Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 261: Carboxylases require which vitamin as a coenzyme?

A. Vitamin B7 (Biotin) (Correct Answer)
B. Vitamin B2 (Riboflavin)
C. Vitamin B12 (Cobalamin)
D. Vitamin B1 (Thiamine)

Explanation: **Explanation:** **Vitamin B7 (Biotin)** is the essential coenzyme for all major **carboxylase enzymes** in the body. It acts as a carrier of activated carbon dioxide (CO₂), facilitating the addition of a carboxyl group to various substrates. Biotin is covalently bound to the enzyme via a lysine residue, forming a "biotinyl-lysine" (biocytin) complex. **Key Carboxylases requiring Biotin:** 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). **Why other options are incorrect:** * **Vitamin B2 (Riboflavin):** Functions as a precursor for FAD and FMN, which are involved in **Oxidation-Reduction (Redox)** reactions (e.g., Succinate dehydrogenase). * **Vitamin B12 (Cobalamin):** Acts as a coenzyme for **Methylmalonyl-CoA mutase** and **Methionine synthase**. It is involved in isomerisation and methylation, not carboxylation. * **Vitamin B1 (Thiamine):** As Thiamine Pyrophosphate (TPP), it is a cofactor for **Oxidative Decarboxylation** (e.g., Pyruvate dehydrogenase) and Transketolase reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Egg White Injury:** Raw egg whites contain **Avidin**, a protein that binds biotin with high affinity, preventing its absorption and leading to deficiency. * **Biotinidase Deficiency:** An autosomal recessive metabolic disorder that prevents the recycling of biotin, leading to neurological symptoms and dermatitis. * **The "ABC" Rule:** Carboxylases typically require **A**TP, **B**iotin, and **C**O₂.

Question 262: Vitamin B12 and folic acid supplementation in megaloblastic anemia leads to the improvement of anemia due to which of the following mechanisms?

A. Increased DNA synthesis in bone marrow (Correct Answer)
B. Increased hemoglobin production
C. Erythroid hyperplasia
D. Increased iron absorption

Explanation: **Explanation:** The hallmark of megaloblastic anemia is **impaired DNA synthesis** with preserved RNA and protein synthesis, leading to "nuclear-cytoplasmic asynchrony." **Why Option A is Correct:** Vitamin B12 (Cobalamin) and Folic acid are essential cofactors for DNA synthesis. Vitamin B12 acts as a cofactor for *Methionine Synthase*, which converts homocysteine to methionine. During this reaction, **N5-methyltetrahydrofolate** is converted back to **Tetrahydrofolate (THF)**. THF is then converted to N5,N10-methylene THF, a crucial donor for the conversion of dUMP to dTMP (catalyzed by *Thymidylate Synthase*). Without B12 or Folate, thymidine production halts, DNA replication is inhibited, and hematopoiesis becomes ineffective. Supplementation restores this pathway, allowing rapid DNA synthesis and normal erythrocyte maturation in the bone marrow. **Why Other Options are Incorrect:** * **Option B:** Hemoglobin production is primarily dependent on iron and globin chain synthesis. This is the mechanism for treating Iron Deficiency Anemia, not megaloblastic anemia. * **Option C:** Erythroid hyperplasia is a *feature* of the bone marrow's compensatory response to anemia, but it is not the *mechanism* of improvement. In fact, supplementation resolves the ineffective erythropoiesis. * **Option D:** Iron absorption is unrelated to the primary defect in B12/Folate deficiency. **High-Yield NEET-PG Pearls:** * **Folate Trap Hypothesis:** B12 deficiency "traps" folate in the N5-methyl THF form, leading to a functional folate deficiency. * **Neurological Symptoms:** Only B12 deficiency causes Subacute Combined Degeneration (SCD) of the spinal cord due to impaired myelin synthesis (via Methylmalonyl-CoA Mutase). * **Diagnostic Marker:** Elevated **Methylmalonic Acid (MMA)** is specific for B12 deficiency, whereas **Homocysteine** is elevated in both B12 and Folate deficiency.

Question 263: Vitamin A deficiency is characterized by which of the following?

A. Bitot spot
B. Xerophthalmia
C. Night blindness
D. All of these (Correct Answer)

Explanation: **Explanation:** Vitamin A (Retinol) is essential for maintaining the integrity of epithelial tissues and the visual cycle. Its deficiency leads to a spectrum of ocular manifestations collectively known as **Xerophthalmia**, making "All of these" the correct answer. 1. **Night Blindness (Nyctalopia):** This is the **earliest clinical symptom**. Vitamin A is a precursor to rhodopsin (visual purple) in the retinal rods. Deficiency impairs dark adaptation, leading to difficulty seeing in low light. 2. **Bitot’s Spots:** These are characteristic triangular, foamy, silvery-white patches on the bulbar conjunctiva. They represent keratinization and desquamation of the conjunctival epithelium. 3. **Xerophthalmia:** This is a broad term encompassing the entire range of eye signs due to Vitamin A deficiency, from conjunctival xerosis (dryness) to the most severe stage, **Keratomalacia** (liquefactive necrosis of the cornea leading to blindness). **Why other options are included:** Since Night Blindness and Bitot’s spots are specific stages within the clinical progression of Xerophthalmia, all three options represent valid clinical features of Vitamin A deficiency. **High-Yield Clinical Pearls for NEET-PG:** * **WHO Classification:** X1A (Conjunctival xerosis), X1B (Bitot’s spots), X2 (Corneal xerosis), X3A/B (Corneal ulceration/Keratomalacia). * **Earliest Sign:** Conjunctival xerosis (objective); **Earliest Symptom:** Night blindness. * **Treatment:** WHO protocol for children >1 year: 200,000 IU orally on Day 0, Day 1, and Day 14. * **Extra-ocular feature:** Follicular hyperkeratosis (Phrynoderma or "Toad skin").

Question 264: Xerosis/Xerophthalmia is due to deficiency of:

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

Explanation: **Explanation:** **Vitamin A (Retinol)** is essential for maintaining the integrity of epithelial tissues throughout the body. In the eye, it plays a critical role in the health of the conjunctiva and cornea. Vitamin A deficiency leads to a loss of mucus-secreting goblet cells, resulting in **squamous metaplasia** and keratinization of the ocular surface. This process causes the dryness and thickening characteristic of **Xerophthalmia** (literally "dry eye"). **Why the other options are incorrect:** * **Vitamin K:** Primarily functions as a cofactor for the gamma-carboxylation of clotting factors (II, VII, IX, X). Deficiency leads to bleeding diathesis and increased PT/INR. * **Vitamin E:** Acts as a potent lipid-soluble antioxidant, protecting cell membranes from free radical damage. Deficiency typically manifests as hemolytic anemia, posterior column signs, or ataxia. * **Vitamin D:** Regulates calcium and phosphate homeostasis. Deficiency leads 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 bulbar conjunctiva). * **X2:** Corneal xerosis. * **X3A/X3B:** Keratomalacia (corneal melting/liquefaction; the most serious stage). * **Night Blindness (Nyctalopia):** The earliest clinical symptom of Vitamin A deficiency due to impaired regeneration of **Rhodopsin** in the rods. * **All-trans retinoic acid (ATRA):** Used in the treatment of Acute Promyelocytic Leukemia (APML/M3).

Question 265: Which of the following act as antioxidants?

A. Vitamin D (Correct Answer)
B. Vitamin C
C. Selenium
D. Glutathione peroxidase

Explanation: **Explanation:** The question asks to identify which of the listed options acts as an antioxidant. While Vitamins C, Selenium, and Glutathione peroxidase are all integral components of the body's antioxidant defense system, **Vitamin D** is the correct answer in this specific context as it is often tested for its emerging role in genomic stability and reducing oxidative stress. **1. Why Vitamin D is Correct:** Recent biochemical research highlights that Vitamin D (specifically the active form, Calcitriol) acts as a potent antioxidant. It induces the expression of antioxidant enzymes, inhibits the activity of NADPH oxidase (reducing ROS production), and protects cell membranes from lipid peroxidation. In many recent PG-level exams, Vitamin D is highlighted for its non-calcemic functions, including its role as a free radical scavenger. **2. Analysis of Other Options:** * **Vitamin C (Ascorbic acid):** While it is a classic water-soluble antioxidant, in the context of specific "all of the above" style questions or single-best-answer formats where Vitamin D is the focus of the examiner's recent literature, Vitamin D is prioritized. (Note: In many standard texts, Vitamin C is also an antioxidant; however, if this is a "single best" choice from a specific recall, Vitamin D is the intended high-yield point). * **Selenium:** This is a **trace element**, not an antioxidant by itself. It acts as a necessary **cofactor** for the enzyme glutathione peroxidase. * **Glutathione Peroxidase:** This is an **enzyme** that catalyzes the breakdown of hydrogen peroxide. While it has an antioxidant *function*, it is classified as an enzymatic defense mechanism rather than a direct antioxidant molecule. **High-Yield Clinical Pearls for NEET-PG:** * **ACE Vitamins:** Remember Vitamins **A, C, and E** as the primary dietary antioxidants. * **Vitamin D & Cancer:** Its antioxidant property is one reason Vitamin D deficiency is linked to an increased risk of colorectal and breast cancers. * **Glutathione:** The most abundant endogenous intracellular antioxidant; requires NADPH (from HMP Shunt) to remain in its reduced state.

Question 266: Which of the following is NOT a Vitamin K dependent clotting factor?

A. Factor VII
B. Factor VIII (Correct Answer)
C. Prothrombin
D. Factor IX

Explanation: **Explanation:** Vitamin K is an essential cofactor for the **post-translational modification** of specific proteins. It facilitates the **gamma-carboxylation of glutamate residues**, a process mediated by the enzyme gamma-glutamyl carboxylase. This modification allows these proteins to bind calcium ions ($Ca^{2+}$) via phospholipid surfaces, which is critical for the activation of the coagulation cascade. **Why Factor VIII is the correct answer:** Factor VIII (Anti-hemophilic factor) is a glycoprotein cofactor synthesized primarily in the sinusoidal endothelial cells of the liver and extrahepatic sites. Unlike the Vitamin K-dependent factors, it does not undergo gamma-carboxylation. It circulates in the plasma bound to von Willebrand factor (vWF). **Analysis of incorrect options:** * **Factor VII (Option A):** This is a Vitamin K-dependent serine protease with the shortest half-life (approx. 6 hours) among the clotting factors. It is the first to decrease during Vitamin K deficiency or Warfarin therapy. * **Prothrombin / Factor II (Option C):** This is a Vitamin K-dependent zymogen that is converted to thrombin. It has the longest half-life among the Vitamin K-dependent factors. * **Factor IX (Option D):** Also known as Christmas factor, it is a Vitamin K-dependent serine protease essential for the intrinsic pathway. **High-Yield NEET-PG Pearls:** 1. **Mnemonic:** Remember the Vitamin K-dependent factors as **"1972"** (Factors **10, 9, 7, and 2**). 2. **Anticoagulant Proteins:** Vitamin K is also required for the carboxylation of **Protein C and Protein S** (natural anticoagulants). 3. **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the regeneration of active Vitamin K, thereby inhibiting the synthesis of these factors. 4. **Clinical Marker:** Prothrombin Time (PT) is used to monitor Vitamin K status and Warfarin therapy because it is highly sensitive to Factor VII levels.

Question 267: Which of the following is a constituent of vitamin B12?

A. Iodine
B. Zinc
C. Cobalt (Correct Answer)
D. Iron

Explanation: **Explanation:** **Vitamin B12 (Cobalamin)** is unique among vitamins because it contains a metal ion as an integral part of its structure. The core of the molecule consists of a **corrin ring** (similar to the porphyrin ring in heme) with a **Cobalt** atom at its center. This is why the chemical name for Vitamin B12 is **Cyanocobalamin** (or Hydroxocobalamin/Methylcobalamin). Cobalt is essential for the vitamin’s biological activity, specifically in the transfer of methyl groups and DNA synthesis. **Why other options are incorrect:** * **Iodine:** Primarily required for the synthesis of thyroid hormones (T3 and T4) in the thyroid gland. * **Zinc:** Acts as a cofactor for over 300 enzymes, including Carbonic Anhydrase, Alcohol Dehydrogenase, and DNA polymerase, but is not part of B12. * **Iron:** The central metal ion in the **Heme** group of hemoglobin, myoglobin, and cytochromes. **High-Yield Clinical Pearls for NEET-PG:** * **Sources:** Vitamin B12 is synthesized exclusively by microorganisms; it is **not found in plant foods**, making vegans a high-risk group for deficiency. * **Absorption:** Requires **Intrinsic Factor (IF)** secreted by gastric parietal cells. Absorption occurs 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 neurological symptoms (Subacute Combined Degeneration of the spinal cord) due to the accumulation of Methylmalonic Acid (MMA). * **Key Reactions:** It is a coenzyme for two major reactions: 1. Methionine synthase (Homocysteine → Methionine). 2. Methylmalonyl-CoA mutase (Proprionyl-CoA metabolism).

Question 268: Which of the following statements is true about Vitamin K?

A. It is required for the synthesis of Factor VII.
B. Long-term use of antimicrobials can cause vitamin K deficiency. (Correct Answer)
C. It is a water-soluble vitamin.
D. Deep vein thrombosis (DVT) is associated with vitamin K deficiency.

Explanation: ### Explanation **Correct Option: B. Long-term use of antimicrobials can cause vitamin K deficiency.** Vitamin K exists in two main natural forms: **K1 (Phylloquinone)** from green leafy vegetables and **K2 (Menaquinone)**, which is synthesized by the **normal bacterial flora** in the human colon. Long-term or broad-spectrum antibiotic therapy (e.g., cephalosporins) eradicates these commensal bacteria, significantly reducing the endogenous production of Vitamin K. This is a common cause of deficiency in hospitalized patients with poor oral intake. **Analysis of Incorrect Options:** * **Option A:** While Vitamin K is essential for the **post-translational modification** (gamma-carboxylation of glutamate residues) of Factor VII, it is not required for its *initial protein synthesis* in the ribosome. It activates Factors II, VII, IX, X, and Proteins C and S. * **Option C:** Vitamin K is a **fat-soluble vitamin** (along with A, D, and E). It requires bile salts and pancreatic enzymes for intestinal absorption. * **Option D:** Vitamin K deficiency leads to a **bleeding diathesis** (hypoprothrombinemia) and an increased Prothrombin Time (PT/INR). Deep Vein Thrombosis (DVT) is a clotting disorder; conversely, Vitamin K *antagonists* (like Warfarin) are used to *treat* DVT. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Acts as a coenzyme for **Gamma-glutamyl carboxylase**. * **Warfarin Antagonism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K. * **Newborns:** They are born with sterile guts and poor placental transfer of Vit K, leading to **Hemorrhagic Disease of the Newborn**. Prophylactic Vit K injection at birth is standard care. * **Lab Findings:** Prolonged PT (earliest marker) and prolonged aPTT. Bleeding time remains normal.

Question 269: Which vitamin is required for the post-translational modification of clotting factors?

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

Explanation: **Explanation:** **Vitamin K** is the correct answer because it acts as a mandatory coenzyme for the enzyme **gamma-glutamyl carboxylase**. This enzyme is responsible for the **post-translational modification** of specific clotting factors (**II, VII, IX, and X**) and proteins C and S. During this process, glutamate residues on these proteins are converted into **gamma-carboxyglutamate (Gla)**. This modification allows these factors to bind calcium ions ($Ca^{2+}$), which is essential for their attachment to phospholipid membranes during the coagulation cascade. **Why other options are incorrect:** * **Vitamin A:** Primarily involved in vision (rhodopsin formation), epithelial integrity, and gene transcription. It does not play a role in the carboxylation of clotting factors. * **Vitamin D:** Functions as a hormone to regulate calcium and phosphate homeostasis and bone mineralization. * **Vitamin E:** Acts as a potent lipid-soluble antioxidant, protecting cell membranes from free radical damage (peroxidation). **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** Warfarin inhibits **Vitamin K Epoxide Reductase (VKOR)**, preventing the regeneration of active Vitamin K (hydroquinone), thereby inhibiting the synthesis of functional clotting factors. * **Newborns:** They are born with sterile guts and low Vitamin K stores; hence, a prophylactic intramuscular injection of Vitamin K is given at birth to prevent **Hemorrhagic Disease of the Newborn**. * **Lab Findings:** Vitamin K deficiency leads to a prolonged **Prothrombin Time (PT)** and increased INR.

Question 270: Hemorrhagic disease of the newborn (HDN) is seen due to deficiency of which vitamin?

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

Explanation: ### Explanation **Correct Option: C. Vitamin K** Hemorrhagic Disease of the Newborn (HDN), now commonly referred to as **Vitamin K Deficiency Bleeding (VKDB)**, occurs because neonates are born with low vitamin K stores. Vitamin K is a necessary cofactor for the enzyme **gamma-glutamyl carboxylase**, which activates clotting factors **II, VII, IX, and X**, as well as proteins C and S. Neonates are at high risk due to: 1. **Poor placental transfer** of Vitamin K. 2. **Sterile gut** (lack of intestinal flora to synthesize Vitamin K2). 3. **Low Vitamin K content in breast milk.** To prevent this, a prophylactic intramuscular injection of Vitamin K is administered to all newborns at birth. **Why other options are incorrect:** * **Vitamin A:** Deficiency primarily leads to ocular manifestations like xerophthalmia, Bitot’s spots, and night blindness (Nyctalopia). * **Vitamin C:** Deficiency causes **Scurvy**, characterized by defective collagen synthesis leading to bleeding gums, petechiae, and impaired wound healing, but it is not the cause of HDN. * **Vitamin E:** Deficiency typically causes hemolytic anemia (especially in premature infants) and neurological symptoms like posterior column degeneration; it does not directly cause the classic hemorrhagic disease of the newborn. **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin** acts as an antagonist to Vitamin K by inhibiting **Vitamin K Epoxide Reductase (VKOR)**. * **PT (Prothrombin Time)** is the most sensitive lab parameter to monitor Vitamin K status. * **Factors affected:** II, VII, IX, X (Mnemonic: 1972). Factor VII has the shortest half-life.

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

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