Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 471: Which vitamin deficiency causes glossitis and cheilosis?

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

Explanation: **Explanation:** **Riboflavin (Vitamin B2)** is the correct answer because its deficiency specifically manifests as **Ariboflavinosis**. This clinical syndrome is characterized by mucosal and epithelial changes, most notably **glossitis** (magenta-colored tongue), **cheilosis** (fissuring of the lips), and **angular stomatitis** (cracks at the corners of the mouth). Riboflavin is a precursor for FMN and FAD, which are essential for redox reactions in energy metabolism; tissues with high turnover rates, like the oral mucosa, are the first to show signs of deficiency. **Analysis of Incorrect Options:** * **Thiamin (B1):** Deficiency primarily leads to **Beriberi** (Dry: peripheral neuropathy; Wet: high-output heart failure) or **Wernicke-Korsakoff syndrome** in alcoholics. It does not typically present with isolated cheilosis. * **Folic Acid (B9):** While deficiency can cause glossitis, its hallmark is **megaloblastic anemia** and neural tube defects. It is not the classic cause of the "cheilosis-glossitis" dyad associated with B2. * **Vitamin A:** Deficiency is primarily associated with ocular symptoms such as **Nyctalopia** (night blindness), Xerophthalmia, and Bitot’s spots, or skin changes like follicular hyperkeratosis. **High-Yield Clinical Pearls for NEET-PG:** * **Magenta Tongue:** A classic buzzword for Riboflavin deficiency (distinguish from the "Beefy Red" tongue of B12/Niacin deficiency). * **Corneal Neovascularization:** Another specific sign of B2 deficiency often tested alongside oral symptoms. * **Enzyme Link:** Riboflavin is required by **Glutathione Reductase**; its activity in RBCs is used to biochemically diagnose B2 deficiency.

Question 472: What is the similarity between Vitamin C and Vitamin K?

A. Both aid in the conversion of proline to hydroxyproline.
B. Both are involved in post-translational modification. (Correct Answer)
C. Both possess anti-infective activity.
D. Both are involved in the coagulation cascade.

Explanation: **Explanation:** The correct answer is **B. Both are involved in post-translational modification.** Post-translational modification refers to the chemical modification of a protein after its synthesis on the ribosome. Both Vitamin C and Vitamin K act as essential cofactors for enzymes that modify specific amino acid residues: 1. **Vitamin C (Ascorbic Acid):** Acts as a coenzyme for **prolyl hydroxylase** and **lysyl hydroxylase**. It facilitates the hydroxylation of proline and lysine residues in procollagen. This is crucial for the cross-linking and structural stability of triple-helical collagen. 2. **Vitamin K:** Acts as a cofactor for **$\gamma$-glutamyl carboxylase**. It is essential for the carboxylation of glutamate residues to $\gamma$-carboxyglutamate (Gla) on proteins like Factors II, VII, IX, X, and Proteins C and S. This modification allows these proteins to bind calcium and anchor to phospholipid membranes. **Analysis of Incorrect Options:** * **Option A:** This describes the specific function of **Vitamin C** only. Vitamin K does not affect proline hydroxylation. * **Option C:** **Vitamin A** is traditionally known as the "anti-infective" vitamin due to its role in maintaining epithelial integrity and immune function. * **Option D:** This is the specific function of **Vitamin K**. Vitamin C deficiency leads to scurvy (bleeding gums/petechiae) due to capillary fragility, but it is not a part of the enzymatic coagulation cascade. **NEET-PG High-Yield Pearls:** * **Vitamin C:** Deficiency leads to **Scurvy** (corkscrew hair, easy bruising, poor wound healing). It also aids in **Iron absorption** (reduces $Fe^{3+}$ to $Fe^{2+}$). * **Vitamin K:** Antagonized by **Warfarin**. It is necessary for the synthesis of "1972" factors (10, 9, 7, 2). Newborns receive a prophylactic dose because breast milk is low in Vitamin K and their sterile guts cannot synthesize it.

Question 473: Keshan disease is seen in deficiency of which element?

A. Selenium (Correct Answer)
B. Chromium
C. Molybdenum
D. Copper

Explanation: **Explanation:** **Keshan disease** is a congestive cardiomyopathy primarily caused by a deficiency of **Selenium**. It was first identified in the Keshan province of China, where the soil is notoriously low in this trace element. **Why Selenium is the correct answer:** Selenium is a vital component of the enzyme **Glutathione Peroxidase**, which protects cells from oxidative damage by neutralizing hydrogen peroxide. In selenium deficiency, the antioxidant defense system fails, leading to lipid peroxidation and subsequent myocardial necrosis and fibrosis. While selenium deficiency is the primary trigger, it often acts in synergy with a viral trigger (like the Coxsackievirus B) to manifest as cardiomyopathy. **Why other options are incorrect:** * **Chromium:** Deficiency is associated with **impaired glucose tolerance** and insulin resistance, as chromium is a component of the Glucose Tolerance Factor (GTF). * **Molybdenum:** It is a cofactor for enzymes like Xanthine Oxidase and Sulfite Oxidase. Deficiency is rare but can lead to neurological abnormalities and lens dislocation. * **Copper:** Deficiency leads to **Menkes Kinky Hair Syndrome**, microcytic anemia, and skeletal abnormalities. It is a cofactor for Lysyl Oxidase and Cytochrome c Oxidase. **High-Yield Clinical Pearls for NEET-PG:** * **Kashin-Beck Disease:** Another selenium deficiency disorder characterized by osteoarthropathy (cartilage degeneration). * **Selenocysteine:** Known as the **21st amino acid**, it is the form in which selenium is incorporated into proteins (Selenoproteins). * **Toxicity:** Excess selenium (Selenosis) causes garlic breath, hair loss (alopecia), and nail dystrophy.

Question 474: What is the primary function of cyanocobalamin?

A. Post-translational carboxylation of proteins
B. Acts as coenzymes for methionine synthase and L-methylmalonyl-CoA synthase
C. A coenzyme for one-carbon transfer in nucleic acid metabolism (Correct Answer)
D. A cofactor for decarboxylation of ketoacids

Explanation: ### Explanation **Correct Option: C (A coenzyme for one-carbon transfer in nucleic acid metabolism)** Cyanocobalamin (Vitamin B12) is essential for DNA synthesis. It acts as a mandatory cofactor for the enzyme **Methionine Synthase**, which converts homocysteine to methionine. During this reaction, B12 accepts a methyl group from N5-methyltetrahydrofolate (N5-methyl-THF). Without B12, folate remains "trapped" as N5-methyl-THF (the **Folate Trap**), making it unavailable for the synthesis of purines and dTMP. This impairment of one-carbon metabolism leads to megaloblastic anemia due to defective nucleic acid synthesis. **Analysis of Incorrect Options:** * **Option A:** This refers to **Vitamin K**, which is the cofactor for γ-glutamyl carboxylase, essential for activating clotting factors II, VII, IX, and X. * **Option B:** While B12 is a coenzyme for Methionine Synthase and **Methylmalonyl-CoA Mutase** (not synthase), Option C is the "primary" functional role emphasized in the context of its most common clinical manifestation (anemia) and its role in the folate cycle. * **Option D:** This refers to **Thiamine (Vitamin B1)**, which acts as a cofactor for enzymes like pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. **High-Yield Clinical Pearls for NEET-PG:** * **Absorption:** Requires **Intrinsic Factor** (secreted by parietal cells) and occurs in the **terminal ileum**. * **Deficiency Symptoms:** Megaloblastic anemia + **Subacute Combined Degeneration (SCD)** of the spinal cord (due to accumulation of methylmalonic acid affecting myelin). * **Schilling Test:** Historically used to differentiate causes of B12 malabsorption. * **Biochemical Markers:** Deficiency leads to elevated levels of both **Homocysteine** and **Methylmalonic acid (MMA)**. (Note: Folate deficiency only elevates homocysteine).

Question 475: Which of the following is the least useful anti-rachitic agent?

A. 1,25-dihydroxycholecalciferol (calcitriol)
B. Cholecalciferol (Correct Answer)
C. 25-hydroxycholecalciferol
D. Calcium

Explanation: **Explanation:** The term **"anti-rachitic"** refers to the ability of a substance to prevent or treat rickets (and osteomalacia) by promoting bone mineralization. To understand why **Cholecalciferol (Vitamin D3)** is the least useful agent among the options, one must look at the metabolic activation pathway of Vitamin D. 1. **Why Cholecalciferol is the correct answer:** Cholecalciferol is the inactive, storage form of Vitamin D. It requires two successive hydroxylations—first in the **liver** (to 25-OH-D3) and then in the **kidney** (to 1,25-(OH)₂-D3)—to become biologically active. In clinical scenarios involving renal failure or hepatic dysfunction, cholecalciferol cannot be converted into its active form, making it the least potent and least "useful" immediate anti-rachitic agent compared to its metabolites or direct mineral supplementation. 2. **Analysis of Incorrect Options:** * **1,25-dihydroxycholecalciferol (Calcitriol):** This is the most potent anti-rachitic agent. It is the active hormone that directly stimulates intestinal calcium absorption and bone mineralization. * **25-hydroxycholecalciferol (Calcidiol):** This is the major circulating form. While less active than calcitriol, it is one step further in the activation pathway than cholecalciferol and is more effective in patients with liver bypass needs. * **Calcium:** Rickets is fundamentally a failure of osteoid mineralization due to low calcium-phosphate product. Providing calcium is a direct and essential therapeutic step in treating nutritional rickets. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The 1-alpha-hydroxylation in the kidney (catalyzed by 1-alpha-hydroxylase) is the most strictly regulated step in Vitamin D synthesis. * **Vitamin D-Dependent Rickets Type 1:** Caused by a deficiency in renal 1-alpha-hydroxylase; treated with **Calcitriol**. * **Vitamin D-Dependent Rickets Type 2:** Caused by defective Vitamin D receptors (VDR); characterized by high levels of calcitriol but end-organ resistance. * **Storage form:** 25-hydroxycholecalciferol (Calcidiol) is the form measured to assess a patient's Vitamin D status due to its long half-life.

Question 476: Avidin influences which of the following vitamins?

A. Biotin (Correct Answer)
B. Niacin
C. Thiamine
D. Phylloquinone

Explanation: **Explanation:** The correct answer is **A. Biotin (Vitamin B7)**. **Why Biotin is correct:** Avidin is a heat-labile glycoprotein found in **raw egg whites**. It has an extraordinarily high affinity for Biotin. When consumed, avidin binds tightly to biotin in the gastrointestinal tract, forming a non-absorbable complex that prevents the vitamin's absorption. This can lead to "egg white injury," characterized by dermatitis, alopecia, and neurological symptoms. Cooking denatures avidin, rendering it unable to bind biotin. **Why other options are incorrect:** * **Niacin (B3):** Its deficiency (Pellagra) is associated with maize-based diets (low tryptophan) or Hartnup disease, not avidin. * **Thiamine (B1):** Deficiency is typically linked to chronic alcoholism or polished rice diets, leading to Beriberi or Wernicke-Korsakoff syndrome. * **Phylloquinone (K1):** This is a fat-soluble vitamin involved in blood clotting. Its absorption is influenced by bile salts and fat intake, not egg white proteins. **High-Yield Clinical Pearls for NEET-PG:** 1. **Biochemical Role:** Biotin acts as a coenzyme for **carboxylation reactions** (e.g., Pyruvate carboxylase, Acetyl-CoA carboxylase, and Propionyl-CoA carboxylase). 2. **The "CO2 Carrier":** Biotin functions as a carrier of activated carbon dioxide. 3. **Clinical Presentation:** To develop a deficiency via diet, a patient must consume a massive amount of raw egg whites (e.g., 12–20 per day) over a prolonged period. 4. **Valproate Link:** Long-term use of the anticonvulsant Valproate can sometimes interfere with biotinidase activity, leading to functional biotin deficiency.

Question 477: Post-translational modification of lysine and proline is facilitated by which vitamin?

A. Vitamin A
B. Vitamin C (Correct Answer)
C. Vitamin B1
D. Vitamin D

Explanation: **Explanation:** **Vitamin C (Ascorbic Acid)** is the correct answer because it acts as a vital co-factor for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**. These enzymes are responsible for the post-translational hydroxylation of proline and lysine residues during **collagen synthesis**. The underlying biochemical mechanism involves the reduction of ferric iron ($Fe^{3+}$) back to the active ferrous state ($Fe^{2+}$) within the enzyme's active site. This hydroxylation is essential for the formation of stable triple-helical collagen fibers; without it, collagen fibers lack tensile strength and are easily degraded. **Why other options are incorrect:** * **Vitamin A:** Primarily involved in vision (retinal), gene transcription (retinoic acid), and epithelial cell differentiation, not collagen hydroxylation. * **Vitamin B1 (Thiamine):** Acts as a coenzyme (TPP) for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase) and the HMP shunt (Transketolase). * **Vitamin D:** Functions as a hormone regulating calcium and phosphate homeostasis and bone mineralization. **High-Yield Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to defective collagen synthesis, manifesting as "corkscrew hair," petechiae, splinter hemorrhages, and **swollen, bleeding gums**. * **Wound Healing:** Vitamin C is critical for the proliferative phase of wound healing due to its role in collagen cross-linking. * **Iron Absorption:** Vitamin C enhances the absorption of non-heme iron from the gut by maintaining it in the ferrous ($Fe^{2+}$) state. * **Antioxidant:** It is a potent water-soluble antioxidant that regenerates Vitamin E.

Question 478: What is the active form of vitamin D?

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

Explanation: **Explanation:** The active form of Vitamin D is **1, 25-dihydroxyvitamin D**, also known as **Calcitriol**. Vitamin D synthesis is a multi-step process. In the skin, 7-dehydrocholesterol is converted to Cholecalciferol (Vitamin D3) by UV light. This undergoes two essential hydroxylations: 1. **First Hydroxylation:** Occurs in the **Liver** by the enzyme 25-hydroxylase to form 25-hydroxyvitamin D [25(OH)D]. 2. **Second Hydroxylation:** Occurs in the **Kidney** (proximal convoluted tubule) by the enzyme **1α-hydroxylase** to form 1, 25-dihydroxyvitamin D. This is the biologically active hormone that binds to nuclear receptors to regulate calcium and phosphate homeostasis. **Analysis of Options:** * **Option A (Cholecalciferol):** This is the inactive precursor synthesized in the skin or ingested; it has no biological activity until hydroxylated. * **Option B (24, 25-dihydroxyvitamin D):** This is an **inactive metabolite** formed when 1α-hydroxylase is inhibited. It is the body's way of diverting Vitamin D away from the active form when calcium levels are sufficient. * **Option D (25-hydroxyvitamin D):** Known as **Calcidiol**, this is the major **circulating form** and the best indicator of a patient’s overall Vitamin D status (nutritional reserve), but it is not the active form. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The 1α-hydroxylation in the kidney is the most tightly regulated step, stimulated by **PTH** and low serum phosphate. * **Storage:** Vitamin D is stored primarily in **adipose tissue**. * **Clinical Marker:** Always measure **25(OH)D** to diagnose deficiency, not Calcitriol, due to its longer half-life.

Question 479: Thiamine acts as a cofactor in which of the following reactions?

A. Conversion of pyruvate to acetyl-CoA (Correct Answer)
B. Transamination reactions
C. Oxidation in the respiratory chain
D. Conversion of pyridoxal to pyridoxal phosphate

Explanation: **Explanation:** **1. Why Option A is Correct:** Thiamine (Vitamin B1) is converted into its active form, **Thiamine Pyrophosphate (TPP)**. TPP acts as a crucial cofactor for enzymes involved in oxidative decarboxylation. The conversion of **Pyruvate to Acetyl-CoA** is catalyzed by the **Pyruvate Dehydrogenase (PDH) complex**, which requires TPP (along with FAD, NAD, CoA, and Lipoic acid). TPP is also a cofactor for: * $\alpha$-ketoglutarate dehydrogenase (TCA cycle) * Branched-chain $\alpha$-ketoacid dehydrogenase (BCAA metabolism) * Transketolase (HMP Shunt) **2. Why Other Options are Incorrect:** * **Option B:** Transamination reactions require **Pyridoxal Phosphate (PLP)**, the active form of Vitamin B6. * **Option C:** Oxidation in the respiratory chain (Electron Transport Chain) primarily utilizes **FAD/FMN** (derived from Vitamin B2/Riboflavin) and **NAD/NADP** (derived from Vitamin B3/Niacin) as electron carriers. * **Option D:** The conversion of pyridoxal to pyridoxal phosphate is a phosphorylation reaction catalyzed by pyridoxal kinase, requiring ATP, not thiamine. **3. High-Yield Clinical Pearls for NEET-PG:** * **Wernicke-Korsakoff Syndrome:** Characterized by the triad of ataxia, ophthalmoplegia, and confusion. It is common in chronic alcoholics due to thiamine deficiency. * **Beriberi:** "Dry" beriberi involves peripheral neuropathy; "Wet" beriberi involves high-output heart failure. * **Diagnostic Marker:** Erythrocyte **transketolase activity** is the most reliable biochemical test to assess thiamine status. * **Clinical Rule:** Always administer thiamine *before* glucose in malnourished patients to prevent precipitating Wernicke encephalopathy (as glucose oxidation consumes remaining thiamine).

Question 480: Zinc deficiency causes which of the following conditions?

A. Acrodermatitis enteropathica (Correct Answer)
B. Scaly dermatitis
C. Follicular keratosis
D. Skeletal fluorosis

Explanation: **Explanation:** **1. Why Acrodermatitis Enteropathica is Correct:** Acrodermatitis enteropathica (AE) is the classic clinical manifestation of severe **Zinc deficiency**. It can be inherited (autosomal recessive mutation in the *SLC39A4* gene, which encodes a zinc transporter) or acquired (due to malnutrition or malabsorption). Zinc is a vital cofactor for over 300 enzymes, including Carbonic Anhydrase and Alkaline Phosphatase. Deficiency leads to the characteristic triad of **periorificial and acral dermatitis** (rash around the mouth, anus, and limbs), **alopecia**, and **diarrhea**. **2. Analysis of Incorrect Options:** * **B. Scaly dermatitis:** While zinc deficiency involves skin changes, "scaly dermatitis" is a non-specific term. However, it is specifically associated with **Vitamin B6 (Pyridoxine)** or **Biotin** deficiency. * **C. Follicular keratosis:** Also known as "Phrynoderma" (toad skin), this is a hallmark of **Vitamin A deficiency** (and sometimes Vitamin C or essential fatty acids). * **D. Skeletal fluorosis:** This is caused by **chronic fluoride toxicity** (excess), not a deficiency. It leads to increased bone density and calcification of ligaments. **3. High-Yield Clinical Pearls for NEET-PG:** * **Zinc & Wound Healing:** Zinc is essential for DNA synthesis and cell division; deficiency causes **delayed wound healing** and **hypogeusia** (decreased taste acuity). * **Immunity:** Zinc is crucial for T-lymphocyte function; deficiency leads to cell-mediated immune dysfunction. * **Growth:** In children, zinc deficiency is a major cause of **growth retardation** and hypogonadism. * **Enzyme Marker:** Serum **Alkaline Phosphatase** levels often decrease in zinc deficiency as it is a zinc-dependent enzyme.

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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