Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 641: A patient presents with pellagra, parkinsonism, convulsions, anemia, and kidney stones. This presentation is most indicative of a deficiency in which of the following?

A. Coenzyme A
B. FADH
C. Niacin
D. Pyridoxal phosphate (Correct Answer)

Explanation: **Explanation:** The correct answer is **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6. This multi-system presentation is explained by the diverse roles of PLP as a coenzyme in amino acid metabolism: 1. **Pellagra:** PLP is a required cofactor for the enzyme *Kynureninase* in the pathway converting Tryptophan to Niacin. Deficiency leads to secondary Niacin deficiency, causing pellagra. 2. **Parkinsonism & Convulsions:** PLP is essential for the decarboxylation of amino acids to synthesize neurotransmitters. Deficiency reduces **Dopamine** (leading to parkinsonian symptoms) and **GABA** (leading to seizures/convulsions). 3. **Anemia:** PLP is the cofactor for **ALAS (δ-aminolevulinate synthase)**, the rate-limiting step in heme synthesis. Deficiency results in *Sideroblastic anemia*. 4. **Kidney Stones:** PLP is a cofactor for *Alanine-glyoxylate aminotransferase*. Lack of PLP leads to increased conversion of glyoxylate to **oxalate**, causing calcium oxalate nephrolithiasis. **Incorrect Options:** * **Coenzyme A (B5):** Primarily involved in acyl group transfer (e.g., Fatty acid synthesis). Deficiency is rare and typically presents with "Burning Foot Syndrome." * **FADH (B2):** Involved in redox reactions. Deficiency causes cheilosis, glossitis, and corneal neovascularization, but not stones or seizures. * **Niacin (B3):** While deficiency causes Pellagra (3 Ds: Dermatitis, Diarrhea, Dementia), it does not explain the sideroblastic anemia or oxalate stones seen in this patient. **High-Yield Clinical Pearls for NEET-PG:** * **Drug-Induced Deficiency:** **Isoniazid (INH)** therapy for TB is the most common cause of B6 deficiency (it inhibits pyridoxine kinase). Always co-administer B6 with INH. * **Homocystinuria:** B6 is a cofactor for *Cystathionine β-synthase*; some types of homocystinuria are "B6-responsive." * **Microscopy:** Sideroblastic anemia shows "Ringed Sideroblasts" on Prussian Blue staining of bone marrow.

Question 642: Which of the following enzymes require Vitamin C for their activity?

A. Procollagen proline hydroxylase (Correct Answer)
B. Procollagen amino peptidase
C. Procollagen carboxyl peptidase
D. Lysyl oxidase

Explanation: **Explanation:** **1. Why Option A is Correct:** Vitamin C (Ascorbic acid) acts as a vital reducing agent in the post-translational modification of collagen. The enzymes **Prolyl hydroxylase** (Procollagen proline hydroxylase) and **Lysyl hydroxylase** require Vitamin C to maintain the iron cofactor in its reduced **ferrous (Fe²⁺) state**. This hydroxylation of proline and lysine residues is essential for the interchain hydrogen bonding that stabilizes the collagen triple helix. Without Vitamin C, collagen fibers are weak and unstable, leading to the clinical manifestation of Scurvy. **2. Why Other Options are Incorrect:** * **Options B & C (Procollagen amino/carboxyl peptidases):** These enzymes are responsible for cleaving the N-terminal and C-terminal propeptides of procollagen in the extracellular space to form tropocollagen. They do not require Vitamin C for their catalytic activity. * **Option D (Lysyl oxidase):** While this enzyme is critical for collagen cross-linking, it is a **Copper-dependent** enzyme, not Vitamin C-dependent. It oxidatively deaminates lysine and hydroxylysine residues to form reactive aldehydes (allysine). **3. NEET-PG High-Yield Pearls:** * **Vitamin C Functions:** Beyond collagen synthesis, it is required for **Dopamine β-hydroxylase** (Dopamine → Norepinephrine) and **7-α-hydroxylase** (Bile acid synthesis). * **Scurvy Symptoms:** Remember the "4 Hs": Hemorrhage (petechiae, bleeding gums), Hyperkeratosis, Hypochondriasis, and Hematologic abnormalities (anemia). * **Iron Absorption:** Vitamin C enhances the absorption of non-heme iron by reducing it from the ferric (Fe³⁺) to the ferrous (Fe²⁺) state in the stomach.

Question 643: Which is the most potent chain-breaking antioxidant?

A. Vitamin C
B. Alpha-tocopherol (Correct Answer)
C. Catalase
D. Glutathione peroxidase

Explanation: **Explanation:** **Why Alpha-tocopherol is correct:** Vitamin E (specifically **alpha-tocopherol**) is the most potent **lipid-soluble, chain-breaking antioxidant** in the human body. Its primary role is to protect polyunsaturated fatty acids (PUFAs) in cell membranes from lipid peroxidation. When a free radical (like a hydroxyl radical) attacks a membrane lipid, it initiates a self-propagating chain reaction. Alpha-tocopherol intercepts this process by donating a hydrogen atom to the lipid peroxyl radical, neutralizing it and forming a relatively stable tocopheroxyl radical. This "breaks the chain" of oxidative damage, preserving membrane integrity. **Why the other options are incorrect:** * **Vitamin C (Ascorbic acid):** While a powerful antioxidant, it is **water-soluble** and acts primarily in the cytosol and extracellular fluid. Its main role in this context is to regenerate (reduce) the tocopheroxyl radical back into active Vitamin E. * **Catalase:** This is an **enzymatic antioxidant** that specifically decomposes hydrogen peroxide ($H_2O_2$) into water and oxygen. It does not directly "break" the lipid peroxidation chain. * **Glutathione peroxidase:** This is a selenium-dependent enzyme that reduces lipid hydroperoxides to alcohols and $H_2O_2$ to water. While crucial for preventing the *initiation* of damage, it is not classified as a "chain-breaking" molecule like Vitamin E. **High-Yield NEET-PG Pearls:** * **Location:** Vitamin E is primarily found in the **mitochondrial membrane** and **endoplasmic reticulum**. * **Synergy:** Vitamin E and Vitamin C work together; Vitamin C regenerates Vitamin E, while Vitamin E protects against the oxidation of Vitamin A. * **Deficiency:** Leads to hemolytic anemia (due to fragile RBC membranes), posterior column degeneration, and ataxia. * **Selenium Link:** Selenium and Vitamin E act synergistically; selenium is a cofactor for Glutathione peroxidase, which reduces the requirement for Vitamin E.

Question 644: Which of the following is a Vitamin K-dependent clotting factor?

A. Factor VII (Correct Answer)
B. Fibrinogen (Factor I)
C. Factor XI
D. Factor XII

Explanation: **Explanation:** **1. Why Factor VII is Correct:** 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. This process adds a negative charge to the factors, allowing them to bind calcium ions ($Ca^{2+}$) and attach to phospholipid surfaces, which is critical for the coagulation cascade. The Vitamin K-dependent clotting factors are **Factors II (Prothrombin), VII, IX, and X**, as well as anticoagulant **Proteins C and S**. Factor VII has the shortest half-life among these, making it the first to decrease during Vitamin K deficiency or Warfarin therapy. **2. Why the Other Options are Incorrect:** * **Factor I (Fibrinogen):** It is a soluble plasma glycoprotein synthesized in the liver but does not require Vitamin K for its synthesis or functional activation. * **Factor XI (Plasma Thromboplastin Antecedent):** Part of the intrinsic pathway, its synthesis is independent of Vitamin K. * **Factor XII (Hageman Factor):** This factor initiates the intrinsic pathway upon contact with negatively charged surfaces; it is not modified by Vitamin K-dependent carboxylation. **3. High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K and thus inhibiting the activation of Factors II, VII, IX, and X. * **Monitoring:** Prothrombin Time (PT/INR) is used to monitor Vitamin K status and Warfarin therapy because it is most sensitive to **Factor VII** levels. * **Newborns:** They are Vitamin K deficient due to sterile guts and poor placental transfer; hence, a prophylactic Vitamin K injection is given at birth to prevent **Hemorrhagic Disease of the Newborn**. * **Mnemonic:** Remember "1972" (Factors **10, 9, 7, 2**) to recall the Vitamin K-dependent factors.

Question 645: All of the following are sources of vitamin A except?

A. Egg
B. Milk
C. Sunflower seeds (Correct Answer)
D. Tomato

Explanation: **Explanation:** Vitamin A is found in two primary forms: **Retinoids** (preformed Vitamin A) found in animal sources and **Carotenoids** (provitamin A) found in plant sources. **Why Sunflower Seeds is the Correct Answer:** Sunflower seeds are a rich source of **Vitamin E (Tocopherol)** and healthy fats, but they contain negligible amounts of Vitamin A. In the context of NEET-PG, it is crucial to remember that most seeds and vegetable oils (except Red Palm Oil) are characterized by their Vitamin E content rather than Vitamin A. **Analysis of Incorrect Options:** * **Egg:** An excellent source of preformed Vitamin A (retinol), specifically concentrated in the **egg yolk**. * **Milk:** Whole milk and dairy products are significant animal sources of retinol. In many regions, milk is also fortified with Vitamin A. * **Tomato:** Contains **Lycopene** and **Beta-carotene**. Beta-carotene is a provitamin carotenoid that the body converts into retinal, making tomatoes a valid plant source. **NEET-PG High-Yield Pearls:** 1. **Richest Source:** The richest natural source of Vitamin A is **Halibut liver oil**, followed by Cod liver oil. 2. **Plant Sources:** The richest plant source is **Red Palm Oil** (contains high concentrations of alpha and beta-carotenes). 3. **Storage:** Vitamin A is stored in the liver in **Ito cells** (Stellate cells) as retinyl palmitate. 4. **Golden Rice:** A genetically modified variety of rice produced to biosynthesize beta-carotene to prevent Vitamin A deficiency in developing nations.

Question 646: Which vitamin's functions are carried out through coenzyme A?

A. Vitamin B5 (Correct Answer)
B. Vitamin B9
C. Vitamin B12
D. All of the above

Explanation: **Explanation:** **1. Why Vitamin B5 is Correct:** Vitamin B5, also known as **Pantothenic acid**, is the direct precursor for the synthesis of **Coenzyme A (CoA)**. The name is derived from the Greek word *"pantothen,"* meaning "from everywhere," reflecting its wide distribution in foods. Pantothenate is phosphorylated and combined with ATP and cysteine to form CoA. * **Mechanism:** Coenzyme A acts as a universal carrier of **acyl groups** (like acetyl, succinyl, or fatty acyl groups). It is essential for the TCA cycle (Acetyl-CoA, Succinyl-CoA), fatty acid synthesis and oxidation, and cholesterol synthesis. **2. Why Other Options are Incorrect:** * **Vitamin B9 (Folic Acid):** Its active coenzyme form is **Tetrahydrofolate (THF)**. It is primarily involved in **one-carbon metabolism**, essential for DNA synthesis (purines and thymidine) and amino acid metabolism. * **Vitamin B12 (Cobalamin):** It functions through two specific coenzymes: **Methylcobalamin** (homocysteine to methionine conversion) and **Deoxyadenosylcobalamin** (methylmalonyl-CoA to succinyl-CoA conversion). **3. High-Yield Clinical Pearls for NEET-PG:** * **Acyl Carrier Protein (ACP):** Besides CoA, Vitamin B5 is also a constituent of the ACP domain of the Fatty Acid Synthase multienzyme complex. * **Deficiency:** Rare due to wide availability, but severe deficiency leads to **"Burning Feet Syndrome"** (Gopalan’s syndrome). * **Key Reaction:** The conversion of Pyruvate to Acetyl-CoA by the Pyruvate Dehydrogenase (PDH) complex requires five cofactors, one of which is CoA (derived from B5). * **Mnemonic:** Remember **"Panto"** sounds like **"Penta"** (5) = Vitamin B5.

Question 647: What is the triad of biotin deficiency?

A. Dermatitis, glossitis, Steatorrhea
B. Dermatitis, glossitis, Alopecia (Correct Answer)
C. Mental changes, diarrhoea, Alopecia
D. Dermatitis, dementia, diarrhoea

Explanation: **Explanation:** Biotin (Vitamin B7) acts as a vital coenzyme for **carboxylation reactions**, including those catalyzed by pyruvate carboxylase, acetyl-CoA carboxylase, and propionyl-CoA carboxylase. These reactions are essential for gluconeogenesis, fatty acid synthesis, and amino acid metabolism. **1. Why Option B is Correct:** The classic clinical triad of biotin deficiency consists of **Dermatitis** (typically periorificial, scaly, and erythematous), **Glossitis** (inflammation of the tongue), and **Alopecia** (hair loss). These symptoms arise because biotin is crucial for the synthesis of fatty acids and the maintenance of rapidly dividing cells in the skin and hair follicles. **2. Analysis of Incorrect Options:** * **Option A:** While dermatitis and glossitis occur, **Steatorrhea** (fatty stools) is not a feature of biotin deficiency; it is more commonly associated with malabsorption syndromes or Vitamin E/K deficiencies. * **Option C:** Mental changes (lethargy, hallucinations) can occur in severe biotin deficiency, but **Diarrhoea** is not a primary component of the triad. * **Option D:** This describes the **"3 Ds" of Pellagra** (Niacin/Vitamin B3 deficiency): Dermatitis, Dementia, and Diarrhoea. **High-Yield NEET-PG Pearls:** * **Avidin Connection:** Consumption of large amounts of **raw egg whites** can induce biotin deficiency because the protein avidin binds biotin with high affinity, preventing its absorption. * **Key Enzymes:** Remember the mnemonic **"ABC"** for carboxylases—they require **A**TP, **B**iotin, and **C**O₂. * **Metabolic Marker:** Deficiency leads to the accumulation of odd-chain fatty acids and organic aciduria (e.g., 3-hydroxyisovaleric acid).

Question 648: A patient presents with severe energy deficiency. Which of the following vitamins might be deficient in this patient?

A. Pyridoxine
B. Riboflavin
C. Thiamin (Correct Answer)
D. Folic acid

Explanation: **Explanation:** **Thiamin (Vitamin B1)** is the correct answer because its active form, **Thiamin Pyrophosphate (TPP)**, is a critical coenzyme for key enzymes in carbohydrate metabolism and energy production. Specifically, it is required by: 1. **Pyruvate Dehydrogenase (PDH):** Converts Pyruvate to Acetyl-CoA (linking glycolysis to the TCA cycle). 2. **$\alpha$-Ketoglutarate Dehydrogenase:** A rate-limiting step in the TCA cycle. 3. **Branched-chain $\alpha$-ketoacid Dehydrogenase:** Essential for metabolizing leucine, isoleucine, and valine. A deficiency in Thiamin leads to a "metabolic logjam," preventing the efficient oxidation of glucose and resulting in a severe ATP (energy) deficit, clinically manifesting as **Beriberi** or **Wernicke-Korsakoff syndrome**. **Analysis of Incorrect Options:** * **Pyridoxine (B6):** Primarily involved in amino acid metabolism (transamination and decarboxylation) and heme synthesis. While important, its deficiency does not present primarily as a generalized energy crisis. * **Riboflavin (B2):** Precursor for FAD and FMN. While involved in the electron transport chain, Thiamin deficiency is more classically associated with acute energy failure and lactic acidosis in clinical vignettes. * **Folic Acid (B9):** Essential for one-carbon metabolism and DNA synthesis. Deficiency leads to megaloblastic anemia, not acute energy deficiency. **NEET-PG High-Yield Pearls:** * **The "Dehydrogenase" Rule:** TPP is a coenzyme for most oxidative decarboxylation reactions. * **Clinical Marker:** Erythrocyte **Transketolase** activity is the most reliable diagnostic test for Thiamin deficiency. * **Wernicke’s Encephalopathy Triad:** Confusion, Ataxia, and Ophthalmoplegia. * **Management Tip:** Always administer Thiamin *before* Glucose in a malnourished patient to prevent precipitating Wernicke’s encephalopathy.

Question 649: Perifollicular and petechial hemorrhages are characteristic of which condition?

A. Acrodermatitis enteropathica
B. Pellagra
C. Scurvy (Correct Answer)
D. Phrynoderma

Explanation: **Explanation:** **Correct Answer: C. Scurvy** Scurvy is caused by a deficiency of **Vitamin C (Ascorbic acid)**. Vitamin C is a crucial co-factor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**, which are responsible for the hydroxylation of proline and lysine residues during collagen synthesis. In its absence, defective collagen (lacking cross-linking) leads to increased capillary fragility. * **Perifollicular hemorrhages** (bleeding around hair follicles) and **petechiae** (small red/purple spots) are hallmark signs of this vascular fragility. Other classic features include "corkscrew hairs," swollen spongy gums, and impaired wound healing. **Incorrect Options:** * **A. Acrodermatitis enteropathica:** This is an autosomal recessive disorder of **Zinc absorption**. It presents with a triad of periorificial/acral dermatitis, alopecia, and diarrhea, rather than hemorrhagic spots. * **B. Pellagra:** Caused by **Niacin (Vitamin B3)** deficiency. It is characterized by the "4 Ds": Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. The dermatitis is typically photosensitive and symmetrical. * **D. Phrynoderma:** Also known as "toad skin," this is follicular hyperkeratosis associated with **Vitamin A** deficiency (and sometimes Essential Fatty Acids). It presents as dry, firm papules on the extensor surfaces, not hemorrhages. **NEET-PG High-Yield Pearls:** * **Vitamin C and Iron:** Vitamin C enhances the absorption of non-heme iron by keeping it in the ferrous ($Fe^{2+}$) state. * **Infantile Scurvy (Barlow’s Disease):** Look for "ground glass appearance" of bones and "Frankel’s line" (white line of calcification) on X-ray. * **Collagen Type:** Scurvy primarily affects the stability of the triple helix in **Type I and Type III collagen**.

Question 650: Xerosis is seen in which deficiency?

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

Explanation: **Explanation:** **Vitamin A (Retinol)** is essential for maintaining the integrity of epithelial tissues. It acts as a regulator of gene expression, ensuring that epithelial cells differentiate into mucus-secreting cells. In Vitamin A deficiency, these cells undergo **squamous metaplasia** and keratinization. **Xerosis** (pathological dryness) occurs because the conjunctiva and cornea lose their ability to produce mucus and tears, leading to a dry, lusterless appearance. This is a hallmark of Xerophthalmia. **Analysis of Incorrect Options:** * **Vitamin B deficiency:** Depending on the specific B vitamin, deficiency typically presents with neurological symptoms (B1, B12), dermatitis/glossitis (B2, B3, B6), or megaloblastic anemia (B9, B12), but not primary ocular xerosis. * **Vitamin C deficiency (Scurvy):** Leads to defective collagen synthesis. Clinical features include perifollicular hemorrhages, bleeding gums, and impaired wound healing. * **Vitamin D deficiency:** Primarily affects calcium homeostasis and bone mineralization, leading to Rickets in children and Osteomalacia in adults. **High-Yield Clinical Pearls for NEET-PG:** * **WHO Classification of Xerophthalmia:** * **X1A:** Conjunctival xerosis (earliest sign). * **X1B:** Bitot’s spots (triangular, foamy patches on the conjunctiva). * **X2:** Corneal xerosis. * **X3A/X3B:** Keratomalacia (liquefactive necrosis of the cornea). * **Night Blindness (Nyctalopia):** The earliest symptom of Vitamin A deficiency due to the depletion of Rhodopsin in rod cells. * **Dark Adaptation Time:** This is increased in Vitamin A deficiency and is a sensitive functional test.

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

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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

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Vitamin B6 and Transamination

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Folate and Vitamin B12 in One-Carbon Metabolism

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Vitamin C and Collagen Synthesis

Practice Questions

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