Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 171: What food group has the highest vitamin A content?

A. Lemon
B. Green leafy vegetables (Correct Answer)
C. Tomato
D. Ragi

Explanation: **Explanation:** Vitamin A exists in two main forms: **Preformed Vitamin A** (Retinol), found in animal sources like liver and fish oils, and **Provitamin A carotenoids** (mainly Beta-carotene), found in plant sources. Among the options provided, **Green Leafy Vegetables (GLVs)** like spinach, amaranth, and coriander are the richest sources of Beta-carotene. * **Why Green Leafy Vegetables are correct:** In the plant kingdom, dark green leaves contain high concentrations of carotenoids. Although the green chlorophyll masks the yellow-orange pigment, these vegetables are the primary dietary source of Vitamin A for a large portion of the population in developing countries. * **Why other options are incorrect:** * **Lemon:** Primarily a source of Vitamin C (Ascorbic acid); it contains negligible Vitamin A. * **Tomato:** Contains Lycopene and some Beta-carotene, but the concentration per gram is significantly lower than that found in dark green leafy vegetables. * **Ragi (Finger Millet):** While rich in Calcium and dietary fiber, cereals and millets are generally poor sources of Vitamin A. **High-Yield Clinical Pearls for NEET-PG:** * **Storage:** Vitamin A is stored in the **Ito cells** (Stellate cells) of the liver. * **Deficiency Sequence:** The earliest symptom is **Nyctalopia** (Night blindness), followed by Conjunctival xerosis, **Bitot’s spots** (keratinized plaques), and eventually Keratomalacia (liquefactive necrosis of the cornea). * **Golden Rice:** A genetically modified variety of rice produced to biosynthesize Beta-carotene to prevent Vitamin A deficiency. * **Toxicity:** Hypervitaminosis A can lead to pseudotumor cerebri (increased intracranial pressure) and is highly **teratogenic** (avoided in pregnancy).

Question 172: Which of the following is common to both acyl carrier protein and coenzyme A?

A. Located in mitochondria as well as cytoplasm
B. Carriers of folate
C. Phosphopantotheine is the reactive group (Correct Answer)
D. Phosphorylated to tyrosine

Explanation: **Explanation:** The correct answer is **C. Phosphopantotheine is the reactive group.** Both **Coenzyme A (CoA)** and **Acyl Carrier Protein (ACP)** are essential molecules in lipid metabolism. They share a common functional unit: **4'-phosphopantotheine**, which is derived from Vitamin B5 (Pantothenic acid). This prosthetic group contains a terminal **thiol (-SH) group** that forms a high-energy thioester bond with acyl groups, allowing them to act as carriers of fatty acid chains during synthesis and oxidation. **Analysis of Options:** * **A. Location:** CoA is found in both mitochondria (Beta-oxidation, TCA cycle) and cytoplasm. However, ACP is primarily a component of the **Fatty Acid Synthase (FAS) multienzyme complex**, which is located exclusively in the **cytosol** in humans. * **B. Carriers of folate:** This is incorrect. Folate (Vitamin B9) carriers are typically related to one-carbon metabolism (e.g., Tetrahydrofolate). CoA and ACP carry **acyl/acetyl groups**, not folate. * **D. Phosphorylated to tyrosine:** This is biologically irrelevant to these cofactors. Phosphopantotheine is linked to a **serine residue** in ACP, not tyrosine. **High-Yield NEET-PG Pearls:** * **Vitamin Link:** Pantothenic acid (B5) is the precursor for both CoA and ACP. * **Functional Difference:** CoA is a "soluble" carrier used in various pathways (TCA, Heme synthesis, Beta-oxidation), whereas ACP is a "fixed" carrier within the Fatty Acid Synthase complex for de novo lipogenesis. * **The "Business End":** The reactive part of both molecules is the **Sulfhydryl (-SH) group**, which is why CoA is often written as CoA-SH.

Question 173: Thiamine deficiency is best detected by estimating the activity of which enzyme?

A. Transketolase (Correct Answer)
B. Transaldolase
C. Hexokinase
D. Pyruvate carboxylase

Explanation: **Explanation:** The gold standard for detecting thiamine (Vitamin B1) deficiency is the measurement of **Erythrocyte Transketolase Activity (ETKA)**. **Why Transketolase is the correct answer:** Thiamine pyrophosphate (TPP) is a vital cofactor for several enzymes, including Transketolase, Pyruvate Dehydrogenase, and α-ketoglutarate dehydrogenase. Transketolase is a key enzyme in the **Hexose Monophosphate (HMP) Shunt**, responsible for the interconversion of sugars. In clinical practice, the activity of this enzyme is measured in red blood cells before and after the addition of TPP. An increase in enzyme activity (>15–25%) upon adding TPP confirms a functional thiamine deficiency. **Analysis of Incorrect Options:** * **B. Transaldolase:** While also part of the HMP shunt, it does **not** require TPP as a cofactor; it uses a Schiff base mechanism instead. * **C. Hexokinase:** This is the first enzyme of glycolysis. It requires Magnesium ($Mg^{2+}$) but is independent of thiamine. * **D. Pyruvate Carboxylase:** This enzyme converts pyruvate to oxaloacetate (gluconeogenesis). It requires **Biotin (B7)**, not thiamine. (Note: Pyruvate *Dehydrogenase* requires thiamine, but *Carboxylase* does not). **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad of Wernicke’s Encephalopathy:** Ophthalmoplegia, Ataxia, and Confusion. * **Dry Beriberi:** Polyneuritis and muscle wasting. * **Wet Beriberi:** High-output heart failure and edema. * **Key TPP-dependent enzymes:** Remember the mnemonic **"APT"**: **A**lpha-ketoglutarate dehydrogenase, **P**yruvate dehydrogenase, and **T**ransketolase.

Question 174: What is the Recommended Dietary Allowance (RDA) of thiamine per 1000 kcal of energy per day?

A. 0.5 mg (Correct Answer)
B. 0.6 mg
C. 0.7 mg
D. 0.8 mg

Explanation: **Explanation:** **1. Why Option A is correct:** Thiamine (Vitamin B1) acts as a crucial coenzyme in the form of **Thiamine Pyrophosphate (TPP)** for key enzymes in carbohydrate metabolism, such as Pyruvate Dehydrogenase and α-ketoglutarate dehydrogenase. Because thiamine is essential for oxidative decarboxylation in energy production, its requirement is directly proportional to caloric intake. The established RDA for thiamine is **0.5 mg per 1000 kcal** of energy consumed. This ensures that the metabolic machinery has sufficient cofactors to process the glucose load. **2. Why other options are incorrect:** * **Options B, C, and D (0.6, 0.7, 0.8 mg):** These values exceed the standard recommended baseline for healthy adults. While requirements may increase slightly during physiological stress, pregnancy, or lactation (usually an additional 0.2–0.3 mg/day), the standard benchmark used in medical examinations for energy-linked calculation is strictly 0.5 mg/1000 kcal. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Enzymes requiring TPP:** Remember the mnemonic **ATP** (α-ketoglutarate dehydrogenase, Transketolase, and Pyruvate dehydrogenase). * **Diagnostic Marker:** The best method to assess thiamine status is measuring **Erythrocyte Transketolase activity**. * **Wernicke-Korsakoff Syndrome:** Classically seen in alcoholics. **Rule:** Always administer thiamine *before* glucose in a malnourished patient to prevent precipitating acute Wernicke encephalopathy. * **Deficiency:** Leads to **Beriberi**. * *Dry Beriberi:* Polyneuritis and muscle wasting. * *Wet Beriberi:* High-output heart failure and edema. * **Dietary Note:** Polishing rice removes the thiamine-rich aleurone layer, leading to deficiency in populations reliant on white rice.

Question 175: Which of the following reactions is affected in patients with scurvy?

A. Carboxylation
B. Acetylation
C. Hydroxylation (Correct Answer)
D. Transamination

Explanation: **Explanation:** **Scurvy** is caused by a deficiency of **Vitamin C (Ascorbic Acid)**. Vitamin C acts as a vital co-factor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes are responsible for the **hydroxylation** of proline and lysine residues in pre-procollagen. 1. **Why Hydroxylation is Correct:** Hydroxylation is essential for the cross-linking of collagen fibers, which provides structural integrity and tensile strength to connective tissues. In the absence of Vitamin C, the iron cofactor in these enzymes remains in the ferric ($Fe^{3+}$) state; Vitamin C reduces it back to the ferrous ($Fe^{2+}$) state, maintaining enzymatic activity. Without this, collagen remains unstable, leading to the fragile capillaries and poor wound healing seen in scurvy. 2. **Why Other Options are Incorrect:** * **Carboxylation:** This is primarily associated with **Vitamin K** (gamma-carboxylation of clotting factors) and **Biotin** (ABC enzymes: Acetyl-CoA, Propionyl-CoA, and Pyruvate carboxylase). * **Acetylation:** This involves **Coenzyme A** (derived from Vitamin B5/Pantothenic acid). * **Transamination:** This is the hallmark reaction of **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6. **Clinical Pearls for NEET-PG:** * **Classic Triad of Scurvy:** Gum bleeding, petechiae/ecchymosis, and perifollicular hemorrhages ("Corkscrew hairs"). * **Infantile Scurvy (Barlow’s Disease):** Presents with subperiosteal hemorrhage and "frog-leg" position. * **Radiological Signs:** Frankel’s line (dense provisional calcification) and Wimberger’s ring sign.

Question 176: What specific disease is caused by vitamin B1 deficiency?

A. Pellagra
B. Angular cheilitis
C. Megaloblastic anemia
D. Peripheral polyneuritis (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Peripheral polyneuritis** **Medical Concept:** Vitamin B1 (Thiamine) serves as a precursor for **Thiamine Pyrophosphate (TPP)**, a vital coenzyme for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase and α-ketoglutarate dehydrogenase). Thiamine deficiency impairs glucose metabolism, particularly affecting high-energy-demand tissues like the brain and peripheral nerves. **Dry Beriberi** is the clinical manifestation characterized by symmetrical **peripheral polyneuritis**, involving sensory and motor impairments, muscle wasting, and "wrist drop" or "foot drop." **Analysis of Incorrect Options:** * **A. Pellagra:** Caused by a deficiency of **Vitamin B3 (Niacin)**. It is characterized by the "4 Ds": Dermatitis, Diarrhea, Dementia, and Death. * **B. Angular cheilitis:** Primarily associated with **Vitamin B2 (Riboflavin)** deficiency. It presents as fissuring and inflammation at the corners of the mouth. * **C. Megaloblastic anemia:** Caused by a deficiency of **Vitamin B12 (Cobalamin)** or **Vitamin B9 (Folic acid)**, leading to impaired DNA synthesis and large, immature red blood cells. **High-Yield Clinical Pearls for NEET-PG:** * **Wet Beriberi:** Thiamine deficiency with cardiovascular involvement (high-output heart failure and edema). * **Wernicke-Korsakoff Syndrome:** Seen in chronic alcoholics; presents with the triad of Ataxia, Ophthalmoplegia, and Confusion (Wernicke’s) plus confabulation and amnesia (Korsakoff’s). * **Transketolase Activity:** Erythrocyte transketolase activity is the gold-standard biochemical marker to diagnose thiamine deficiency. * **Infantile Beriberi:** Occurs in infants breastfed by thiamine-deficient mothers; presents with aphonia (silent cry) and heart failure.

Question 177: Thiamine acts as a cofactor in which of the following reactions?

A. Pyruvate to oxaloacetate
B. Malonate to oxaloacetate
C. Succinate to fumarate
D. Pyruvate to acetyl CoA (Correct Answer)

Explanation: **Explanation:** **Thiamine (Vitamin B1)**, in its active form **Thiamine Pyrophosphate (TPP)**, serves as a vital cofactor for enzymes involved in oxidative decarboxylation and the pentose phosphate pathway. 1. **Why Option D is Correct:** The conversion of **Pyruvate to Acetyl CoA** is catalyzed by the **Pyruvate Dehydrogenase (PDH) Complex**. This multi-enzyme complex requires five cofactors: **T**hiamine (TPP), **L**ipoic acid, **C**oenzyme A (B5), **F**AD (B2), and **N**AD+ (B3). TPP specifically facilitates the decarboxylation step of pyruvate. 2. **Why Other Options are Incorrect:** * **Option A (Pyruvate to Oxaloacetate):** This is catalyzed by *Pyruvate Carboxylase*. All carboxylases (except Vitamin K-dependent ones) require **Biotin (B7)** as a cofactor, not Thiamine. * **Option B (Malonate to Oxaloacetate):** This is not a standard metabolic pathway. However, Malonate is a classic competitive inhibitor of Succinate Dehydrogenase. * **Option C (Succinate to Fumarate):** This reaction is part of the TCA cycle, catalyzed by *Succinate Dehydrogenase*, which requires **FAD (Riboflavin/B2)**. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for TPP-dependent enzymes:** "**A**ny **A**lpha **P**erson **T**akes **T**hiamine" 1. **A**lpha-ketoglutarate dehydrogenase (TCA cycle) 2. **A**lpha-ketoacid dehydrogenase (Branched-chain amino acid metabolism; deficient in Maple Syrup Urine Disease) 3. **P**yruvate dehydrogenase (Link reaction) 4. **T**ransketolase (HMP Shunt; used to diagnose Thiamine deficiency via RBC transketolase activity) * **Clinical Correlation:** Thiamine deficiency leads to **Beriberi** (Dry/Wet) and **Wernicke-Korsakoff Syndrome**. Always administer Thiamine *before* Glucose in alcoholic patients to prevent precipitating Wernicke’s encephalopathy.

Question 178: Vitamin H is also known as which of the following?

A. Cobalamin
B. Biotin (Correct Answer)
C. Pyridoxine
D. Folate

Explanation: **Explanation:** **Biotin (Vitamin B7)** is historically known as **Vitamin H**, derived from the German words *Haar und Haut* (Hair and Skin), as deficiency was first associated with dermatitis and alopecia. It acts as a vital coenzyme for **carboxylation reactions**, where it serves as a carrier of activated carbon dioxide (CO₂). Key enzymes requiring Biotin include Pyruvate carboxylase (gluconeogenesis), Acetyl-CoA carboxylase (fatty acid synthesis), and Propionyl-CoA carboxylase. **Analysis of Incorrect Options:** * **Cobalamin (Vitamin B12):** Essential for DNA synthesis and myelin formation. Its deficiency leads to megaloblastic anemia and subacute combined degeneration of the spinal cord. * **Pyridoxine (Vitamin B6):** Acts as a coenzyme (PLP) for transamination, decarboxylation, and heme synthesis. * **Folate (Vitamin B9):** Crucial for one-carbon metabolism and prevents neural tube defects in the fetus. **High-Yield Clinical Pearls for NEET-PG:** * **Egg White Injury:** Raw egg whites contain a glycoprotein called **Avidin**, which binds tightly to Biotin and prevents its absorption, leading to deficiency. * **Carboxylase Link:** Remember the mnemonic "ABC enzymes" (ATP, Biotin, and CO₂) for carboxylases. * **Clinical Presentation:** Biotin deficiency presents with dermatitis, glossitis, alopecia, and neurological symptoms like depression and hallucinations. * **Holocarboxylase Synthetase Deficiency:** A rare genetic disorder where Biotin cannot be attached to its apoenzymes, treated with high-dose Biotin supplementation.

Question 179: Which of the following is a feature of manganese deficiency?

A. Anemia
B. Alopecia
C. Ataxia
D. Gonadal atrophy (Correct Answer)

Explanation: **Explanation:** Manganese ($Mn^{2+}$) is an essential trace element that acts as a cofactor for several critical enzymes, including **pyruvate carboxylase**, **superoxide dismutase (Mn-SOD)**, and enzymes involved in glycosyltransferase activity. **Why Gonadal Atrophy is Correct:** Manganese is vital for the synthesis of cholesterol (a precursor to steroid hormones) and the proper functioning of the reproductive system. Experimental studies and clinical observations have shown that manganese deficiency leads to impaired reproductive function, characterized by **gonadal atrophy**, reduced spermatogenesis, and sterility. It also plays a role in bone matrix formation; thus, deficiency can lead to skeletal abnormalities. **Analysis of Incorrect Options:** * **A. Anemia:** Typically associated with deficiencies in **Iron** (microcytic), **Vitamin B12/Folate** (megaloblastic), or **Copper** (due to impaired iron mobilization via ceruloplasmin). * **B. Alopecia:** A classic sign of **Zinc** deficiency (often seen in Acrodermatitis enteropathica) or **Biotin** deficiency. * **C. Ataxia:** While manganese *toxicity* (Manganism) causes Parkinsonian-like neurological symptoms and ataxia, deficiency is more specifically linked to impaired growth and reproductive failure. **NEET-PG High-Yield Pearls:** * **Mn-Superoxide Dismutase:** Located in the **mitochondria** (distinguish from Cu-Zn SOD in the cytosol). * **Manganism:** Occupational hazard in miners; presents as "Psychosis" followed by Parkinsonian tremors due to accumulation in the **basal ganglia**. * **Key Enzyme:** Manganese is the specific activator for **glycosyltransferases** required for cartilage synthesis.

Question 180: Which vitamins play an important role in the citric acid cycle?

A. Thiamine, riboflavin, niacin, pantothenic acid (Correct Answer)
B. Thiamine, biotin, riboflavin, lipoic acid
C. Thiamine, pyridoxine, riboflavin, niacin, pantothenic acid
D. Thiamine, mecobalamin, pantothenic acid

Explanation: The Citric Acid Cycle (TCA cycle) is the central metabolic pathway that requires four specific B-complex vitamins to function as essential coenzymes. ### 1. Why Option A is Correct The correct answer is **Thiamine (B1), Riboflavin (B2), Niacin (B3), and Pantothenic acid (B5)**. These vitamins are precursors to the following vital cofactors: * **Thiamine (B1):** Converted to **TPP** (Thiamine Pyrophosphate), a cofactor for **$\alpha$-ketoglutarate dehydrogenase**. * **Riboflavin (B2):** Forms **FAD**, the prosthetic group for **Succinate dehydrogenase**. * **Niacin (B3):** Forms **NAD+**, the electron acceptor for **Isocitrate dehydrogenase**, **$\alpha$-ketoglutarate dehydrogenase**, and **Malate dehydrogenase**. * **Pantothenic acid (B5):** A structural component of **Coenzyme A (CoA-SH)**, essential for forming Acetyl-CoA and Succinyl-CoA. ### 2. Why Other Options are Incorrect * **Option B:** While **Lipoic acid** is a cofactor for $\alpha$-ketoglutarate dehydrogenase, it is technically a vitamin-like sulfur compound, not a vitamin. **Biotin** is required for carboxylation (e.g., Pyruvate carboxylase) but is not directly involved in the TCA cycle steps. * **Option C:** **Pyridoxine (B6)** is primarily involved in transamination and heme synthesis; it has no direct role in the TCA cycle. * **Option D:** **Mecobalamin (B12)** is involved in the conversion of Propionyl-CoA to Succinyl-CoA (anaplerotic reaction), but it is not a core requirement for the cycle's internal steps. ### 3. High-Yield Clinical Pearls for NEET-PG * **The PDH Complex:** The conversion of Pyruvate to Acetyl-CoA (the link reaction) requires the same four vitamins plus Lipoic acid. * **Arsenic Poisoning:** Arsenite inhibits enzymes requiring Lipoic acid (like $\alpha$-ketoglutarate dehydrogenase), leading to a backup of substrates and lactic acidosis. * **Mnemonic:** Remember **"The Rhythm Nearly Paused"** (Thiamine, Riboflavin, Niacin, Pantothenate).

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