Vitamins and Coenzymes Practice Questions

Q: Which of the following vitamins is NOT obtained from a plant source?

Cobalamine. **Explanation:** The correct answer is **Cobalamine (Vitamin B12)**. **1. Why Cobalamine is the correct answer:** Vitamin B12 is unique among vitamins because it is synthesized exclusively by **microorganisms** (bacteria and archaea). It is not produced by plants or animals. Animals obtain it by ingesting contaminated food or through ruminal bacterial synthesis. Consequently, B12 is found naturally only in **animal-sourced foods** (meat, eggs, dairy, and shellfish). Strict vegetarians (vegans) are at a high risk of deficiency because plants do not contain B12 unless they are fortified or contaminated with nitrogen-fixing bacteria. **2. Why the other options are incorrect:** * **Riboflavin (B2):** Widely distributed in both plants and animals. Rich plant sources include green leafy vegetables and enriched cereals. * **Thiamine (B1):** Found abundantly in whole grains, legumes, and nuts. The outer layer of rice (bran) is a major source. * **Vitamin A:** While preformed Vitamin A (retinol) is found in animal products, plants provide **Provitamin A (Beta-carotene)**, which is converted to Vitamin A in the human body. Sources include carrots, spinach, and sweet potatoes. **Clinical Pearls for NEET-PG:** * **Absorption:** B12 requires **Intrinsic Factor (IF)** from gastric parietal cells for absorption 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 **Subacute Combined Degeneration (SCD)** of the spinal cord due to defective myelin synthesis (accumulation of methylmalonic acid). * **Key Enzyme:** B12 is a coenzyme for **Methionine Synthase** and **Methylmalonyl-CoA Mutase**.

Q: Megaloblastic anemia occurs due to deficiency of which vitamin?

Vitamin B12. **Explanation:** **Megaloblastic anemia** is characterized by the presence of large, nucleated red blood cell precursors (megaloblasts) in the bone marrow. This occurs primarily due to impaired **DNA synthesis**, while RNA and protein synthesis remain unaffected, leading to "nuclear-cytoplasmic asynchrony." **Why Vitamin B12 is correct:** Vitamin B12 (Cobalamin) is a crucial cofactor for the enzyme **Methionine Synthase**. This enzyme converts homocysteine to methionine and simultaneously converts 5-methyltetrahydrofolate (5-methyl THF) back into **Tetrahydrofolate (THF)**. In B12 deficiency, folate remains trapped in the 5-methyl THF form (the **"Folate Trap"**). Since THF is essential for purine and pyrimidine (dTMP) synthesis, its unavailability halts DNA replication, resulting in megaloblastic changes. **Why other options are incorrect:** * **Niacin (B3):** Deficiency leads to **Pellagra**, characterized by the 3 Ds: Dermatitis, Diarrhea, and Dementia. * **Vitamin C:** Deficiency causes **Scurvy**, characterized by defective collagen synthesis, leading to bleeding gums and poor wound healing. * **Vitamin B1 (Thiamine):** Deficiency leads to **Beriberi** (Dry/Wet) or **Wernicke-Korsakoff syndrome**, affecting the cardiovascular and nervous systems. **High-Yield Clinical Pearls for NEET-PG:** * **Folate (B9) Deficiency** also causes megaloblastic anemia, but unlike B12 deficiency, it **does not** cause neurological symptoms (Subacute Combined Degeneration of the Spinal Cord). * **Schilling Test:** Historically used to determine the cause of B12 malabsorption (e.g., Pernicious Anemia). * **Biochemical Markers:** In B12 deficiency, both **Methylmalonic Acid (MMA)** and **Homocysteine** levels are elevated. In Folate deficiency, only Homocysteine is elevated.

Q: Which of the following vitamins is involved in the post-translational modification of glutamate residues and plays an important role in the coagulation pathway?

Vitamin K. **Explanation:** **Correct Option: B (Vitamin K)** Vitamin K acts as a crucial coenzyme for the enzyme **$\gamma$-glutamyl carboxylase**. This enzyme adds a carboxyl group to specific glutamate (Glu) residues on certain proteins, converting them into **$\gamma$-carboxyglutamate (Gla)** residues. This post-translational modification is essential because the two adjacent carboxylic acid groups in Gla residues create a high-affinity binding site for **Calcium ($Ca^{2+}$)**. This "calcium bridge" allows clotting factors to bind to the phospholipid membranes of platelets, a vital step in the coagulation cascade. **Why other options are incorrect:** * **Vitamin C:** Involved in the post-translational **hydroxylation** of proline and lysine residues during collagen synthesis, not carboxylation. * **Vitamin D:** Primarily functions as a hormone regulating calcium and phosphate homeostasis; it does not act as a coenzyme for glutamate modification. * **Vitamin B12:** Acts as a coenzyme for methionine synthase and methylmalonyl-CoA mutase; its deficiency leads to megaloblastic anemia and neurological issues. **High-Yield NEET-PG Pearls:** 1. **Vitamin K-dependent factors:** Factors **II, VII, IX, X**, and Proteins **C and S**. 2. **Warfarin Mechanism:** Inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the regeneration of active (reduced) Vitamin K, thereby inhibiting the $\gamma$-carboxylation process. 3. **Clinical Marker:** Prothrombin Time (PT) is the most sensitive lab test to monitor Vitamin K status/Warfarin therapy. 4. **Newborns:** They are born with a sterile gut and low Vitamin K stores, necessitating a prophylactic Vitamin K injection at birth to prevent **Hemorrhagic Disease of the Newborn**.

Q: In vitamin A deficiency, which of the following is NOT a typical complaint?

Phosphoric calciuria. **Explanation:** Vitamin A (Retinol) is essential for vision, epithelial integrity, and immune function. The correct answer is **Phosphoric calciuria** because it is not associated with Vitamin A deficiency; rather, it is a clinical feature related to **Vitamin D** metabolism or parathyroid disorders. **Why Phosphoric calciuria is the correct choice:** Phosphoric calciuria (excessive excretion of calcium and phosphate in urine) is typically seen in conditions like Vitamin D toxicity or hyperparathyroidism. Vitamin A does not play a direct role in the renal handling of phosphate or calcium. **Analysis of Incorrect Options:** * **Night Blindness (Nyctalopia):** This is the earliest symptom of Vitamin A deficiency. It occurs due to the failure to regenerate **Rhodopsin** (visual purple) in the rod cells of the retina, which is necessary for low-light vision. * **Xerophthalmia:** This refers to a spectrum of ocular manifestations. Deficiency leads to the keratinization of the conjunctiva and cornea, causing dryness (xerosis), Bitot’s spots, and eventually keratomalacia (corneal melting). * **Keratosis:** Vitamin A is vital for maintaining healthy epithelial tissues. Deficiency leads to **Follicular Hyperkeratosis** (Phrynoderma or "Toad skin"), where skin becomes rough and scaly due to excessive keratin plugs in hair follicles. **High-Yield Clinical Pearls for NEET-PG:** * **WHO Classification:** The first clinical sign of Vitamin A deficiency is Night Blindness (XN), while the first objective sign is Conjunctival Xerosis (X1A). * **Storage:** Vitamin A is stored in the **Ito cells** (Stellate cells) of the liver. * **Transport:** It is transported in the blood bound to **Retinol Binding Protein (RBP)** and Pre-albumin. * **Teratogenicity:** Isotretinoin (a Vitamin A derivative) is highly teratogenic; a negative pregnancy test is mandatory before prescription.

Q: Which enzyme is thiamine dependent?

Transketolase. **Explanation:** Thiamine (Vitamin B1) functions in its active form, **Thiamine Pyrophosphate (TPP)**, as a vital coenzyme for oxidative decarboxylation and carbon-transfer reactions. **Why Transketolase is Correct:** Transketolase is a key enzyme in the **Pentose Phosphate Pathway (HMP Shunt)**. It requires TPP to transfer two-carbon units between sugar phosphates. In clinical practice, measuring erythrocyte transketolase activity is the gold standard for diagnosing thiamine deficiency. **Analysis of Incorrect Options:** * **B. Transaldolase:** Also part of the HMP Shunt, but it does **not** require a cofactor. It uses a lysine residue at its active site to form a Schiff base with the substrate. * **C. Amino transferase (Transaminase):** These enzymes (e.g., ALT, AST) require **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6, for the transfer of amino groups. * **D. Enolase:** An enzyme in glycolysis that converts 2-phosphoglycerate to phosphoenolpyruvate. It is dependent on **Magnesium (Mg²⁺)** and is inhibited by fluoride. **High-Yield NEET-PG Pearls:** * **The "T-A-P" Mnemonic:** Thiamine is a cofactor for four major enzymes: 1. **T**ransketolase (HMP Shunt) 2. **A**lpha-ketoglutarate dehydrogenase (TCA Cycle) 3. **P**yruvate dehydrogenase (Link reaction) 4. **B**ranched-chain alpha-ketoacid dehydrogenase (Metabolism of Leucine, Isoleucine, Valine). * **Clinical Correlation:** Deficiency leads to **Beriberi** (Dry/Wet) and **Wernicke-Korsakoff Syndrome**. Always administer thiamine *before* glucose in malnourished patients to prevent precipitating Wernicke’s encephalopathy.

Q: Which of the following is a lipid-soluble plasma membrane-associated antioxidant?

Vitamin E. **Explanation:** The correct answer is **Vitamin E (α-tocopherol)**. **1. Why Vitamin E is correct:** Vitamin E is a potent **lipid-soluble** antioxidant. Its primary physiological role is to act as a "chain-breaker" in the process of **lipid peroxidation**. Because of its hydrophobic nature, it integrates into the phospholipid bilayer of plasma membranes and lipoproteins. It protects the membrane from oxidative damage by scavenging free radicals (like superoxide and hydroxyl radicals), specifically preventing the oxidation of polyunsaturated fatty acids (PUFAs). **2. Why the other options are incorrect:** * **Ubiquitin (A):** This is a small regulatory protein, not an antioxidant. It marks misfolded or damaged proteins for degradation via the **Proteasome pathway** (Ubiquitin-Proteasome System). * **Vitamin C (C):** While a powerful antioxidant, Vitamin C (Ascorbic acid) is **water-soluble**. It acts in the aqueous compartments of the cell (cytosol) and is responsible for regenerating the reduced form of Vitamin E. * **Glutathione (D):** This is a tripeptide (Glu-Cys-Gly) that serves as the major **intracellular, water-soluble** antioxidant. It acts as a substrate for Glutathione Peroxidase to neutralize hydrogen peroxide. **3. High-Yield Clinical Pearls for NEET-PG:** * **Synergy:** Vitamin C and Vitamin E work together; Vitamin C reduces the tocopheroxyl radical back to active Vitamin E. * **Deficiency:** Vitamin E deficiency leads to **hemolytic anemia** (due to fragile RBC membranes) and posterior column neurological defects (mimicking Friedreich’s ataxia). * **Toxicity:** Excess Vitamin E can interfere with Vitamin K metabolism, leading to an increased risk of bleeding (prolonged PT/INR). * **Selenium:** This trace element is a cofactor for Glutathione Peroxidase and works synergistically with Vitamin E.

Q: Which of the following foods contains the highest quantity of vitamin C?

Indian gooseberry. **Explanation:** Vitamin C (Ascorbic acid) is a water-soluble vitamin essential for collagen synthesis, antioxidant defense, and iron absorption. While all citrus fruits are excellent sources, the **Indian Gooseberry (Amla)** is renowned in biochemistry and nutrition as one of the richest natural sources of Vitamin C. * **Why Indian Gooseberry is Correct:** Amla contains approximately **600–700 mg of Vitamin C per 100g**, which is nearly 10 to 20 times higher than the concentration found in oranges. It also contains tannins that prevent the oxidation of the vitamin, making it more stable even when processed. * **Why Others are Incorrect:** * **Orange and Lemon:** These are classic citrus sources but contain significantly less Vitamin C (approx. 30–50 mg/100g) compared to Amla. * **Grapes:** While they contain antioxidants like resveratrol, their Vitamin C content is relatively low (approx. 4–10 mg/100g). **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **Biochemical Function:** Vitamin C acts as a coenzyme for **prolyl and lysyl hydroxylase**, which are essential for the post-translational modification of procollagen. Deficiency leads to **Scurvy** (characterized by spongy gums, petechiae, and poor wound healing). 2. **Iron Absorption:** Vitamin C enhances the absorption of **non-heme iron** by reducing it from the ferric ($Fe^{3+}$) to the ferrous ($Fe^{2+}$) state. 3. **Antioxidant Role:** It regenerates Vitamin E and acts as a scavenger of free radicals. 4. **Highest Source Overall:** While Amla is the highest among the given options, the **Barbados cherry (Acerola)** actually holds the highest concentration of Vitamin C in the plant kingdom.

Question 1

Which of the following vitamins is NOT obtained from a plant source?

Accepted Answer

Cobalamine

Answer

Riboflavin

Answer

Thiamine

Answer

Vitamin A

Question 2

Megaloblastic anemia occurs due to deficiency of which vitamin?

Accepted Answer

Vitamin B12

Answer

Niacin

Answer

Vitamin C

Answer

Vitamin B1

Question 3

Which of the following vitamins is involved in the post-translational modification of glutamate residues and plays an important role in the coagulation pathway?

Accepted Answer

Vitamin K

Answer

Vitamin C

Answer

Vitamin D

Answer

Vitamin B12

Question 4

In vitamin A deficiency, which of the following is NOT a typical complaint?

Accepted Answer

Phosphoric calciuria

Answer

Night blindness

Answer

Xerophthalmia

Answer

Keratosis

Question 5

Which enzyme is thiamine dependent?

Accepted Answer

Transketolase

Answer

Transaldolase

Answer

Amino transferase

Answer

Eno lase

Question 6

Which of the following is a lipid-soluble plasma membrane-associated antioxidant?

Accepted Answer

Vitamin E

Answer

Ubiquitin

Answer

Vitamin C

Answer

Glutathione

Question 7

Which of the following foods contains the highest quantity of vitamin C?

Accepted Answer

Indian gooseberry

Answer

Orange

Answer

Lemon

Answer

Grapes

Question 8

Which vitamin deficiency leads to lactic acidosis?

Accepted Answer

Thiamine

Answer

Riboflavin

Answer

Niacin

Answer

Pantothenic acid

Question 9

The laboratory diagnosis of thiamine deficiency is by:

Accepted Answer

Transketolase assay measured before and after the addition of thiamine pyrophosphate

Answer

Transaldolase assay measured before and after the addition of thiamine pyrophosphate

Answer

Thiamine pyrophosphate assay in RBC after adding sugar

Answer

Serum LDH levels

Question 10

Hangover following alcohol consumption can be treated with which of the following vitamins?

Accepted Answer

Thiamine

Answer

Pyridoxine

Answer

Riboflavin

Answer

Niacin

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?