Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 151: Angular oral fissures are most commonly treated with which of the following?

A. Riboflavin (Correct Answer)
B. Pyridoxine
C. Cyanocobalamin
D. Vitamin C

Explanation: **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.

Question 152: Which of the following vitamins enhances the absorption of iron?

A. Vitamin K
B. Vitamin C (Correct Answer)
C. Vitamin B1
D. Folic acid

Explanation: **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.

Question 153: All of the following are free radicals except?

A. Superoxide
B. Hydrogen peroxide (Correct Answer)
C. Peroxyl
D. Hydroperoxyl

Explanation: ### 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 154: All of the following are antioxidants except?

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

Explanation: ### Explanation The correct answer is **Vitamin B**. **1. Why Vitamin B is the correct answer:** Antioxidants are substances that neutralize free radicals (Reactive Oxygen Species) to prevent cellular damage. While **Vitamin B complex** members (like B1, B2, B3, B6, and B12) are essential coenzymes in metabolic pathways (e.g., Krebs cycle, DNA synthesis), they do not function directly as primary antioxidants. Some B vitamins act as "indirect" antioxidants (e.g., Riboflavin is a cofactor for Glutathione Reductase), but they are not classified as dietary antioxidants in the same category as Vitamins A, C, and E. **2. Why the other options are incorrect:** * **Vitamin A (Beta-carotene):** This is a lipid-soluble antioxidant that protects against lipid peroxidation and is particularly effective at quenching singlet oxygen. * **Vitamin C (Ascorbic Acid):** A potent water-soluble antioxidant. It directly scavenges free radicals and is crucial for regenerating the reduced form of Vitamin E. * **Vitamin E (Tocopherol):** The most powerful naturally occurring lipid-soluble antioxidant. It protects cell membranes from oxidative stress by breaking the chain reaction of lipid peroxidation. **3. NEET-PG High-Yield Clinical Pearls:** * **The "ACE" Mnemonic:** Remember **A, C, and E** as the primary antioxidant vitamins. * **Synergy:** Vitamin C and Vitamin E work together; Vitamin C "recycles" oxidized Vitamin E back to its active form. * **Enzymatic Antioxidants:** Apart from vitamins, the body uses enzymes like **Superoxide Dismutase (SOD)** (requires Zn, Cu, Mn), **Catalase** (requires Fe), and **Glutathione Peroxidase** (requires Selenium). * **Selenium:** Often tested as the essential trace element that functions as an antioxidant via the Glutathione Peroxidase system.

Question 155: All of the following are Vitamin B6 dependent syndromes, EXCEPT?

A. Pyridoxine dependent seizures
B. Xanthurenic aciduria
C. Cystanthioninuria
D. Darier disease (Correct Answer)

Explanation: ### Explanation The correct answer is **Darier disease** because it is a genetic skin disorder, not a metabolic syndrome related to Vitamin B6 (Pyridoxine) deficiency or dependency. #### 1. Why Darier Disease is the Correct Answer **Darier disease** (Keratosis follicularis) is an autosomal dominant genodermatosis caused by a mutation in the **ATP2A2 gene**, which encodes the SERCA2 calcium pump. This leads to impaired intracellular calcium signaling and loss of adhesion between keratinocytes (acantholysis). It presents with greasy, warty papules in seborrheic areas and has no biochemical link to Vitamin B6. #### 2. Analysis of Vitamin B6 Dependent Syndromes Vitamin B6 (as Pyridoxal Phosphate - PLP) is a crucial coenzyme for transamination, decarboxylation, and sulfur amino acid metabolism. * **Pyridoxine-dependent seizures:** A rare genetic condition where a mutation in the *ALDH7A1* gene leads to the accumulation of metabolites that inactivate PLP. It presents as intractable neonatal seizures that respond only to high doses of Vitamin B6. * **Xanthurenic aciduria:** Occurs due to a defect in **kynureninase** (a PLP-dependent enzyme in the Tryptophan-Niacin pathway). In B6 deficiency, tryptophan metabolism is diverted, leading to the excretion of xanthurenic acid in the urine. * **Cystanthioninuria:** Caused by a deficiency of **cystathionase**, the PLP-dependent enzyme that converts cystathionine to cysteine. This results in the accumulation and urinary excretion of cystathionine. #### 3. Clinical Pearls for NEET-PG * **Sideroblastic Anemia:** B6 is a cofactor for **ALA synthase** (the rate-limiting step in heme synthesis). Deficiency or inhibition (e.g., by Isoniazid) leads to microcytic anemia with ringed sideroblasts. * **Homocystinuria (Type I):** Due to cystathionine β-synthase deficiency; some patients are "B6 responsive." * **Isoniazid (INH) Therapy:** Always co-prescribe B6 to prevent peripheral neuropathy, as INH induces B6 deficiency.

Question 156: Excess of leucine causes:

A. Pellagra (Correct Answer)
B. Constipation
C. Hypervitaminosis
D. Progeria

Explanation: **Explanation:** The correct answer is **Pellagra (Option A)**. **Mechanism:** Pellagra is traditionally associated with a deficiency of Niacin (Vitamin B3) or its precursor, the amino acid Tryptophan. However, a high intake of **Leucine** (commonly seen in populations consuming Jowar/Sorghum as a staple diet) can precipitate Pellagra through two primary mechanisms: 1. **Inhibition of Quinolinate Phosphoribosyl Transferase (QPRT):** Leucine inhibits this key enzyme in the *Kynurenine pathway*, which converts Tryptophan to Niacin (NAD/NADP). 2. **Increased Tryptophan Oxygenase activity:** Excess leucine increases the activity of this enzyme, leading to the rapid breakdown of tryptophan, further reducing its availability for niacin synthesis. **Analysis of Incorrect Options:** * **B. Constipation:** Pellagra is actually characterized by the "3 Ds"—Dermatitis, Dementia, and **Diarrhea**. Constipation is not a clinical feature of leucine excess or niacin deficiency. * **C. Hypervitaminosis:** This refers to vitamin toxicity (usually fat-soluble vitamins like A or D). Leucine is an amino acid, and its excess leads to a functional *deficiency* of a vitamin, not an excess. * **D. Progeria:** This is a rare genetic condition characterized by premature aging (Hutchinson-Gilford syndrome), unrelated to amino acid intake or niacin metabolism. **NEET-PG High-Yield Pearls:** * **The 4 Ds of Pellagra:** Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death (if untreated). * **Dietary Link:** Maize (corn) diets lack tryptophan and contain bound niacin (niacytin); Sorghum (Jowar) diets contain high leucine. Both lead to Pellagra. * **Hartnup Disease:** A genetic defect in neutral amino acid transport (including tryptophan) that also presents with pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause pellagra because tryptophan is diverted to produce massive amounts of Serotonin (5-HT) instead of Niacin.

Question 157: The dark adaptation time in Vitamin A deficiency is:

A. Increased (Correct Answer)
B. Decreased
C. Unchanged
D. Fluctuating

Explanation: **Explanation:** **Why the correct answer is right:** Dark adaptation is the process by which the eyes become more sensitive to light in low-illumination settings. This process depends on the regeneration of **Rhodopsin** (visual purple) in the rod cells of the retina. Rhodopsin is a conjugated protein consisting of **11-cis-retinal** (a Vitamin A derivative) and the protein opsin. In Vitamin A deficiency, there is a decreased availability of 11-cis-retinal, leading to a failure in the rapid resynthesis of Rhodopsin. Consequently, it takes a significantly longer time for the rods to reach the threshold of sensitivity required to see in the dark. Therefore, the **dark adaptation time is increased**. This is the earliest clinical manifestation of Vitamin A deficiency, leading to Nyctalopia (night blindness). **Why the incorrect options are wrong:** * **Decreased:** A decreased time would imply faster-than-normal adaptation, which is physiologically impossible in a deficiency state where the essential precursor for the visual cycle is missing. * **Unchanged:** Vitamin A is the structural backbone of the visual cycle; its absence directly impairs the biochemical pathway, making an "unchanged" status impossible. * **Fluctuating:** The deficiency leads to a progressive and consistent decline in Rhodopsin regeneration, not an erratic or fluctuating pattern. **High-Yield NEET-PG Pearls:** * **Wald’s Visual Cycle:** The biochemical pathway describing the interconversion of Vitamin A isomers in the retina. * **Earliest Symptom:** Night blindness (Nyctalopia) is the earliest symptom of Vitamin A deficiency. * **Earliest Sign:** Conjunctival xerosis (followed by Bitot’s spots). * **WHO Classification:** X1A (Conjunctival xerosis), X1B (Bitot’s spots), X2 (Corneal xerosis), X3A/B (Keratomalacia). * **Retinoic Acid:** This form of Vitamin A is involved in gene expression and epithelial maintenance but **cannot** be used for the visual cycle (only Retinal is used).

Question 158: Which of the following vitamins contains sulfur in its structure?

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

Explanation: **Explanation:** The correct answer is **Thiamine (Vitamin B1)**. Thiamine is chemically composed of two rings: a **pyrimidine ring** and a **thiazole ring**, linked by a methylene bridge. The "thia-" in thiamine specifically refers to the **sulfur atom** contained within the thiazole ring. **Analysis of Options:** * **Thiamine (B1):** Contains sulfur. Its active form, Thiamine Pyrophosphate (TPP), is a vital coenzyme for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase complex). * **Riboflavin (B2):** Contains a dimethyl isoalloxazine ring attached to ribitol. It does not contain sulfur. * **Folic Acid (B9):** Composed of p-aminobenzoic acid (PABA), a pteridine ring, and glutamic acid. It does not contain sulfur. * **Niacin (B3):** Derived from tryptophan and consists of a simple pyridine ring structure. It does not contain sulfur. **High-Yield Clinical Pearls for NEET-PG:** 1. **Sulfur-containing Vitamins:** There are only two—**Thiamine (B1)** and **Biotin (B7)**. (Note: Lipoic acid also contains sulfur and acts as a co-factor, though it is not strictly a vitamin). 2. **Sulfur-containing Amino Acids:** Methionine, Cysteine, and Cystine. 3. **Deficiency:** Thiamine deficiency leads to **Beriberi** (Dry/Wet) and **Wernicke-Korsakoff syndrome**. 4. **Key Enzyme:** Thiamine is a cofactor for **Transketolase**; measuring erythrocyte transketolase activity is the best diagnostic test for B1 deficiency.

Question 159: Which of the following vitamins is not associated with toxicity?

A. Pantothenic acid (Correct Answer)
B. Pyridoxine
C. Vitamin A
D. Vitamin E

Explanation: ### Explanation The correct answer is **Pantothenic Acid (Vitamin B5)**. **1. Why Pantothenic Acid is the correct answer:** Pantothenic acid is a water-soluble vitamin that serves as a precursor for **Coenzyme A (CoA)** and the **Acyl Carrier Protein (ACP)**. It is widely distributed in foods (Greek: *pantos* meaning "everywhere"). Because it is water-soluble, any excess intake is rapidly excreted in the urine. To date, there is no known clinical toxicity or "Upper Intake Level" (UL) established for Vitamin B5 in humans, even at high supplemental doses. **2. Why the other options are incorrect:** * **Vitamin A (Retinol):** This is a fat-soluble vitamin stored in the liver. Excessive intake leads to **Hypervitaminosis A**, characterized by increased intracranial pressure (pseudotumor cerebri), hepatomegaly, and skin desquamation. It is also highly teratogenic. * **Vitamin E (Tocopherol):** While generally safe, very high doses of Vitamin E can interfere with Vitamin K metabolism, leading to an increased risk of **hemorrhage** (due to antagonism of vitamin K-dependent clotting factors). * **Pyridoxine (Vitamin B6):** Although water-soluble, chronic high doses of B6 (usually >200 mg/day) can cause severe **sensory neuropathy** and nerve degeneration. This makes it a classic "exception" among B-complex vitamins regarding toxicity. **3. NEET-PG High-Yield Pearls:** * **Fat-soluble vitamins (A, D, E, K)** are generally more toxic than water-soluble vitamins because they are stored in adipose tissue and the liver. * **Vitamin B6 (Pyridoxine)** is the most common water-soluble vitamin associated with sensory neurotoxicity. * **Vitamin C** toxicity can lead to **oxalate kidney stones** due to its metabolism to oxalate. * **Niacin (B3)** toxicity causes "flushing" due to prostaglandin-mediated vasodilation.

Question 160: All of the following are vitamers of Vitamin B6, except?

A. Pyridoxine
B. Pyridoxone (Correct Answer)
C. Pyridoxal
D. Pyridoxamine

Explanation: **Explanation:** Vitamin B6 is a water-soluble vitamin that exists in three natural forms, known as **vitamers**. These vitamers are metabolic precursors to the active coenzyme form, **Pyridoxal Phosphate (PLP)**. **1. Why Pyridoxone is the correct answer:** **Pyridoxone** is a distractor term and does not exist as a natural form or vitamer of Vitamin B6. It is often confused with "Pyridoxine" by students during exams. Because it is not a biological precursor to PLP, it is the "except" in this list. **2. Analysis of incorrect options (The true vitamers):** * **Pyridoxine (Option A):** An alcohol form found primarily in plants. It is the most stable form and is commonly used in vitamin supplements. * **Pyridoxal (Option C):** An aldehyde form found primarily in animal-derived foods. * **Pyridoxamine (Option D):** An amine form also found primarily in animal products. All three vitamers are absorbed in the intestine and converted in the liver to **Pyridoxal-5-phosphate (PLP)** via the enzyme pyridoxal kinase. **High-Yield Clinical Pearls for NEET-PG:** * **Key Function:** PLP is the essential coenzyme for **Transamination** (e.g., ALT, AST), **Decarboxylation** (e.g., synthesis of GABA, Histamine, Serotonin), and **Heme synthesis** (ALA synthase). * **Drug Interaction:** **Isoniazid (INH)**, used in TB treatment, inhibits pyridoxal kinase, leading to B6 deficiency. This manifests as **peripheral neuropathy** and **sideroblastic anemia**. * **Homocystinuria:** B6 is a cofactor for Cystathionine β-synthase; deficiency can lead to elevated homocysteine levels. * **Xanthurenic Acid:** In B6 deficiency, tryptophan metabolism is impaired, leading to the urinary excretion of xanthurenic acid (a diagnostic marker).

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

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free