Vitamins and Coenzymes Practice Questions

Q: A child has been diagnosed with vitamin D dependent rickets. What is the most appropriate vitamin D preparation for this condition?

Calcitriol. **Explanation:** **Vitamin D Dependent Rickets (VDDR)** is a genetic disorder that interferes with the activation of Vitamin D. There are two primary types: * **Type I:** Deficiency of the enzyme **1-alpha-hydroxylase** in the kidney, which converts calcifediol to the active form [1]. * **Type II:** Resistance to the action of Vitamin D due to **mutations in the Vitamin D receptor (VDR)** [1]. **Why Calcitriol is the Correct Answer:** In both types of VDDR, the body cannot effectively produce or utilize the active metabolite of Vitamin D. **Calcitriol (1,25-dihydroxyvitamin D3)** is the active form of Vitamin D [1]. By administering Calcitriol, we bypass the need for renal 1-alpha-hydroxylation (essential for Type I) and provide high doses of the active ligand to overcome receptor resistance (in Type II) [1]. **Analysis of Incorrect Options:** * **A & B (Calciferol/Cholecalciferol):** These are inactive precursors (Vitamin D2/D3). They require both hepatic and renal hydroxylation to become active. * **C (Calcifediol):** This is 25-hydroxyvitamin D3. While it has undergone hepatic hydroxylation, it still requires the 1-alpha-hydroxylase enzyme in the kidney to become Calcitriol. **High-Yield Clinical Pearls for NEET-PG:** * **Active Form:** Calcitriol is the preferred preparation in **Chronic Kidney Disease (CKD)** and **Hypoparathyroidism** because these conditions also lack 1-alpha-hydroxylase activity [1]. * **Storage Form:** 25-OH Vitamin D (Calcifediol) is the major circulating form and the best indicator of a patient's Vitamin D status [2]. * **Nutritional Rickets:** Treated with Cholecalciferol (D3), as the metabolic machinery is intact [2].

Q: Which of the following are NOT vitamin K dependent clotting factors?

Factor V and VIII. **Explanation:** Vitamin K is an essential cofactor for the enzyme **gamma-glutamyl carboxylase**. This enzyme catalyzes the post-translational carboxylation of glutamic acid residues on specific proteins, allowing them to bind calcium ions ($Ca^{2+}$) and attach to phospholipid membranes—a critical step in the coagulation cascade. **Why Option B is Correct:** Factors **V and VIII** are not vitamin K-dependent. Factor V (Proaccelerin) and Factor VIII (Anti-hemophilic factor) act as **cofactors** rather than enzymatic zymogens in the clotting cascade. They do not undergo gamma-carboxylation. Factor VIII is notably produced by endothelial cells and travels bound to von Willebrand factor, whereas vitamin K-dependent factors are synthesized exclusively in the liver. **Why Other Options are Incorrect:** The Vitamin K-dependent clotting factors are **II (Prothrombin), VII, IX, and X**. * **Option A, C, and D** are incorrect because they all contain combinations of these four factors. * **Mnemonic:** Remember the year **1972** (Factors 10, 9, 7, and 2). **High-Yield NEET-PG Pearls:** * **Proteins C and S:** These are also Vitamin K-dependent but function as **anticoagulants**. * **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K and thus inhibiting the synthesis of these factors. * **Factor VII:** Has the shortest half-life among the clotting factors, which is why the Prothrombin Time (PT) is the first to rise in Vitamin K deficiency or early Warfarin therapy. * **Calcium Binding:** Gamma-carboxylation creates "Gla domains" which are essential for calcium-mediated binding to platelet surfaces.

Q: Hypervitaminosis A manifests as all of the following except?

Peripheral neuritis. **Explanation:** Hypervitaminosis A occurs due to the excessive intake of preformed Vitamin A (Retinol), leading to toxicity. The correct answer is **Peripheral neuritis**, as it is not a feature of Vitamin A toxicity; rather, it is classically associated with deficiencies of B-complex vitamins, most notably **Thiamine (B1)** and **Pyridoxine (B6)**. **Why the other options are incorrect (Features of Hypervitaminosis A):** * **Alopecia (Option A):** Chronic toxicity leads to skin and mucosal changes, including dry, itchy skin (pruritus), desquamation, and significant hair loss (alopecia). * **Anorexia (Option B):** General systemic symptoms of toxicity include loss of appetite (anorexia), irritability, and weight loss. * **Pseudotumor Cerebri (Option C):** This is a high-yield clinical manifestation. Excessive Vitamin A causes increased intracranial pressure, leading to headaches, papilledema, and blurred vision, mimicking a brain tumor (Idiopathic Intracranial Hypertension). **High-Yield Clinical Pearls for NEET-PG:** * **Acute Toxicity:** Presents with nausea, vomiting, and **bulging fontanelles** in infants. * **Chronic Toxicity:** Characterized by **hepatosplenomegaly** (due to storage in Ito cells), bone pain, and hyperostosis (excessive bone growth). * **Teratogenicity:** Vitamin A is highly teratogenic (Category X). It can cause craniofacial anomalies and cardiac defects in the fetus; hence, pregnancy tests are mandatory before starting Isotretinoin for acne. * **Carotenemia:** Excessive intake of Beta-carotene (carrots) causes yellowing of the skin but **spares the sclera** (unlike jaundice) and does not cause Vitamin A toxicity.

Q: Vitamin B6 deficiency is associated with all of the following except?

Cyclosporine. **Explanation:** The correct answer is **Cyclosporine**. Vitamin B6 (Pyridoxine) deficiency is frequently drug-induced. The underlying mechanism usually involves drugs that act as **pyridoxine antagonists** or those that form complexes with pyridoxal phosphate (PLP), leading to its depletion. **Why Cyclosporine is the correct answer:** Cyclosporine is an **immunosuppressant** (calcineurin inhibitor) used primarily in organ transplants and autoimmune diseases. Its primary side effects include nephrotoxicity, hypertension, and gingival hyperplasia. It does **not** interfere with Vitamin B6 metabolism. **Why the other options are incorrect:** * **Isoniazid (INH):** This is the most classic cause of B6 deficiency. INH binds with PLP to form a hydrazone complex, which is excreted in the urine. This leads to peripheral neuropathy, which is why B6 is co-administered with INH. * **Penicillamine:** Used in Wilson’s disease, it acts as an antagonist to Vitamin B6 by forming a thiazolidine derivative with PLP, rendering it inactive. * **Cycloserine:** An antitubercular drug that inhibits alanine racemase; it is a structural analogue of D-alanine and acts as a potent B6 antagonist, often causing neurological side effects. **High-Yield Clinical Pearls for NEET-PG:** 1. **Clinical Presentation:** B6 deficiency manifests as peripheral neuropathy, microcytic hypochromic anemia (due to impaired heme synthesis), and seborrheic dermatitis. 2. **Sideroblastic Anemia:** B6 is a cofactor for **ALA synthase** (the rate-limiting step in heme synthesis). Deficiency leads to ringed sideroblasts in the bone marrow. 3. **Other B6 Antagonists:** Hydralazine (antihypertensive) and Oral Contraceptive Pills (OCPs) are also known to cause B6 depletion. 4. **Xanthurenic Acid:** Increased urinary excretion of xanthurenic acid after a tryptophan load is a sensitive indicator of B6 deficiency.

Q: All the following coenzymes participate in the transfer of hydrogen and electrons, EXCEPT?

PLP. **Explanation:** The core concept here is distinguishing between **redox coenzymes** (involved in electron/hydrogen transfer) and **group-transfer coenzymes**. **Why PLP is the correct answer:** **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6, is primarily involved in **group transfer reactions**, specifically involving amino groups. It acts as a carrier for amino groups in **transamination**, decarboxylation, and deamination reactions. It does not participate in the transport of hydrogen or electrons. **Why the other options are incorrect:** * **NAD+ (Nicotinamide Adenine Dinucleotide):** Derived from Vitamin B3 (Niacin), it acts as a major electron acceptor in catabolic pathways (like Glycolysis and TCA cycle), accepting two electrons and one proton ($H^+$) to become NADH. * **NADP+:** Also derived from Niacin, it functions similarly to NAD+ but is primarily used in reductive biosynthesis (like fatty acid synthesis) and the HMP shunt. * **FAD (Flavin Adenine Dinucleotide):** Derived from Vitamin B2 (Riboflavin), it accepts two hydrogen atoms (two protons and two electrons) to become $FADH_2$, playing a crucial role in the Electron Transport Chain and the TCA cycle (Succinate dehydrogenase reaction). **High-Yield Clinical Pearls for NEET-PG:** * **PLP Requirement:** PLP is a mandatory cofactor for **ALT and AST** (Transaminases) and **Cystathionine beta-synthase** (deficiency leads to Homocystinuria). * **Drug Interaction:** **Isoniazid (INH)**, an anti-TB drug, inhibits pyridoxine kinase, leading to PLP deficiency and subsequent peripheral neuropathy. * **Redox Mnemonic:** NAD/FAD are "Hydrogen Taxis"—their primary job is to shuttle H+ and electrons to the mitochondria.

Q: What is the mechanism of action of Calcitriol?

Increased calcium absorption from the intestine. **Explanation:** Calcitriol ($1,25\text{-(OH)}_2\text{D}_3$) is the active form of Vitamin D. Its primary physiological role is to maintain plasma calcium levels by acting on the intestine, bone, and kidneys. **Why the correct answer is right:** Calcitriol acts as a steroid hormone. It enters intestinal mucosal cells and binds to a nuclear **Vitamin D Receptor (VDR)**. This complex promotes the transcription of **Calbindin** (a calcium-binding protein). Calbindin facilitates the transport of dietary calcium across the intestinal brush border membrane into the circulation, significantly **increasing intestinal calcium absorption**. **Analysis of Incorrect Options:** * **Option A:** Calcitriol actually **increases** bone resorption (in conjunction with PTH) by stimulating osteoclast maturation to mobilize calcium into the blood when levels are low. * **Option C:** Calcitriol **increases** (not decreases) the renal reabsorption of calcium and phosphorus in the distal convoluted tubules, minimizing loss in urine. * **Option D:** This is the opposite of Calcitriol’s primary function. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D by the enzyme **1-alpha-hydroxylase** in the kidney (stimulated by PTH). * **Dual Effect on Phosphorus:** Unlike PTH (which is phosphaturic), Calcitriol increases both Calcium and Phosphorus levels in the blood. * **Deficiency:** Leads to **Rickets** in children (defective mineralization of osteoid) and **Osteomalacia** in adults. * **VDR Mutation:** Mutations in the Vitamin D Receptor lead to Vitamin D-Dependent Rickets Type II.

Q: Which dietary vitamin is required for transaminase reactions?

Pyridoxine. **Explanation:** **Correct Option: C (Pyridoxine)** Pyridoxine (Vitamin B6) is the precursor for **Pyridoxal Phosphate (PLP)**, which serves as the essential coenzyme for all **transamination reactions**. In these reactions, an amino group is transferred from an amino acid to a keto acid (e.g., the conversion of Aspartate to Oxaloacetate via AST). PLP acts as a temporary carrier of the amino group, forming a Schiff base intermediate. Beyond transamination, PLP is also required for decarboxylation, deamination, and heme synthesis. **Why incorrect options are wrong:** * **A. Thiamine (B1):** Its active form, Thiamine Pyrophosphate (TPP), is a cofactor for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase) and the transketolase reaction in the HMP shunt. * **B. Folic Acid (B9):** Functions in **one-carbon metabolism** (transfer of methyl, formyl, or methylene groups), crucial for DNA synthesis and erythropoiesis. * **D. Cyanocobalamin (B12):** Acts as a coenzyme for only two human enzymes: Methionine synthase (homocysteine to methionine) and Methylmalonyl-CoA mutase. **High-Yield Clinical Pearls for NEET-PG:** * **Isoniazid (INH) Therapy:** This anti-tubercular drug inhibits pyridoxine kinase, leading to B6 deficiency. Patients must be co-prescribed B6 to prevent **peripheral neuropathy**. * **ALT and AST:** These are the most clinically relevant transaminases; both are strictly PLP-dependent. * **Cystathionine Synthase:** B6 is a cofactor here; its deficiency can lead to secondary **homocystinuria**. * **Sideroblastic Anemia:** B6 deficiency impairs ALA synthase (the rate-limiting step of heme synthesis), leading to microcytic anemia with ringed sideroblasts.

Q: Which of the following vitamins would most likely become deficient in a person who develops a completely carnivorous lifestyle?

Vitamin C. **Explanation:** **Vitamin C (Ascorbic Acid)** is the correct answer because it is primarily found in citrus fruits, berries, and green leafy vegetables. Animal tissues, particularly muscle meat, contain negligible amounts of Vitamin C. While organ meats (like liver or adrenals) contain some, a standard carnivorous diet lacks the necessary intake to prevent deficiency. Humans lack the enzyme **L-gulonolactone oxidase**, making Vitamin C an essential dietary requirement. Chronic deficiency leads to **Scurvy**, characterized by defective collagen synthesis (impaired hydroxylation of proline and lysine). **Why the other options are incorrect:** * **Thiamine (B1):** Widely distributed in both animal (pork, liver) and plant tissues. A carnivorous diet provides sufficient B1. * **Niacin (B3):** Abundant in meats, poultry, and fish. Furthermore, the body can synthesize Niacin from the amino acid **Tryptophan**, which is plentiful in animal proteins. * **Cobalamin (B12):** This is the opposite of Vitamin C; B12 is synthesized exclusively by microorganisms and is found **only in animal-derived foods**. A carnivore would have an abundance of B12, whereas a strict vegan would be at risk of deficiency. **Clinical Pearls for NEET-PG:** * **Scurvy Presentation:** "4 H’s" – Hemorrhage (petechiae, ecchymosis), Hyperkeratosis, Hypochondriasis, and Hematologic abnormalities (anemia). * **Key Enzyme Role:** Vitamin C is a co-factor for **prolyl and lysyl hydroxylase**, essential for the post-translational modification of collagen. * **Absorption:** Vitamin C enhances the absorption of **non-heme iron** by reducing it from the ferric ($Fe^{3+}$) to the ferrous ($Fe^{2+}$) state.

Question 1

A child has been diagnosed with vitamin D dependent rickets. What is the most appropriate vitamin D preparation for this condition?

Accepted Answer

Calcitriol

Answer

Calciferol

Answer

Cholecalciferol

Answer

Calcifediol

Question 2

Which of the following are NOT vitamin K dependent clotting factors?

Accepted Answer

Factor V and VIII

Answer

Factor II and IX

Answer

Factor IX and VII

Answer

Factor IX and X

Question 3

Hypervitaminosis A manifests as all of the following except?

Accepted Answer

Peripheral neuritis

Answer

Alopecia

Answer

Anorexia

Answer

Pseudotumor cerebri

Question 4

Vitamin B6 deficiency is associated with all of the following except?

Accepted Answer

Cyclosporine

Answer

Penicillamine

Answer

Isoniazid (INH)

Answer

Cycloserine

Question 5

All the following coenzymes participate in the transfer of hydrogen and electrons, EXCEPT?

Accepted Answer

PLP

Answer

FAD

Answer

NAD+

Answer

NADP+

Question 6

A 40-year-old woman, who has lost 36 kg over the past 2 years, presents with hair loss. She reports following a strict fat-free diet. Her alopecia is probably related to a deficiency of which of the following vitamins?

Accepted Answer

Vitamin A

Answer

Vitamin C

Answer

Vitamin D

Answer

Vitamin E

Question 7

A 75-year-old woman who eats sparingly due to limited income presents with pain in her right leg for the past 2 weeks. Physical examination reveals marked tenderness over the lateral aspect of the right shin, a poorly healed cut on her right hand, and a diffuse hyperkeratotic skin rash. Radiography shows a right tibial diaphyseal subperiosteal hematoma, and laboratory studies show a hemoglobin level of 11.3 g/dL. A deficiency of which of the following nutrients is most likely to explain these findings?

Accepted Answer

Ascorbic acid

Answer

Folate

Answer

Niacin

Answer

Riboflavin

Question 8

What is the mechanism of action of Calcitriol?

Accepted Answer

Increased calcium absorption from the intestine

Answer

Decreased calcium resorption from bone

Answer

Decreased calcium reabsorption by the kidney

Answer

Decreased calcium absorption from the intestine

Question 9

Which dietary vitamin is required for transaminase reactions?

Accepted Answer

Pyridoxine

Answer

Thiamine

Answer

Folic acid

Answer

Cyanocobalamin

Question 10

Which of the following vitamins would most likely become deficient in a person who develops a completely carnivorous lifestyle?

Accepted Answer

Vitamin C

Answer

Thiamine

Answer

Niacin

Answer

Cobalamin

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?