontinuation. **High-Yield Clinical Pearls for NEET-PG:** * **The "Niacin Flush":** The most common side effect, mediated by **Prostaglandin D2 and E2**. It can be blunted by taking **Aspirin** 30 minutes prior. * **Hyperuricemia:** Niacin competes with uric acid for excretion in the kidneys, potentially precipitating **Gout**. * **Lipid Profile:** Niacin is the most effective agent for **increasing HDL** levels. * **Pellagra:** Deficiency of Niacin leads to the 3 Ds: Dermatitis (Casal’s necklace), Diarrhea, and Dementia.

Question 484: Which of the following vitamins is required for the synthesis of blood clotting factors?

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

Explanation: **Explanation:** **Vitamin K** is the correct answer because it acts as a vital coenzyme for the **gamma-glutamyl carboxylase** enzyme. This enzyme is responsible for the post-translational modification (gamma-carboxylation) of glutamic acid residues on specific proteins. This modification allows these proteins to bind calcium ions ($Ca^{2+}$), which is essential for their activation and binding to phospholipid membranes during the coagulation cascade. The specific blood clotting factors dependent on Vitamin K are **Factors II (Prothrombin), VII, IX, and X**, as well as the anticoagulant proteins **Protein C and Protein S**. **Why the other options are incorrect:** * **Vitamin C (Ascorbic Acid):** Primarily functions in collagen synthesis (hydroxylation of proline and lysine) and acts as an antioxidant. Deficiency leads to Scurvy. * **Riboflavin (Vitamin B2):** A precursor for FMN and FAD, which are essential for redox reactions in the TCA cycle and Electron Transport Chain. * **Cobalamin (Vitamin B12):** Essential for DNA synthesis (via folate metabolism) and myelin maintenance. Deficiency causes Megaloblastic anemia and Subacute Combined Degeneration of the spinal cord. **Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** Warfarin acts as a Vitamin K antagonist by inhibiting **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K. * **Newborn Prophylaxis:** Neonates are born with sterile guts and low Vitamin K stores; hence, a prophylactic intramuscular injection of Vitamin K is given at birth to prevent **Hemorrhagic Disease of the Newborn**. * **Lab Findings:** Vitamin K deficiency results in a prolonged **Prothrombin Time (PT)** and potentially a prolonged aPTT.

Question 485: What is a known consequence of Vitamin A toxicity?

A. Wernicke's encephalopathy
B. Night blindness
C. Increased intracranial pressure (Correct Answer)
D. Decreased intracranial pressure

Explanation: **Explanation:** Vitamin A (Retinol) is a fat-soluble vitamin essential for vision and epithelial integrity. However, because it is stored in the liver (Ito cells), excessive intake can lead to toxicity. **Why the correct answer is right:** **Hypervitaminosis A** (Vitamin A toxicity) can lead to a condition known as **Pseudotumor Cerebri** (Idiopathic Intracranial Hypertension). The mechanism involves the impairment of cerebrospinal fluid (CSF) resorption by the arachnoid granulations, leading to **increased intracranial pressure**. Clinically, this manifests as severe headaches, nausea, vomiting, and papilledema. In infants, this may present as a bulging fontanelle. **Why the incorrect options are wrong:** * **A. Wernicke's encephalopathy:** This is caused by a deficiency of **Vitamin B1 (Thiamine)**, typically seen in chronic alcoholics. * **B. Night blindness (Nyctalopia):** This is the earliest clinical manifestation of Vitamin A **deficiency**, not toxicity. It results from a lack of 11-cis-retinal to form rhodopsin. * **D. Decreased intracranial pressure:** Vitamin A toxicity causes an increase, not a decrease, in pressure. **High-Yield Clinical Pearls for NEET-PG:** * **Acute Toxicity:** Presents with nausea, vomiting, vertigo, and blurred vision. * **Chronic Toxicity:** Characterized by bone pain, alopecia, dry/fissured skin, hepatomegaly, and hypercalcemia. * **Teratogenicity:** Isotretinoin (a Vitamin A derivative) is highly teratogenic; it can cause craniofacial, cardiac, and CNS defects. A negative pregnancy test and effective contraception are mandatory before starting therapy. * **Diagnostic Clue:** Always look for a history of excessive intake of liver, fish liver oils, or retinoid medications for acne.

Question 486: Activity of glutathione reductase in RBCs gives an idea about the functional index of which of the following vitamins?

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

Explanation: **Explanation:** The correct answer is **Riboflavin (Vitamin B2)**. The functional status of vitamins is often assessed by measuring the activity of specific enzymes that require them as cofactors. Riboflavin is the precursor for **Flavin Adenine Dinucleotide (FAD)** and **Flavin Mononucleotide (FMN)**. **Glutathione Reductase (GR)** is an FAD-dependent enzyme found in Red Blood Cells (RBCs) that maintains the pool of reduced glutathione, which is essential for protecting cells against oxidative stress. In clinical practice, the **Erythrocyte Glutathione Reductase Activation Coefficient (EGRAC)** test is used; if the enzyme activity increases significantly upon adding FAD *in vitro*, it indicates a functional deficiency of Riboflavin in the patient. **Why other options are incorrect:** * **Pyridoxine (B6):** Its functional status is measured via **Erythrocyte Transaminase** activity (AST/ALT). * **Niacin (B3):** While Niacin provides NADPH (a co-substrate for GR), it is not the prosthetic group of the enzyme. Niacin status is typically assessed by measuring urinary metabolites (N-methylnicotinamide). * **Biotin (B7):** Biotin acts as a coenzyme for carboxylation reactions (e.g., Pyruvate carboxylase). It is not involved in the glutathione redox cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Thiamine (B1):** Functional index is measured by **Erythrocyte Transketolase** activity. * **Riboflavin Deficiency:** Characterized by the "3 Cs"—**C**heilosis, **C**orneal vascularization, and **C**asual (Magenta) tongue. * **Glutathione Reductase** requires **NADPH** (from the HMP shunt) as a reducing equivalent and **FAD** (from B2) as a prosthetic group.

Question 487: A 5-year-old female patient complains of deformed legs and is a strict vegetarian. Lab investigations reveal elevated alkaline phosphatase. Which vitamin deficiency is most likely causing these symptoms, and what is its recommended dietary allowance?

A. 300 IU
B. 400 IU (Correct Answer)
C. 500 IU
D. 600 IU

Explanation: ### Explanation **Clinical Correlation:** The patient presents with **Rickets**, characterized by skeletal deformities ("deformed legs") and elevated **Alkaline Phosphatase (ALP)**. In children, Vitamin D deficiency leads to impaired mineralization of the osteoid matrix. Elevated ALP is a hallmark biochemical marker indicating increased osteoblastic activity attempting to compensate for the weak bone structure. Strict vegetarians are at higher risk if they lack adequate sunlight exposure, as Vitamin D is primarily found in animal products (D3) or synthesized via UV rays. **Why Option B is Correct:** According to the Indian Council of Medical Research (ICMR) and international guidelines (like the AAP), the **Recommended Dietary Allowance (RDA) for Vitamin D** in infants and children (up to 18 years) is **400 IU/day**. This dose is sufficient to maintain serum 25(OH)D levels above 20 ng/mL, preventing rickets and supporting calcium absorption. **Why Other Options are Incorrect:** * **Option A (300 IU):** This is below the established threshold for preventing nutritional rickets in the pediatric population. * **Option C (500 IU):** While safe, this is not the standard RDA defined by major health organizations for this age group. * **Option D (600 IU):** This is the RDA for adults (19–70 years) and pregnant/lactating women according to some guidelines (e.g., US IOM), but 400 IU remains the specific target for pediatric prevention. **High-Yield Clinical Pearls for NEET-PG:** * **Active Form:** 1,25-dihydroxycholicalciferol (Calcitriol). * **Storage Form:** 25-hydroxyvitamin D (Calcidiol) – measured to assess status. * **Biochemical Triad of Rickets:** Low/Normal Calcium, Low Phosphate, and **High Alkaline Phosphatase**. * **Radiological Sign:** Cupping, splaying, and fraying of metaphyses (best seen at the wrist). * **Vegetarian Source:** Vitamin D2 (Ergocalciferol) from fungi/yeast; Vitamin D3 (Cholecalciferol) is animal-derived.

Question 488: Megaloblastic anemia is caused by deficiency of which vitamin?

A. Vitamin B6
B. Thiamine
C. Folic acid (Correct Answer)
D. Riboflavin

Explanation: **Explanation:** **Megaloblastic anemia** is a macrocytic anemia characterized by impaired DNA synthesis in hematopoietic precursor cells. The correct answer is **Folic acid (Vitamin B9)** because it is essential for the synthesis of thymidine triphosphate (dTTP). A deficiency in folate leads to "nuclear-cytoplasmic asynchrony," where the cell's nucleus matures slowly while the cytoplasm grows normally, resulting in large, immature red blood cells (megaloblasts). **Why the other options are incorrect:** * **Vitamin B6 (Pyridoxine):** Deficiency typically leads to **sideroblastic anemia** or microcytic anemia, as it is a cofactor for ALA synthase in heme synthesis. * **Thiamine (B1):** Deficiency causes **Beriberi** (Dry/Wet) or Wernicke-Korsakoff syndrome, not megaloblastic anemia. * **Riboflavin (B2):** Deficiency manifests as **ariboflavinosis**, characterized by cheilosis, glossitis, and corneal vascularization. **High-Yield Clinical Pearls for NEET-PG:** 1. **Dual Deficiency:** Megaloblastic anemia is most commonly caused by deficiencies in either **Folic acid (B9)** or **Vitamin B12 (Cobalamin)**. 2. **The Folate Trap:** B12 is required to convert N5-methyl THF back to THF. Without B12, folate remains "trapped" in its methylated form, leading to functional folate deficiency. 3. **Differential Diagnosis:** To distinguish between the two, check **Methylmalonic Acid (MMA)** levels. MMA is elevated in B12 deficiency but **normal** in folate deficiency. 4. **Neurological Symptoms:** B12 deficiency presents with Subacute Combined Degeneration (SCD) of the spinal cord; folate deficiency does **not** have neurological involvement. 5. **Drug-Induced:** Drugs like **Methotrexate** (DHFR inhibitor) and **Phenytoin** can cause folate-deficiency megaloblastic anemia.

Question 489: Which vitamin acts as a reducing agent?

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

Explanation: **Explanation:** **Vitamin C (Ascorbic Acid)** is the correct answer because it acts as a powerful **reducing agent** (electron donor) in several essential biochemical reactions. Its primary mechanism involves maintaining metal ions, such as iron ($Fe^{2+}$) and copper ($Cu^{2+}$), in their reduced states. This is critical for the enzyme **prolyl and lysyl hydroxylase**, which are required for collagen synthesis. By donating electrons, Vitamin C also acts as an antioxidant, neutralizing free radicals and protecting cells from oxidative damage. **Analysis of Incorrect Options:** * **Vitamin D:** Acts primarily as a **hormone** rather than a coenzyme or reducing agent. Its chief role is maintaining calcium and phosphorus homeostasis by increasing intestinal absorption. * **Vitamin B1 (Thiamine):** Its active form, Thiamine Pyrophosphate (TPP), acts as a coenzyme for **oxidative decarboxylation** reactions (e.g., Pyruvate Dehydrogenase complex) and transketolase reactions, but it does not function as a direct reducing agent. * **Vitamin B12 (Cobalamin):** Functions as a coenzyme for only two reactions in humans: the conversion of homocysteine to methionine and the conversion of methylmalonyl-CoA to succinyl-CoA. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to defective collagen synthesis, characterized by "corkscrew hair," petechiae, and bleeding gums. * **Iron Absorption:** Vitamin C enhances the absorption of non-heme iron from the gut by reducing it from the ferric ($Fe^{3+}$) to the **ferrous ($Fe^{2+}$)** state. * **Antioxidant Synergy:** Vitamin C helps regenerate the reduced form of **Vitamin E**.

Question 490: Which of the following vitamin deficiencies is associated with hyperplasia and keratinization of salivary glands?

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

Explanation: **Explanation:** **Vitamin A (Retinol)** is essential for maintaining the integrity and differentiation of epithelial tissues throughout the body. It acts as a regulator of gene expression by binding to nuclear receptors (RAR/RXR). In the absence of Vitamin A, specialized epithelia (like the columnar epithelium of the salivary glands, respiratory tract, and urinary tract) undergo **squamous metaplasia**. This leads to **hyperplasia** of the basal cells and excessive production of keratin (**keratinization**), causing the glands to become plugged and lose their secretory function. This same process in the eye leads to Xerophthalmia. **Why other options are incorrect:** * **Vitamin B:** Deficiencies typically manifest as neurological symptoms (B1, B12), dermatitis/glossitis (B2, B3, B6), or megaloblastic anemia (B9, B12), but do not cause generalized squamous metaplasia of glandular tissue. * **Vitamin C:** Deficiency (Scurvy) leads to defective collagen synthesis, resulting in gingival bleeding, perifollicular hemorrhages, and poor wound healing, rather than keratinization. * **Vitamin K:** This is essential for the gamma-carboxylation of clotting factors (II, VII, IX, X). Deficiency leads to bleeding diathesis and increased PT/INR. **High-Yield Clinical Pearls for NEET-PG:** * **Bitot’s Spots:** Triangular, pearly-white foamy patches on the conjunctiva due to squamous metaplasia (pathognomonic for Vitamin A deficiency). * **Follicular Hyperkeratosis:** Also known as "Phrynoderma" or toad skin; seen in severe Vitamin A deficiency. * **All-trans retinoic acid (ATRA):** Used in the treatment of Acute Promyelocytic Leukemia (M3). * **Teratogenicity:** Isotretinoin (used for acne) is highly teratogenic; a negative pregnancy test is mandatory before prescription.

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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