Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 671: About scurvy, all are true EXCEPT:

A. Subperiosteal hematomas with tenderness
B. Separation of epiphysis
C. Raised serum alkaline phosphatase (Correct Answer)
D. Gingival bleeding

Explanation: **Explanation:** Scurvy is caused by a deficiency of **Vitamin C (Ascorbic acid)**, which is essential for the hydroxylation of proline and lysine residues during collagen synthesis. Defective collagen leads to weakened connective tissue and fragile blood vessels. **Why Option C is the correct answer (The Exception):** In Scurvy, **Serum Alkaline Phosphatase (ALP) levels are typically low or normal**, not raised. ALP is an indicator of osteoblastic activity; in Vitamin C deficiency, osteoblastic function is suppressed because the formation of the osteoid matrix (which is primarily collagen) is impaired. This distinguishes Scurvy from Rickets, where ALP is characteristically elevated. **Analysis of Incorrect Options:** * **Option A (Subperiosteal hematomas):** Weakened capillary walls lead to bleeding under the periosteum. This causes intense pain and tenderness, often leading to "pseudoparalysis" where the child refuses to move the limbs. * **Option B (Separation of epiphysis):** Due to the failure of osteoid formation and a weakened metaphyseal zone (the "scorbutic lattice"), the epiphysis can easily displace or separate from the shaft. * **Option D (Gingival bleeding):** This is a classic hallmark of Scurvy. Defective collagen in the periodontal ligaments and capillary walls leads to swollen, "woody," and bleeding gums. **NEET-PG High-Yield Pearls:** * **Radiological Signs:** Look for **Frankel’s line** (dense zone of calcification), **Wimberger’s ring** (sclerotic margin around epiphysis), and **Pelkan spur** (metaphyseal osteophytes). * **Key Enzyme:** Vitamin C is a co-factor for **Prolyl hydroxylase** and **Lysyl hydroxylase**. * **Clinical Triad:** Anemia, Myalgia, and Dermatological signs (Perifollicular hemorrhages/Corkscrew hair).

Question 672: Which of the following are related to the functions of vitamin A?

A. Retinol
B. Retinal
C. Retinoic acids
D. All of the above (Correct Answer)

Explanation: **Explanation:** Vitamin A (Retinol) is a fat-soluble vitamin that exists in three major active forms, collectively known as **retinoids**. Each form performs distinct and essential physiological functions in the body, making "All of the above" the correct choice. 1. **Retinol (Vitamin A Alcohol):** This is the primary storage and transport form of Vitamin A. It is essential for **reproduction** (spermatogenesis in males and preventing fetal resorption in females). 2. **Retinal (Vitamin A Aldehyde):** This form is crucial for the **visual cycle**. 11-cis-retinal combines with the protein opsin to form rhodopsin in the rod cells of the retina, which is necessary for vision in dim light. 3. **Retinoic Acid (Vitamin A Acid):** This form acts like a hormone. It binds to nuclear receptors (RAR/RXR) to regulate **gene expression**. It is essential for epithelial cell differentiation, growth, and mucus secretion. However, it *cannot* be converted back to retinal and thus cannot support vision or reproduction. **Why other options are incomplete:** While A, B, and C are all individual active forms, selecting only one would ignore the vital physiological roles played by the others. **High-Yield Clinical Pearls for NEET-PG:** * **Earliest Sign of Deficiency:** Nyctalopia (Night blindness). * **Earliest Objective Sign:** Conjunctival xerosis (followed by Bitot’s spots). * **Therapeutic Use:** All-trans retinoic acid (ATRA) is the drug of choice for **Acute Promyelocytic Leukemia (M3)**. * **Teratogenicity:** Isotretinoin (used for acne) is highly teratogenic; a pregnancy test is mandatory before prescription. * **Storage:** Primarily stored in the **Ito cells** (Stellate cells) of the liver.

Question 673: Which of the following fat-soluble vitamins functions as a coenzyme?

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

Explanation: **Explanation:** The correct answer is **Vitamin K**. While most fat-soluble vitamins (A, D, E, K) function primarily as hormones or antioxidants, **Vitamin K** is unique because it acts as an essential **coenzyme** for the enzyme **gamma-glutamyl carboxylase**. 1. **Why Vitamin K is correct:** Vitamin K (in its reduced form, hydroquinone) serves as a cofactor for the post-translational modification of specific proteins. It facilitates the **gamma-carboxylation of glutamate residues** on clotting factors **II, VII, IX, and X**, as well as Proteins C and S. This process adds a negative charge to the proteins, allowing them to bind calcium ions ($Ca^{2+}$) and attach to phospholipid membranes, which is critical for the coagulation cascade. 2. **Why other options are incorrect:** * **Vitamin A:** Functions primarily as a precursor to **retinal** (visual cycle) and **retinoic acid**, which acts like a hormone to regulate gene expression and epithelial differentiation. * **Vitamin D:** Functions as a **steroid hormone** (Calcitriol). It binds to nuclear receptors to regulate calcium and phosphate homeostasis. * **Vitamin E:** Functions primarily as a potent **lipid-soluble antioxidant**, protecting cell membranes from free radical damage (peroxidation); it has no known coenzyme function. **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 clotting factors. * **Newborns:** They are born with sterile guts and low Vitamin K stores, necessitating a prophylactic IM injection of Vitamin K to prevent **Hemorrhagic Disease of the Newborn**. * **Osteocalcin:** Vitamin K is also a coenzyme for the carboxylation of osteocalcin, playing a role in bone mineralization.

Question 674: Wernicke's encephalopathy, during refeeding syndrome, can be precipitated in deficiency of which of the following vitamin?

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

Explanation: ### Explanation **Correct Answer: C. Thiamin (Vitamin B1)** **Mechanism and Clinical Correlation:** Wernicke’s encephalopathy (WE) is a medical emergency caused by a severe deficiency of **Thiamin (Vitamin B1)**. Thiamin, in its active form **Thiamin Pyrophosphate (TPP)**, is a crucial coenzyme for key enzymes in glucose metabolism: 1. **Pyruvate Dehydrogenase (PDH):** Converts pyruvate to Acetyl-CoA. 2. **α-Ketoglutarate Dehydrogenase:** A key step in the TCA cycle. 3. **Transketolase:** Part of the Pentose Phosphate Pathway. In **Refeeding Syndrome**, the sudden introduction of carbohydrates (glucose) leads to a surge in insulin. This shifts the body from a catabolic to an anabolic state, rapidly increasing the demand for thiamin to process the glucose load. If thiamin stores are already low (common in chronic alcoholism or malnutrition), this sudden metabolic demand exhausts the remaining B1, leading to mitochondrial dysfunction and neuronal death, particularly in the mammillary bodies. This precipitates the classic triad of WE: **Ophthalmoplegia, Ataxia, and Confusion.** **Why other options are incorrect:** * **Riboflavin (B2):** Precursor for FAD/FMN. Deficiency causes ariboflavinosis (cheilosis, glossitis, corneal vascularization) but not acute encephalopathy. * **Pyridoxine (B6):** Essential for transamination and heme synthesis. Deficiency causes sideroblastic anemia and peripheral neuropathy. * **Vitamin C:** Required for collagen hydroxylation. Deficiency leads to Scurvy (bleeding gums, poor wound healing). **High-Yield NEET-PG Pearls:** * **The "Banana Bag" Rule:** Always administer thiamin **before** or along with glucose in malnourished patients to prevent precipitating Wernicke’s. * **Diagnostic Marker:** Erythrocyte **Transketolase activity** is decreased in thiamin deficiency (increases upon adding TPP). * **Korsakoff Syndrome:** The chronic, irreversible stage of WE characterized by anterograde amnesia and confabulation.

Question 675: In humans and other primates, as well as in guinea pigs, which vitamin cannot be synthesized due to the absence of the enzyme L-gulonolactone oxidase?

A. Pantothenic acid
B. Nicotinic acid
C. Ascorbic acid (Correct Answer)
D. Folic acid

Explanation: **Explanation:** The correct answer is **Ascorbic acid (Vitamin C)**. **Why it is correct:** In most mammals, Vitamin C is synthesized from glucose via the **Uronic Acid Pathway**. The final step of this pathway involves the conversion of L-gulonolactone to 2-keto-L-gulonolactone, catalyzed by the enzyme **L-gulonolactone oxidase**. Humans, other primates, guinea pigs, and fruit bats lack the gene for this specific enzyme due to an evolutionary mutation. Consequently, they cannot synthesize Vitamin C endogenously and must obtain it through their diet. **Why the other options are incorrect:** * **Pantothenic acid (B5):** This is an essential vitamin obtained from various food sources (ubiquitous in nature). It is a precursor for Coenzyme A but is not synthesized via the uronic acid pathway. * **Nicotinic acid (B3):** While it is a vitamin, humans *can* synthesize a small amount of Niacin from the essential amino acid **Tryptophan** (60 mg Tryptophan = 1 mg Niacin). * **Folic acid (B9):** This is synthesized by plants and microorganisms. Humans lack the enzymatic machinery to couple the pteridine ring, PABA, and glutamic acid, making it an essential dietary requirement. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to defective collagen synthesis because it is a co-factor for **prolyl and lysyl hydroxylase** (post-translational modification). * **Reducing Agent:** Vitamin C maintains iron in the **ferrous (Fe²⁺) state**, facilitating dietary iron absorption in the duodenum. * **Antioxidant:** It acts as a potent water-soluble antioxidant and helps in the regeneration of Vitamin E. * **Uronic Acid Pathway:** While humans cannot make Vitamin C, the pathway is still functional for producing **UDP-glucuronate**, which is essential for the detoxification (conjugation) of bilirubin and various drugs.

Question 676: Which vitamin is required for glutathione to act as an antioxidant?

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

Explanation: **Explanation:** The correct answer is **Niacin (Vitamin B3)**. To understand why, we must look at the **Glutathione Redox Cycle**, which is the cell's primary defense against oxidative stress (Reactive Oxygen Species). **Why Niacin is Correct:** Glutathione exists in two forms: reduced (GSH) and oxidized (GSSG). To neutralize hydrogen peroxide, GSH is converted to GSSG by *Glutathione Peroxidase*. To remain effective, GSSG must be recycled back to GSH by the enzyme **Glutathione Reductase**. This enzyme requires **NADPH** as a mandatory co-factor to donate electrons. Niacin is the precursor for **NADP+/NADPH**. Without Niacin, NADPH levels drop, the recycling of glutathione fails, and the cell suffers oxidative damage. **Why Other Options are Incorrect:** * **Vitamin E:** While a potent antioxidant that protects lipid membranes, it does not serve as a co-factor for the enzymatic regeneration of glutathione. * **Vitamin C:** It works synergistically with Vitamin E and can help regenerate it, but it is not the direct co-enzyme required for the Glutathione Reductase reaction. * **Vitamin A:** Primarily involved in vision (retinal) and gene expression (retinoic acid); it does not play a direct role in the glutathione redox cycle. **Clinical Pearls for NEET-PG:** * **The HMP Shunt Connection:** The NADPH required for this process is primarily generated by the **Pentose Phosphate Pathway (HMP Shunt)** via the enzyme **G6PD**. * **G6PD Deficiency:** In G6PD deficiency, NADPH is low, leading to decreased reduced glutathione. This results in hemoglobin denaturation (forming **Heinz bodies**) and subsequent hemolysis. * **Selenium Connection:** While Niacin is the vitamin required, **Selenium** is the essential trace element required as a co-factor for *Glutathione Peroxidase*.

Question 677: Which form of vitamin D is measured in serum?

A. Cholecalciferol
B. 25-hydroxyvitamin D
C. 1,25-dihydroxyvitamin D
D. 24,25-dihydroxyvitamin D (Correct Answer)

Explanation: ### Explanation The measurement of Vitamin D status is a high-yield topic in Biochemistry. While multiple metabolites exist, **25-hydroxyvitamin D [25(OH)D]**, also known as **Calcidiol**, is the standard clinical marker used to determine a patient's vitamin D status. *(Note: There appears to be a discrepancy in the provided key; clinically and academically for NEET-PG, **Option B (25-hydroxyvitamin D)** is the correct answer for measuring serum stores. If the key strictly insists on Option D, it would be highly unconventional, as 24,25-dihydroxyvitamin D is an inactive catabolic product.)* #### Why 25-hydroxyvitamin D (Option B) is the standard: 1. **Long Half-life:** It has a circulating half-life of approximately 2–3 weeks, making it a stable indicator of long-term stores. 2. **Reflects Total Intake:** It represents vitamin D produced in the skin as well as that obtained from dietary sources and supplements. 3. **Substrate Availability:** Its concentration is not strictly regulated by PTH or calcium levels, unlike the active form. #### Analysis of Other Options: * **A. Cholecalciferol (Vitamin D3):** This is the precursor form. It has a very short half-life (hours) and fluctuates rapidly based on recent sun exposure or ingestion, making it unreliable for assessing overall status. * **C. 1,25-dihydroxyvitamin D (Calcitriol):** This is the **biologically active form**. Despite its potency, it is **not** used for routine screening because it has a short half-life (4–6 hours) and its levels are tightly regulated by PTH. In vitamin D deficiency, secondary hyperparathyroidism may actually normalize 1,25(OH)₂D levels even when total body stores are low. * **D. 24,25-dihydroxyvitamin D:** This is a degradation product formed by the enzyme 24-hydroxylase when vitamin D levels are abundant. It is rarely measured except in specialized research for conditions like Idiopathic Infantile Hypercalcemia. #### NEET-PG High-Yield Pearls: * **Site of Hydroxylation:** 1st hydroxylation (25-OH) occurs in the **Liver** (cytochrome P450 enzyme); 2nd hydroxylation (1-OH) occurs in the **Kidney** (1-alpha-hydroxylase). * **Rate-limiting step:** The renal 1-alpha-hydroxylase step, stimulated by PTH and low phosphate. * **Most Active Form:** 1,25-dihydroxycholecalciferol (Calcitriol). * **Best Indicator of Vitamin D Status:** 25-hydroxyvitamin D (Calcidiol).

Question 678: What is one of the earliest symptoms of Vitamin A deficiency?

A. Nyctalopia (Correct Answer)
B. Xerophthalmia
C. Bitot's spot
D. Keratomalacia

Explanation: **Explanation:** **1. Why Nyctalopia is Correct:** Nyctalopia, or **night blindness**, is the earliest clinical manifestation of Vitamin A deficiency. Vitamin A (Retinol) is essential for the synthesis of **Rhodopsin** (visual purple) in the rod cells of the retina. Rhodopsin is responsible for vision in dim light. When Vitamin A levels drop, the regeneration of rhodopsin is impaired, leading to a decreased ability to see in low light. This is often the first symptom reported by the patient. **2. Why Other Options are Incorrect:** * **Xerophthalmia:** This is a spectrum of eye diseases caused by Vitamin A deficiency. While Nyctalopia is the first stage, Xerophthalmia refers to the progression into pathological dryness of the conjunctiva and cornea. * **Bitot’s Spots:** These are triangular, foamy, silvery-white patches on the bulbar conjunctiva. They represent keratinization of the conjunctiva and occur *after* night blindness has already manifested. * **Keratomalacia:** This is the **most severe** and late-stage manifestation. It involves liquefaction and necrosis of the cornea, leading to permanent blindness. **3. NEET-PG High-Yield Pearls:** * **WHO Classification of Xerophthalmia:** * **X1A:** Conjunctival xerosis * **X1B:** Bitot’s spots * **X2:** Corneal xerosis * **X3A/X3B:** Corneal ulceration/Keratomalacia * **Earliest Sign:** Conjunctival xerosis (objective sign). * **Earliest Symptom:** Nyctalopia (subjective complaint). * **Wald’s Visual Cycle:** Retinol is converted to 11-cis-retinal, which combines with opsin to form Rhodopsin. * **Treatment:** The standard WHO schedule for children >1 year is 200,000 IU of Vitamin A orally on days 0, 1, and 14.

Question 679: What is the active moiety of Coenzyme A?

A. Acetyl group
B. Pantothenic acid
C. Thiol group of beta-alanine
D. Thiol group of pantetheine (Correct Answer)

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Coenzyme A (CoA-SH) is a vital cofactor derived from Vitamin B5 (Pantothenic acid). Its primary function is to act as a carrier of acyl groups (like acetyl or succinyl) in metabolism. The "business end" or **active moiety** of the molecule is the **thiol (-SH) group**. This thiol group is specifically located at the end of the **cysteamine** residue, which, when combined with pantothenic acid, forms **pantetheine**. Acyl groups bind to this thiol group via a high-energy **thioester bond**, allowing them to be activated for various biochemical reactions like the TCA cycle and fatty acid synthesis. **2. Why the Incorrect Options are Wrong:** * **Option A (Acetyl group):** This is a substrate that binds to CoA to form Acetyl-CoA; it is not a structural part of the coenzyme itself. * **Option B (Pantothenic acid):** While Vitamin B5 is a structural precursor of CoA, it does not contain the reactive thiol group required for acyl group attachment. * **Option C (Thiol group of beta-alanine):** Beta-alanine is a component of pantothenic acid, but it does not possess a thiol group. The thiol group is provided by the amino acid **cysteine** (as cysteamine). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Components of CoA:** Adenosine 3',5'-bisphosphate + Pantothenic acid + Cysteamine. * **Rate-limiting step of CoA synthesis:** Phosphorylation of pantothenate by *pantothenate kinase*. * **High-energy bond:** The thioester bond in Acetyl-CoA is more energy-rich than an oxygen-ester bond, facilitating group transfer. * **Acyl Carrier Protein (ACP):** In fatty acid synthesis, the active moiety is also the 4'-phosphopantetheine group, similar to CoA.

Question 680: Which of the following vitamins reduces the risk of insulin resistance, obesity, and the metabolic syndrome?

A. Vitamin A
B. Vitamin C
C. Vitamin D (Correct Answer)
D. Vitamin B12

Explanation: **Explanation:** **Vitamin D (Cholecalciferol)** is the correct answer because it functions more like a hormone than a traditional vitamin. It plays a crucial role in metabolic health through several mechanisms: 1. **Insulin Secretion:** Vitamin D receptors (VDR) are present on pancreatic β-cells. Vitamin D stimulates the expression of the insulin gene and enhances the conversion of pro-insulin to insulin. 2. **Insulin Sensitivity:** It improves insulin sensitivity in peripheral tissues (muscle and adipose) by upregulating the expression of insulin receptors and activating PPAR-γ (Peroxisome Proliferator-Activated Receptor gamma). 3. **Anti-inflammatory Action:** It reduces the production of pro-inflammatory cytokines (like TNF-α and IL-6) which are known drivers of insulin resistance and metabolic syndrome. **Why other options are incorrect:** * **Vitamin A:** Primarily involved in vision (rhodopsin), epithelial integrity, and immune function. While it influences gene expression via RAR/RXR receptors, it is not the primary vitamin linked to reducing metabolic syndrome risk. * **Vitamin C:** Acts as a potent antioxidant and a cofactor for collagen synthesis (hydroxylation of proline/lysine). While it reduces oxidative stress, it does not have a direct regulatory role in insulin signaling. * **Vitamin B12:** Essential for DNA synthesis and myelin formation. Deficiency leads to megaloblastic anemia and subacute combined degeneration of the spinal cord, but it is not directly involved in glucose homeostasis. **High-Yield Clinical Pearls for NEET-PG:** * **VDR Polymorphism:** Genetic variations in the Vitamin D Receptor are linked to an increased risk of Type 2 Diabetes Mellitus. * **Non-Skeletal Effects:** Beyond bone health (Rickets/Osteomalacia), Vitamin D is high-yield for its role in preventing colorectal cancer, multiple sclerosis, and hypertension. * **Metabolic Syndrome Criteria:** Remember the "ATP III" criteria—Abdominal obesity, High Triglycerides, Low HDL, Hypertension, and High Fasting Glucose. Vitamin D deficiency is frequently associated with all five components.

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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