Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 81: Which of the following conditions most rapidly produces a functional deficiency of vitamin K?

A. Premature delivery of the baby
B. Pure vegan diet
C. Broad-spectrum antibiotic consumption
D. Warfarin therapy (Correct Answer)

Explanation: **Explanation:** **1. Why Warfarin therapy is correct:** Warfarin is a competitive inhibitor of the enzyme **Vitamin K Epoxide Reductase (VKOR)**. Under normal conditions, Vitamin K must be recycled from its inactive epoxide form back to its active hydroquinone form to act as a cofactor for the γ-carboxylation of clotting factors (II, VII, IX, X). By blocking this recycling process, Warfarin causes an **immediate functional deficiency** of active Vitamin K. Even if Vitamin K is present in the body, it remains trapped in the inactive form, leading to a rapid rise in Prothrombin Time (PT). **2. Why the other options are incorrect:** * **Premature delivery:** While neonates are born with low Vitamin K stores due to poor placental transfer and a sterile gut, this is a physiological state rather than an "acute/rapidly produced" deficiency compared to the pharmacologic blockade of Warfarin. * **Pure vegan diet:** Vitamin K1 (Phylloquinone) is abundant in green leafy vegetables. Therefore, a vegan diet actually provides high amounts of Vitamin K, making deficiency via this route highly unlikely. * **Broad-spectrum antibiotics:** These drugs eliminate gut flora that synthesize Vitamin K2 (Menaquinone). While this can lead to deficiency, it typically takes several days to weeks to manifest, as it relies on the depletion of existing hepatic stores. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Vitamin K is a cofactor for **γ-glutamyl carboxylase**, which adds a carboxyl group to glutamate residues on Factors **II, VII, IX, X, Protein C, and Protein S**. * **Monitoring:** Warfarin therapy is monitored using **PT/INR** (Extrinsic pathway). * **Antidote:** To reverse Warfarin rapidly, give **Fresh Frozen Plasma (FFP)** or Prothrombin Complex Concentrate (PCC). For non-emergency reversal, give **Vitamin K1**. * **Newborns:** All newborns receive a prophylactic IM injection of Vitamin K to prevent **Hemorrhagic Disease of the Newborn (HDN)**.

Question 82: Which vitamin acts as a co-factor in glycine metabolism?

A. Thiamine
B. Biotin
C. Folic acid (Correct Answer)
D. Pantothenic acid

Explanation: **Explanation:** The correct answer is **Folic acid (Vitamin B9)**. **Why Folic Acid is Correct:** Folic acid, in its active form **Tetrahydrofolate (THF)**, is the primary carrier of one-carbon (1C) units in the body. Glycine metabolism is intricately linked to the 1C pool through two major pathways: 1. **Glycine Cleavage System:** This is the major catabolic pathway for glycine, where it is decomposed into $CO_2$ and $NH_4^+$. During this process, the methylene group of glycine is transferred to THF to form **N5, N10-methylene THF**. 2. **Interconversion with Serine:** Glycine can be reversibly converted to Serine by the enzyme *Serine Hydroxymethyltransferase*, which requires both THF and Pyridoxal Phosphate (B6) as co-factors. **Why Other Options are Incorrect:** * **Thiamine (B1):** Acts as a co-factor (TPP) for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase, $\alpha$-ketoglutarate dehydrogenase) and Transketolase. * **Biotin (B7):** Acts as a co-factor for **carboxylation** reactions (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase). It carries $CO_2$, not 1C units like methyl or methylene groups. * **Pantothenic acid (B5):** A structural component of **Coenzyme A (CoA)**, involved in acyl group transfer and the TCA cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Non-ketotic Hyperglycinemia:** Caused by a defect in the Glycine Cleavage System, leading to high glycine levels in the brain and CSF, resulting in seizures and mental retardation. * **Dual Co-factor Requirement:** Remember that the conversion of Glycine to Serine requires **both B9 (Folic acid) and B6 (PLP)**. * **FIGLU Test:** Histidine metabolism also requires THF; a deficiency in folic acid leads to the excretion of Formiminoglutamic acid (FIGLU) in urine.

Question 83: Deficiency of which vitamin presents classically as 3 'D's (Diarrhea, Dermatitis, Dementia)?

A. Folic acid
B. Pyridoxine
C. Niacin (Correct Answer)
D. Vitamin B12

Explanation: **Explanation:** The correct answer is **Niacin (Vitamin B3)**. The clinical triad of **Diarrhea, Dermatitis, and Dementia** is the hallmark of **Pellagra**, a disease caused by a deficiency of Niacin or its precursor amino acid, **Tryptophan**. If left untreated, a fourth 'D'—Death—ensues. * **Dermatitis:** Classically presents as a symmetrical, photosensitive scaly rash. A characteristic finding is **Casal’s necklace**, a circumferential rash around the lower neck. * **Diarrhea:** Caused by atrophy of the columnar epithelium of the GI tract. * **Dementia:** Results from neuronal degeneration in the brain and spinal cord, often accompanied by irritability and insomnia. **Why incorrect options are wrong:** * **Folic acid (B9):** Deficiency primarily leads to **Megaloblastic anemia** and neural tube defects in fetuses, not the 3 'D's. * **Pyridoxine (B6):** Deficiency causes peripheral neuropathy, sideroblastic anemia, and seborrheic dermatitis, but lacks the specific triad of Pellagra. Note: B6 is required for the conversion of Tryptophan to Niacin. * **Vitamin B12:** Deficiency causes Megaloblastic anemia and **Subacute Combined Degeneration (SCD)** of the spinal cord, characterized by neurological deficits but not the Pellagra triad. **High-Yield Clinical Pearls for NEET-PG:** * **Hartnup Disease:** A genetic disorder affecting neutral amino acid transport (Tryptophan) that can present with Pellagra-like symptoms. * **Carcinoid Syndrome:** Can lead to Niacin deficiency because Tryptophan is diverted to overproduce Serotonin. * **Corn/Maize-based diets:** Predispose populations to Pellagra because the Niacin in corn is bound (niacytin) and unavailable for absorption. * **Coenzyme forms:** Niacin is the precursor for **NAD and NADP**, essential for redox reactions.

Question 84: Niacin deficiency causes what condition?

A. Pigmentation
B. Diarrhea (Correct Answer)
C. Rash
D. Lactic acidosis

Explanation: **Explanation:** Niacin (Vitamin B3) deficiency leads to **Pellagra**, a systemic disease classically characterized by the **"4 Ds"**: **D**ermatitis, **D**iarrhea, **D**ementia, and, if untreated, **D**eath. **Why Diarrhea is the correct answer:** The gastrointestinal tract is highly sensitive to niacin deficiency because it requires high turnover of mucosal cells. Lack of niacin (a precursor to NAD/NADP) impairs cellular repair and energy metabolism, leading to chronic inflammation of the intestinal mucosa, malabsorption, and profuse watery diarrhea. **Analysis of Incorrect Options:** * **A. Pigmentation:** While pellagra involves skin changes, "pigmentation" is too vague. The specific skin finding is a bilateral, symmetrical **photosensitive dermatitis** (Casal’s necklace). * **C. Rash:** Similar to pigmentation, "rash" is a non-specific term. In NEET-PG, examiners look for the most specific clinical manifestation of the "4 Ds." Diarrhea is a primary diagnostic pillar of Pellagra. * **D. Lactic Acidosis:** This is typically associated with **Thiamine (B1) deficiency** (due to failure of the pyruvate dehydrogenase complex) or Biguanide toxicity, not Niacin. **High-Yield Clinical Pearls for NEET-PG:** * **Precursor:** Niacin can be synthesized from the amino acid **Tryptophan** (60 mg Tryptophan = 1 mg Niacin). * **Hartnup Disease:** A defect in neutral amino acid transport (Tryptophan) that can lead to pellagra-like symptoms. * **Carcinoid Syndrome:** Can cause niacin deficiency because tryptophan is diverted to produce excessive Serotonin. * **Corn-based diets:** Maize is low in tryptophan and contains niacin in a bound, unabsorbable form (niacytin).

Question 85: Pernicious anaemia is associated with the deficiency of which vitamin?

A. Folic acid
B. Vitamin B1
C. Vitamin B6
D. Vitamin B12 (Correct Answer)

Explanation: **Explanation:** **Pernicious anaemia** is a specific type of megaloblastic anaemia caused by an **autoimmune destruction of gastric parietal cells**. These cells are responsible for secreting **Intrinsic Factor (IF)**, a glycoprotein essential for the absorption of **Vitamin B12 (Cobalamin)** in the terminal ileum. Without IF, Vitamin B12 cannot be absorbed, leading to a systemic deficiency. **Why Vitamin B12 is the correct answer:** Vitamin B12 is a cofactor for *methionine synthase*. Its deficiency leads to a "folate trap," where folate is stuck in the N5-methyl THF form, preventing DNA synthesis and resulting in megaloblastic changes. Pernicious anaemia is the most common cause of B12 malabsorption and is classically associated with neurological symptoms (Subacute Combined Degeneration of the spinal cord) due to impaired myelin synthesis. **Why other options are incorrect:** * **Folic acid (B9):** While B9 deficiency also causes megaloblastic anaemia, it is usually due to dietary insufficiency or increased demand (pregnancy). It is **not** termed "pernicious" anaemia and does not present with neurological deficits. * **Vitamin B1 (Thiamine):** Deficiency leads to Beriberi or Wernicke-Korsakoff syndrome, primarily affecting energy metabolism and the nervous system, not erythropoiesis. * **Vitamin B6 (Pyridoxine):** Deficiency is associated with sideroblastic anaemia (due to impaired heme synthesis) and peripheral neuropathy, but not megaloblastic anaemia. **NEET-PG High-Yield Pearls:** * **Diagnostic Test:** The **Schilling test** (historically used) or detecting **Anti-Intrinsic Factor antibodies** (highly specific). * **Biochemical Markers:** Elevated levels of both **Methylmalonic Acid (MMA)** and **Homocysteine** are seen in B12 deficiency (only Homocysteine is elevated in Folate deficiency). * **Gastric Findings:** Associated with chronic atrophic gastritis and an increased risk of gastric carcinoma.

Question 86: During the formation of hydroxyproline and hydroxylysine, what essential factor(s) are required?

A. Pyridoxal phosphate
B. Ascorbic acid (Correct Answer)
C. Thiamine pyrophosphate
D. Methylcobalamine

Explanation: ### Explanation **Correct Answer: B. Ascorbic acid** **Underlying Medical Concept:** The hydroxylation of proline and lysine residues in pre-procollagen is a critical post-translational modification required for the stabilization of the collagen triple helix. This reaction is catalyzed by the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes require **molecular oxygen, α-ketoglutarate, and Fe²⁺ (ferrous iron)**. During the reaction, the iron atom is oxidized to the Fe³⁺ (ferric) state, which inactivates the enzyme. **Ascorbic acid (Vitamin C)** acts as a reducing agent, converting Fe³⁺ back to Fe²⁺, thereby maintaining the enzyme in its active form. Without Vitamin C, collagen fibers cannot be cross-linked, leading to decreased tensile strength of connective tissues. **Why Incorrect Options are Wrong:** * **A. Pyridoxal phosphate (B6):** Acts as a coenzyme for transamination, decarboxylation, and heme synthesis (ALA synthase). * **C. Thiamine pyrophosphate (B1):** Essential for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase) and the HMP shunt (Transketolase). * **D. Methylcobalamine (B12):** Involved in the conversion of homocysteine to methionine and the isomerization of methylmalonyl-CoA. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to scurvy, characterized by "corkscrew hair," petechiae, easy bruising, and **bleeding gums** due to defective collagen synthesis. * **Localization:** Hydroxylation occurs within the **Lumen of the Rough Endoplasmic Reticulum (RER)**. * **Amino Acid Markers:** Hydroxyproline and hydroxylysine are unique to collagen; urinary hydroxyproline levels can be used as a marker for bone resorption/collagen breakdown.

Question 87: Vitamin D is maximally found in which of the following?

A. Milk
B. Egg
C. Green leafy vegetables
D. Halibut liver oil (Correct Answer)

Explanation: **Explanation:** Vitamin D (Calciferol) is a fat-soluble vitamin that acts more like a hormone than a nutrient. The primary dietary sources of Vitamin D are animal-based, specifically fatty fish and fish liver oils. **Why Halibut Liver Oil is correct:** Fish liver oils are the richest natural sources of Vitamin D3 (Cholecalciferol). Among these, **Halibut liver oil** contains the highest concentration, followed by Cod liver oil and Shark liver oil. These oils serve as concentrated reservoirs of the vitamin, which the fish accumulate through the marine food chain. **Analysis of Incorrect Options:** * **Milk:** While often fortified in some countries, natural cow’s milk is actually a **poor source** of Vitamin D. It contains negligible amounts, insufficient to meet daily requirements. * **Egg:** The egg yolk contains Vitamin D, but the concentration is significantly lower than that found in fish liver oils. * **Green leafy vegetables:** These are excellent sources of Vitamin K, Folate, and Vitamin C, but they contain **no Vitamin D**. Vitamin D is virtually absent from plant-based foods, with the exception of certain UV-exposed mushrooms (which provide Vitamin D2). **High-Yield Clinical Pearls for NEET-PG:** * **Endogenous Synthesis:** The most important source of Vitamin D is synthesis in the skin (Malpighian layer) from **7-dehydrocholesterol** via UV-B radiation. * **Active Form:** The active form is **1,25-dihydroxycholecalciferol (Calcitriol)**, formed by hydroxylation in the liver (25-position) and then the kidney (1-alpha position). * **Storage:** Vitamin D is stored primarily in **adipose tissue** and the liver. * **Deficiency:** Leads to **Rickets** in children (defective mineralization of osteoid) and **Osteomalacia** in adults (demineralization of formed bone).

Question 88: Which of the following is the major source of Vitamin D?

A. Milk
B. Oily fish
C. Egg yolk
D. Sunlight (Correct Answer)

Explanation: **Explanation:** **1. Why Sunlight is the Correct Answer:** Sunlight is the primary source of Vitamin D for humans. The process begins in the skin, where **7-dehydrocholesterol** (a precursor) is converted into **Cholecalciferol (Vitamin D3)** upon exposure to Ultraviolet B (UVB) radiation. This endogenous synthesis accounts for approximately 80–90% of the body’s Vitamin D requirements, earning it the title "The Sunshine Vitamin." **2. Why Other Options are Incorrect:** * **Milk (A):** Natural cow’s milk is actually a poor source of Vitamin D. While "fortified milk" is common in some countries, it is not a primary natural source. * **Oily Fish (B) and Egg Yolk (C):** These are indeed dietary sources of Vitamin D3. Oily fish (like salmon or mackerel) and cod liver oil are the richest food sources, and egg yolks contain modest amounts. However, dietary intake typically contributes only 10–20% of total Vitamin D levels, making them secondary to sunlight. **3. NEET-PG High-Yield Pearls:** * **Active Form:** The active form of Vitamin D is **1,25-dihydroxycholecalciferol (Calcitriol)**. * **Rate-limiting Step:** The final activation occurs in the kidney via the enzyme **1-alpha-hydroxylase**, which is stimulated by Parathyroid Hormone (PTH). * **Storage Form:** **25-hydroxyvitamin D [25(OH)D]** is the major circulating form and the best indicator of a patient's Vitamin D status. * **Deficiency:** Leads to **Rickets** in children (defective mineralization of osteoid) and **Osteomalacia** in adults (demineralization of pre-existing bone). * **Toxicity:** Vitamin D is the most toxic vitamin in overdose, leading to hypercalcemia and metastatic calcification.

Question 89: What is the first clinical sign of vitamin A deficiency?

A. Poor growth
B. Conjunctival xerosis (Correct Answer)
C. Hydrocephalus
D. Phrynoderma

Explanation: **Explanation:** Vitamin A (Retinol) is essential for maintaining the integrity of epithelial tissues and the synthesis of rhodopsin in the retina. The progression of Vitamin A deficiency (VAD) follows a specific clinical sequence, often tested in NEET-PG. **Why Conjunctival Xerosis is correct:** According to the WHO classification of Xerophthalmia, **Conjunctival Xerosis (X1B)** is considered the **first clinical sign** of Vitamin A deficiency. It is characterized by the conjunctiva becoming dry, lusterless, and non-wettable. While **Night Blindness (XN)** is the **earliest symptom** (reported by the patient), Conjunctival Xerosis is the first objective sign detectable by an examiner. **Analysis of Incorrect Options:** * **A. Poor growth:** While Vitamin A is necessary for skeletal growth and protein synthesis, poor growth is a non-specific feature and usually occurs later or alongside other nutritional deficiencies. * **C. Hydrocephalus:** Acute Vitamin A toxicity (Hypervitaminosis A) can cause increased intracranial pressure (pseudotumor cerebri), but it is not a standard sign of deficiency. * **D. Phrynoderma:** Also known as "Toad Skin" (follicular hyperkeratosis), this is a cutaneous manifestation of VAD. However, it appears after ocular signs and is often associated with combined deficiencies of essential fatty acids and B-complex vitamins. **NEET-PG High-Yield Pearls:** * **Earliest Symptom:** Night Blindness (Nyctalopia). * **First Clinical Sign:** Conjunctival Xerosis. * **Bitot’s Spots (X2):** Triangular, foamy white patches on the bulbar conjunctiva; a hallmark of VAD. * **Keratomalacia (X3):** Liquefactive necrosis of the cornea; a medical emergency leading to irreversible blindness. * **WHO Prophylaxis:** 1 lakh IU (at 9 months with Measles vaccine), followed by 2 lakh IU every 6 months up to age 5 (Total 9 doses/17 lakh IU).

Question 90: Which vitamin is required for the production of thrombin?

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

Explanation: **Explanation:** **Vitamin K** is the correct answer because it serves as an essential cofactor for the enzyme **gamma-glutamyl carboxylase**. This enzyme is responsible for the post-translational modification (gamma-carboxylation) of glutamate residues on specific clotting factors: **II (Prothrombin), VII, IX, and X**, as well as proteins C and S. Thrombin is the activated form of Prothrombin (Factor II). Without Vitamin K, these factors are synthesized in an inactive form (known as **PIVKA**—Proteins Induced by Vitamin K Absence) because they lack the gamma-carboxyglutamate residues required to bind calcium and anchor to phospholipid membranes during the coagulation cascade. **Why other options are incorrect:** * **Vitamin A:** Primarily involved in vision (rhodopsin synthesis), epithelial integrity, and gene expression. * **Vitamin D:** Essential for calcium and phosphate homeostasis and bone mineralization. * **Vitamin E:** Acts as a potent lipid-soluble antioxidant, protecting cell membranes from free radical damage; it does not promote clotting (in fact, high doses can antagonize Vitamin K and increase bleeding risk). **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin Mechanism:** Acts as a Vitamin K antagonist by inhibiting **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K. * **Hemorrhagic Disease of the Newborn:** Neonates have sterile guts and poor placental transfer of Vitamin K, necessitating a prophylactic IM injection at birth to prevent intracranial hemorrhage. * **Laboratory Marker:** Vitamin K deficiency or Warfarin use is primarily monitored using **Prothrombin Time (PT/INR)**.

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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