Vitamins and Coenzymes Practice Questions

Q: Which vitamin is lost on exposure to sunlight?

Vitamin C. **Explanation:** **Vitamin C (Ascorbic Acid)** is the correct answer because it is the most heat-labile and chemically unstable of all vitamins. It is highly sensitive to oxidation, which is accelerated by exposure to **sunlight (UV radiation)**, heat, and alkaline mediums. In the context of food preparation, significant amounts of Vitamin C are lost during drying in the sun, prolonged storage, or open-pan cooking. **Analysis of Incorrect Options:** * **Vitamin A (Retinol):** While Vitamin A is sensitive to oxidation and high temperatures, it is relatively stable compared to Vitamin C. It is primarily lost through rancidification of fats rather than direct sunlight exposure during routine handling. * **Vitamin B1 (Thiamine):** Thiamine is very sensitive to heat (thermolabile) and alkaline pH (e.g., adding baking soda to pulses), but it is not specifically characterized by loss due to sunlight exposure. * **Vitamin B6 (Pyridoxine):** Pyridoxine is fairly stable to heat and acids, though it can be sensitive to light in its pure liquid form; however, in the hierarchy of "vitamins lost on exposure," Vitamin C is the classic medical teaching and the most significant. **Clinical Pearls for NEET-PG:** * **Most Heat-Labile Vitamin:** Vitamin C (destroyed by boiling/canning). * **Scurvy:** The clinical manifestation of Vitamin C deficiency, characterized by "corkscrew hair," petechiae, and bleeding gums. * **Biological Function:** Essential for the post-translational modification of collagen (hydroxylation of proline and lysine). * **Enhancer:** Vitamin C significantly increases the absorption of **non-heme iron** from the gut by reducing it from the ferric ($Fe^{3+}$) to the ferrous ($Fe^{2+}$) state.

Q: A 28-year-old primigravida gave birth to a male child who presented with a midline dimple and a tuft of hair in the lower back. Which vitamin supplementation could have possibly prevented this if it was due to nutritional deficiency in the mother?

Vitamin B9 (Folic Acid). ### Explanation **Correct Option: A. Vitamin B9 (Folic Acid)** The clinical presentation of a **midline dimple and a tuft of hair** over the lumbosacral region is a classic sign of **Spina Bifida Occulta**, the mildest form of Neural Tube Defects (NTDs). NTDs occur due to the failure of the neural tube to close during the 3rd and 4th weeks of gestation (primary neurulation). **Folic acid (Vitamin B9)** is essential for DNA synthesis and methylation. Maternal deficiency of folate is a well-established risk factor for NTDs. Supplementation significantly reduces the incidence of these defects by ensuring proper closure of the neural tube. **Why other options are incorrect:** * **Vitamin B6 (Pyridoxine):** Primarily used to manage intractable seizures in neonates (Pyridoxine-dependent epilepsy) or to prevent peripheral neuropathy in patients taking Isoniazid. It is not linked to NTDs. * **Vitamin B12 (Cobalamin):** While B12 deficiency can theoretically affect the folate cycle, it is not the primary vitamin targeted for the prevention of NTDs in standard obstetric protocols. * **Vitamin B3 (Niacin):** Deficiency leads to Pellagra (Dermatitis, Diarrhea, Dementia, Death). It does not play a role in neural tube closure. --- ### High-Yield Clinical Pearls for NEET-PG: * **Timing:** The neural tube closes by **Day 28** of gestation. Therefore, supplementation must begin **pre-conceptionally**. * **Dosage:** * **Low risk:** 400 mcg (0.4 mg) daily. * **High risk** (previous child with NTD or mother on anticonvulsants like Valproate): 4 mg daily. * **Screening:** Elevated **Alpha-fetoprotein (AFP)** in maternal serum or amniotic fluid suggests an open NTD (Note: AFP is normal in Spina Bifida Occulta). * **Lemon Sign & Banana Sign:** Characteristic ultrasound findings in the fetal skull and cerebellum associated with Spina Bifida.

Q: Which coenzyme is essential for carboxylation reactions?

Biotin. **Explanation:** **Biotin (Vitamin B7)** is the essential coenzyme for all major **carboxylation reactions** in the human body. It acts as a carrier of activated carbon dioxide (CO₂), which is attached to the biotin molecule via an ATP-dependent process before being transferred to the substrate. The four "high-yield" biotin-dependent carboxylases are: 1. **Pyruvate Carboxylase:** Converts pyruvate to oxaloacetate (Gluconeogenesis). 2. **Acetyl-CoA Carboxylase:** Converts Acetyl-CoA to Malonyl-CoA (Fatty acid synthesis). 3. **Propionyl-CoA Carboxylase:** Converts Propionyl-CoA to Methylmalonyl-CoA (Odd-chain fatty acid metabolism). 4. **3-Methylcrotonyl-CoA Carboxylase:** Involved in Leucine catabolism. **Why other options are incorrect:** * **A. Pyridoxine (B6):** Primarily serves as a coenzyme for **transamination** and **decarboxylation** (removal of CO₂), not carboxylation. * **B. Thiamine (B1):** As Thiamine Pyrophosphate (TPP), it is essential for **oxidative decarboxylation** (e.g., Pyruvate Dehydrogenase complex) and transketolase reactions. * **D. Cyanocobalamin (B12):** Involved in **methylation** (Homocysteine to Methionine) and the isomerization of methylmalonyl-CoA to succinyl-CoA. **Clinical Pearls for NEET-PG:** * **Avidin:** A protein in raw egg whites that binds biotin tightly, preventing its absorption and leading to "egg white injury." * **ABC Enzymes:** Remember that most carboxylases require **A**TP, **B**iotin, and **C**O₂. * **Holocarboxylase Synthetase Deficiency:** A rare genetic disorder where biotin cannot be attached to carboxylase enzymes, leading to multiple carboxylase deficiency.

Q: Which of the following is a provitamin?

Beta-carotene. ### Explanation **Correct Answer: C. Beta-carotene** **Understanding the Concept: Provitamins** A **provitamin** (or vitamin precursor) is a substance that has little or no vitamin activity itself but can be converted into an active vitamin form by normal metabolic processes within the body. **Beta-carotene** is the most potent provitamin A. It is a plant-derived carotenoid (found in carrots and leafy greens) that is cleaved in the intestinal mucosa by the enzyme **$\beta$-carotene 15,15'-dioxygenase** to yield two molecules of **Retinal** (Vitamin A). **Analysis of Incorrect Options:** * **A. Ascorbic acid (Vitamin C):** This is the active, water-soluble form of the vitamin itself. It does not require metabolic conversion to function as an antioxidant or a cofactor for prolyl hydroxylase. * **B. Vitamin E (Tocopherol):** This is a group of fat-soluble compounds (primarily $\alpha$-tocopherol) that are biologically active upon ingestion. * **D. Vitamin K:** Whether in the form of Phylloquinone (K1) or Menaquinone (K2), these are active vitamins essential for the $\gamma$-carboxylation of clotting factors (II, VII, IX, X). **High-Yield Clinical Pearls for NEET-PG:** * **Conversion Efficiency:** It takes approximately 6 $\mu$g of beta-carotene to produce 1 $\mu$g of Retinol Activity Equivalent (RAE). * **Hypervitaminosis A:** Unlike preformed Vitamin A (Retinol), excessive intake of Beta-carotene does not cause Vitamin A toxicity because the conversion rate decreases as Vitamin A stores rise. It instead causes **Carotenemia** (yellowish skin discoloration, sparing the sclera). * **Other Provitamins:** Another high-yield provitamin is **7-Dehydrocholesterol**, which is converted to Vitamin D3 (Cholecalciferol) in the skin via UV light.

Q: Angular oral fissures are most commonly treated with which of the following?

Riboflavin. **Explanation:** **1. Why Riboflavin (Vitamin B2) is Correct:** Angular cheilitis (fissures at the corners of the mouth) is the hallmark clinical sign of **Riboflavin deficiency**. Vitamin B2 is a precursor for the coenzymes **FAD and FMN**, which are essential for redox reactions in the electron transport chain and various metabolic pathways. Deficiency typically presents with a "triple C" syndrome: **C**heilosis (angular stomatitis), **C**orneal vascularization, and **C**onjunctivitis/Glossitis (magenta tongue). **2. Why the Other Options are Incorrect:** * **Pyridoxine (B6):** While B6 deficiency can cause glossitis and dermatitis, its primary clinical manifestations are peripheral neuropathy, sideroblastic anemia, and seizures (due to decreased GABA synthesis). * **Cyanocobalamin (B12):** Deficiency leads to Megaloblastic anemia and Subacute Combined Degeneration (SCD) of the spinal cord. While it can cause a sore tongue, it is not the primary treatment for angular fissures. * **Vitamin C:** Deficiency leads to **Scurvy**, characterized by "corkscrew hair," petechiae, and swollen, bleeding gums (scurbutic rosary), rather than isolated angular fissures. **3. NEET-PG High-Yield Pearls:** * **Magenta Tongue:** Classic description for Riboflavin deficiency (vs. "Beefy red tongue" in B12/Niacin deficiency). * **Erythrocyte Glutathione Reductase Activity:** This is the functional gold-standard test to diagnose Riboflavin deficiency. * **Phototherapy Link:** Neonates receiving phototherapy for jaundice are at risk of B2 deficiency because Riboflavin is light-sensitive and degrades easily. * **Warburg’s Yellow Enzyme:** An old name for the first discovered flavoprotein containing Riboflavin.

Q: Which of the following vitamins enhances the absorption of iron?

Vitamin C. **Explanation:** **Vitamin C (Ascorbic Acid)** is the correct answer because it enhances the absorption of **non-heme iron** (found in plant sources) through two primary mechanisms: 1. **Reduction:** It reduces dietary ferric iron ($Fe^{3+}$) to the ferrous state ($Fe^{2+}$), which is the only form capable of being transported across the intestinal epithelium via the Divalent Metal Transporter 1 (DMT1). 2. **Chelation:** It forms a soluble iron-ascorbate complex in the acidic environment of the stomach, preventing the precipitation of iron by phytates or phosphates in the alkaline medium of the duodenum. **Analysis of Incorrect Options:** * **Vitamin K:** Essential for the post-translational gamma-carboxylation of clotting factors II, VII, IX, and X. It has no role in mineral absorption. * **Vitamin B1 (Thiamine):** Acts as a coenzyme (TPP) for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase). Deficiency leads to Beriberi or Wernicke-Korsakoff syndrome. * **Folic Acid:** Crucial for one-carbon metabolism and DNA synthesis. While often co-prescribed with iron to treat nutritional anemia, it does not directly facilitate iron absorption. **High-Yield Clinical Pearls for NEET-PG:** * **Inhibitors of Iron Absorption:** Phytates (cereals), oxalates, polyphenols (tea/coffee), and calcium/antacids decrease iron uptake. * **Storage & Transport:** Iron is stored as **Ferritin** (primary) or Hemosiderin and transported in the blood by **Transferrin**. * **Clinical Correlation:** Patients on oral iron supplements are advised to take them with orange juice (rich in Vitamin C) to maximize bioavailability.

Q: All of the following are free radicals except?

Hydrogen peroxide. ### Explanation The core concept in this question is the definition of a **Free Radical**: an atom or molecule that contains one or more **unpaired electrons** in its outer shell, making it highly unstable and reactive. **Why Hydrogen Peroxide (H₂O₂) is the correct answer:** While Hydrogen peroxide is a **Reactive Oxygen Species (ROS)**, it is **not a free radical**. It does not have any unpaired electrons in its molecular orbitals. It is a stable (though reactive) molecule that can cross biological membranes easily. However, it can lead to the formation of the highly dangerous hydroxyl radical (•OH) via the **Fenton reaction** in the presence of transition metals like Iron (Fe²⁺). **Analysis of Incorrect Options:** * **A. Superoxide (O₂•⁻):** This is a primary free radical formed by the addition of a single electron to molecular oxygen. It is the precursor to most other ROS. * **C. Peroxyl (ROO•):** A free radical intermediate typically formed during the process of lipid peroxidation. * **D. Hydroperoxyl (HOO•):** This is the protonated form of superoxide. It is a free radical and is more lipid-soluble than superoxide, allowing it to initiate membrane damage. **High-Yield Clinical Pearls for NEET-PG:** 1. **Reactive Oxygen Species (ROS):** This category includes both free radicals (Superoxide, Hydroxyl, Nitric Oxide) and non-radicals (Hydrogen peroxide, Singlet oxygen, Ozone). 2. **The Fenton Reaction:** $Fe^{2+} + H_2O_2 \rightarrow Fe^{3+} + \bullet OH + OH^-$ (Crucial for understanding iron toxicity). 3. **Haber-Weiss Reaction:** $O_2^{\bullet-} + H_2O_2 \rightarrow \bullet OH + OH^- + O_2$. 4. **Antioxidant Defense:** **Superoxide Dismutase (SOD)** converts superoxide to $H_2O_2$, which is then neutralized by **Catalase** or **Glutathione Peroxidase**.

Question 1

Which vitamin is lost on exposure to sunlight?

Accepted Answer

Vitamin C

Answer

Vitamin A

Answer

Vitamin B1

Answer

Vitamin B6

Question 2

Vitamin D deficiency causes all except:

Accepted Answer

Microdontia

Answer

Widening of predentin

Answer

Defective calcification

Answer

Interglobular dentin formation

Question 3

A 28-year-old primigravida gave birth to a male child who presented with a midline dimple and a tuft of hair in the lower back. Which vitamin supplementation could have possibly prevented this if it was due to nutritional deficiency in the mother?

Accepted Answer

Vitamin B9 (Folic Acid)

Answer

Vitamin B6 (Pyridoxine)

Answer

Vitamin B12 (Cobalamin)

Answer

Vitamin B3 (Niacin)

Question 4

Which of the following patients requires Vitamin B12 supplementation?

Accepted Answer

Patient on a strict vegetarian diet

Answer

Patient with normal B12 level having dementia

Answer

Patient with elevated homocysteine level

Answer

Patient with general debility

Question 5

Which coenzyme is essential for carboxylation reactions?

Accepted Answer

Biotin

Answer

Pyridoxine

Answer

Thiamine

Answer

Cyanocobalamin

Question 6

Which of the following is a provitamin?

Accepted Answer

Beta-carotene

Answer

Ascorbic acid

Answer

Vitamin E

Answer

Vitamin K

Question 7

Angular oral fissures are most commonly treated with which of the following?

Accepted Answer

Riboflavin

Answer

Pyridoxine

Answer

Cyanocobalamin

Answer

Vitamin C

Question 8

Megaloblastic anemia due to folic acid deficiency is commonly due to what factor?

Accepted Answer

Inadequate dietary intake

Answer

Defective intestinal absorption

Answer

Absence of folic acid binding protein in serum

Answer

Absence of glutamic acid in the intestine

Question 9

Which of the following vitamins enhances the absorption of iron?

Accepted Answer

Vitamin C

Answer

Vitamin K

Answer

Vitamin B1

Answer

Folic acid

Question 10

All of the following are free radicals except?

Accepted Answer

Hydrogen peroxide

Answer

Superoxide

Answer

Peroxyl

Answer

Hydroperoxyl

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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