Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 291: Which vitamin deficiency is commonly seen in vegetarians?

A. Vitamin B12 (Correct Answer)
B. Vitamin B6
C. Vitamin B3
D. Vitamin B2

Explanation: **Explanation:** **Vitamin B12 (Cobalamin)** is the correct answer because it is synthesized exclusively by microorganisms and is found naturally only in **animal-based foods** (meat, fish, poultry, eggs, and dairy). Unlike other water-soluble vitamins, plants do not require B12 and therefore do not produce or store it. Consequently, strict vegetarians (vegans) are at a high risk of deficiency unless they consume fortified foods or supplements. **Analysis of Incorrect Options:** * **Vitamin B6 (Pyridoxine):** Widely distributed in both animal and plant sources (e.g., bananas, potatoes, nuts, and whole grains). Deficiency is usually associated with isoniazid (INH) therapy or alcoholism rather than a vegetarian diet. * **Vitamin B3 (Niacin):** Found in meat, but also abundant in legumes, nuts, and enriched cereals. Deficiency (Pellagra) is typically seen in populations relying on a maize-centric diet. * **Vitamin B2 (Riboflavin):** While dairy is a major source, it is also found in green leafy vegetables and enriched grains. Deficiency is rare in isolation and usually occurs alongside other B-complex deficiencies. **Clinical Pearls for NEET-PG:** * **Storage:** Unlike other B-vitamins, B12 is stored in the **liver** in significant amounts (3–5 mg), meaning deficiency symptoms may take 3–5 years to manifest after stopping intake. * **Absorption:** Requires **Intrinsic Factor (IF)** secreted by gastric parietal cells; absorption occurs in the **terminal ileum**. * **Biochemical Markers:** Deficiency leads to elevated levels of **Methylmalonic Acid (MMA)** and **Homocysteine**. * **Clinical Presentation:** Megaloblastic anemia with neurological symptoms (Subacute Combined Degeneration of the Spinal Cord).

Question 292: Pantothenic acid is utilized in the synthesis of which hormone?

A. Insulin
B. Thyroxine
C. Corticosteroids (Correct Answer)
D. All of the above

Explanation: **Explanation:** **Pantothenic acid (Vitamin B5)** is the essential precursor for the synthesis of **Coenzyme A (CoA)**. Coenzyme A plays a pivotal role in the metabolism of carbohydrates, lipids, and proteins by acting as a carrier of acyl groups (e.g., Acetyl-CoA). **Why Corticosteroids is the correct answer:** The synthesis of all steroid hormones, including **corticosteroids** (cortisol and aldosterone) and sex steroids, begins with **Cholesterol**. The rate-limiting step and the initial building blocks for cholesterol synthesis are derived from **Acetyl-CoA** (via the HMG-CoA reductase pathway). Since Pantothenic acid is a structural component of CoA, it is indispensable for the production of the Acetyl-CoA required to synthesize the steroid nucleus. **Why other options are incorrect:** * **Insulin:** This is a peptide hormone synthesized via protein synthesis (translation) in the beta cells of the pancreas. It does not require a CoA-dependent lipid pathway for its primary structure. * **Thyroxine (T4):** This is an amino acid derivative synthesized from Tyrosine and Iodine within the thyroid follicles. Its synthesis does not involve Coenzyme A. **High-Yield NEET-PG Clinical Pearls:** * **Active Form:** The active form of Vitamin B5 is Coenzyme A and **Acyl Carrier Protein (ACP)**, the latter being crucial for fatty acid synthesis. * **Deficiency:** Though rare, deficiency leads to **"Burning Feet Syndrome"** (Gopalan’s syndrome), characterized by paresthesia and erythema of the distal extremities. * **Key Enzyme Association:** Pantothenic acid is a vital component of the **Pyruvate Dehydrogenase (PDH)** complex and the **Alpha-ketoglutarate dehydrogenase** complex.

Question 293: Which one of the following statements regarding folic acid is incorrect?

A. It prevents megaloblastic anemia.
B. Its deficiency is detected by the Schilling test. (Correct Answer)
C. Its administration should be started in early pregnancy.
D. Its deficiency causes neural tube defects in the fetus.

Explanation: The correct answer is **B. Its deficiency is detected by the Schilling test.** ### Explanation **1. Why Option B is Incorrect (The Correct Answer):** The **Schilling test** is specifically used to diagnose the cause of **Vitamin B12 (Cobalamin)** deficiency, particularly to distinguish between dietary deficiency and Pernicious Anemia (lack of Intrinsic Factor). It has no role in the diagnosis of Folic Acid deficiency. Folic acid deficiency is typically diagnosed by measuring serum folate levels or, more accurately, **Red Cell Folate** levels. **2. Analysis of Other Options:** * **Option A:** Folic acid is essential for DNA synthesis (specifically the conversion of dUMP to dTMP). Deficiency leads to impaired nuclear maturation while cytoplasm grows normally, resulting in **Megaloblastic Anemia**. * **Option C & D:** Folic acid is critical for the closure of the neural tube in the developing embryo. Deficiency during the first trimester is a major risk factor for **Neural Tube Defects (NTDs)** like Spina Bifida and Anencephaly. Therefore, periconceptional supplementation (starting before or in early pregnancy) is standard medical practice. ### NEET-PG High-Yield Pearls * **FIGLU Excretion Test:** The specific biochemical test for folate deficiency is the measurement of Formiminoglutamic acid (FIGLU) in urine after a Histidine load. * **The "Folate Trap":** Vitamin B12 deficiency leads to a functional folate deficiency because folate remains trapped as N5-methyl THF. * **Supplementation Dose:** For a normal pregnancy, 400 µg/day is recommended; for a mother with a previous child having NTD, the dose is increased to 4 mg/day. * **Drug-Induced Deficiency:** Phenytoin, Methotrexate, and Trimethoprim are common drugs that interfere with folate metabolism.

Question 294: Which of the following vitamins is metabolically produced by the body?

A. Biotin
B. Vitamin K
C. Niacin (Correct Answer)
D. Pyridoxine

Explanation: **Explanation:** The correct answer is **Niacin (Vitamin B3)**. While most vitamins must be obtained entirely through diet, Niacin is unique because it can be endogenously synthesized in the liver from the essential amino acid **Tryptophan**. **Why Niacin is Correct:** The metabolic pathway for Niacin synthesis is the **Kynurenine pathway**. Approximately **60 mg of Tryptophan** is required to produce **1 mg of Niacin**. This process requires Vitamin B6 (Pyridoxine) as a cofactor for the enzyme kynureninase and Riboflavin (B2). Because the body can produce it, Niacin is technically not a "strict" vitamin if dietary tryptophan intake is sufficient. **Why Other Options are Incorrect:** * **Biotin (B7) and Vitamin K:** These are synthesized by **intestinal bacterial flora**, not by human metabolic pathways. While they are available to the body, they are considered products of the microbiome rather than endogenous human metabolism. * **Pyridoxine (B6):** This is an essential vitamin that must be obtained strictly from dietary sources (meat, whole grains, vegetables). The human body lacks the enzymes to synthesize the pyridine ring. **NEET-PG High-Yield Pearls:** * **Hartnup Disease:** A defect in the transport of neutral amino acids (like Tryptophan) leads to Niacin deficiency, presenting as **Pellagra** (Dermatitis, Diarrhea, Dementia, Death). * **Carcinoid Syndrome:** Tumor cells divert tryptophan to produce excessive Serotonin, leading to secondary Niacin deficiency and Pellagra. * **Cofactor Requirement:** Iron, Riboflavin (B2), and Pyridoxine (B6) are essential for the conversion of Tryptophan to Niacin. Deficiency in B6 can therefore manifest as Pellagra-like symptoms.

Question 295: In which organ is Vitamin C present in the largest amount?

A. Eye
B. Kidney
C. Testis
D. Adrenal cortex (Correct Answer)

Explanation: **Explanation:** **Correct Answer: D. Adrenal cortex** Vitamin C (Ascorbic acid) is found in the highest concentration in the **Adrenal glands**, specifically the adrenal cortex. The concentration here is approximately 100 times higher than that in the plasma. **Underlying Medical Concept:** The adrenal cortex requires high levels of Vitamin C for two primary reasons: 1. **Steroidogenesis:** It acts as a cofactor for enzymes involved in the synthesis of corticosteroid hormones. 2. **Antioxidant Protection:** The process of steroid synthesis generates significant reactive oxygen species (ROS). Vitamin C acts as a potent antioxidant to protect the glandular tissue from oxidative damage. During stress, ACTH stimulates the depletion of Vitamin C from the adrenal cortex as it is utilized rapidly. **Analysis of Incorrect Options:** * **A. Eye:** While the aqueous humor and lens contain high concentrations of Vitamin C (to protect against light-induced oxidative damage), the absolute concentration per gram of tissue is lower than that of the adrenal cortex. * **B. Kidney:** The kidneys are involved in the excretion and reabsorption of Vitamin C but do not store it in significant concentrations compared to endocrine organs. * **C. Testis:** While Vitamin C is present in the testes to protect sperm from oxidative DNA damage, the concentration does not reach the levels found in the adrenal glands. **High-Yield NEET-PG Pearls:** * **Storage Hierarchy:** Adrenal Glands > Pituitary Gland > Corpus Luteum > Retina > Brain. * **Biochemical Role:** Vitamin C is essential for the **hydroxylation of Proline and Lysine** residues during collagen synthesis. * **Absorption:** It is absorbed in the distal ileum via **SVCT1 and SVCT2** (Sodium-dependent Vitamin C Transporters). * **Clinical Sign:** The earliest sign of Scurvy is often **follicular hyperkeratosis**, followed by swollen, spongy gums and "corkscrew" hairs.

Question 296: Vitamin B12 is required for a number of metabolic processes. Which of the following lesions would NOT lead to a deficiency of this vitamin?

A. Chronic gastritis resulting in achlorhydria
B. Autoimmune destruction of gastric parietal cells
C. Surgical resection of the jejunum (Correct Answer)
D. Surgical resection of the ileum

Explanation: **Explanation:** Vitamin B12 (Cobalamin) absorption is a complex, multi-step process that relies on specific anatomical sites and physiological secretions. The correct answer is **Surgical resection of the jejunum** because the jejunum is not a primary site for B12 absorption or processing. **1. Why Option C is correct:** Vitamin B12 absorption occurs specifically in the **terminal ileum**. While the jejunum is the primary site for the absorption of most nutrients (including Folate), its resection does not directly impair the B12-Intrinsic Factor (IF) complex uptake. **2. Why the other options are incorrect:** * **A & B (Gastric Factors):** Gastric parietal cells secrete **Intrinsic Factor (IF)** and **Hydrochloric acid (HCl)**. HCl is required to release B12 from dietary proteins, while IF is essential for B12 absorption in the ileum. Achlorhydria (lack of HCl) and autoimmune destruction of parietal cells (Pernicious Anemia) both lead to B12 deficiency. * **D (Ileal Resection):** The terminal ileum contains the specific **cubilin receptors** required to internalize the B12-IF complex. Resection of this segment directly prevents B12 absorption. **High-Yield NEET-PG Pearls:** * **Site of Absorption:** B12 = Terminal Ileum; Folate = Jejunum; Iron = Duodenum (**"I Just Drank"** mnemonic: Iron, Jejunum/Folate, Duodenum). * **Transport Proteins:** B12 binds to **R-binder** (Haptocorrin) in the stomach and **Intrinsic Factor** in the duodenum (after pancreatic proteases degrade R-binder). In the blood, it is transported by **Transcobalamin II**. * **Storage:** Unlike other water-soluble vitamins, B12 is stored in the liver for 3–5 years; hence, deficiency takes years to manifest. * **Biochemical Markers:** B12 deficiency leads to elevated levels of both **Homocysteine** and **Methylmalonic Acid (MMA)**.

Question 297: A 'magenta tongue' is a clinical finding associated with which of the following conditions?

A. Riboflavin deficiency (Correct Answer)
B. Scarlet fever
C. Familial dysautonomia
D. Median rhomboid glossitis

Explanation: **Explanation:** **1. Why Riboflavin (Vitamin B2) Deficiency is Correct:** Riboflavin deficiency, also known as **ariboflavinosis**, typically presents with a constellation of oral-ocular-genital symptoms. The **'magenta tongue'** is a classic clinical sign characterized by a purplish-red discoloration and edema of the tongue, often accompanied by the loss of filiform papillae (glossitis). This occurs due to vascular congestion and changes in the overlying epithelium. Other hallmark features include **cheilosis** (fissuring of lips), **angular stomatitis** (cracks at the corners of the mouth), and corneal neovascularization. **2. Analysis of Incorrect Options:** * **Scarlet Fever:** Associated with a **'Strawberry tongue'**. Initially, it appears as a 'white strawberry tongue' (white coat with red papillae), which later desquamates to become a 'red strawberry tongue'. * **Familial Dysautonomia (Riley-Day Syndrome):** Characterized by a **total absence of fungiform papillae**, resulting in a smooth, pale tongue. * **Median Rhomboid Glossitis:** A fungal condition (Candidiasis) presenting as a persistent, asymptomatic, **diamond-shaped (rhomboid) red patch** on the central posterior dorsal tongue. **3. NEET-PG High-Yield Pearls:** * **Coenzyme Forms:** Riboflavin is the precursor for **FMN** (Flavin Mononucleotide) and **FAD** (Flavin Adenine Dinucleotide), essential for redox reactions (e.g., Succinate dehydrogenase in TCA cycle). * **Diagnostic Test:** The functional status of Riboflavin is assessed by measuring **Erythrocyte Glutathione Reductase activity**. * **The "Three S's" of Tongue Colors:** * **Magenta:** Vitamin B2 deficiency. * **Beefy Red:** Vitamin B12 or Niacin (B3) deficiency. * **Strawberry:** Scarlet fever or Kawasaki disease.

Question 298: Excess intake of retinol causes which of the following?

A. Nausea
B. Hepatomegaly
C. Anorexia
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. All of the above**. This question tests the clinical manifestations of **Hypervitaminosis A** (Vitamin A toxicity). **Underlying Medical Concept:** Retinol (Vitamin A) is a fat-soluble vitamin stored primarily in the **Ito cells (stellate cells)** of the liver. Because it is not easily excreted in the urine, excessive intake—either through acute massive doses or chronic supplementation—leads to accumulation and systemic toxicity. **Analysis of Options:** * **A. Nausea:** Acute toxicity often presents with signs of increased intracranial pressure (**Pseudotumor cerebri**), leading to nausea, vomiting, headache, and papilledema. * **B. Hepatomegaly:** Since the liver is the primary storage site, chronic excess leads to hypertrophy of Ito cells, fibrosis, and hepatomegaly. In severe cases, this can progress to cirrhosis and portal hypertension. * **C. Anorexia:** General systemic symptoms of chronic toxicity include anorexia (loss of appetite), weight loss, fatigue, and dry, peeling skin (desquamation). **High-Yield Clinical Pearls for NEET-PG:** 1. **Teratogenicity:** Vitamin A is highly teratogenic. It can cause craniofacial abnormalities and cardiac defects in the fetus. Pregnancy is a strict contraindication for high-dose retinoids (e.g., Isotretinoin). 2. **Bone Changes:** Chronic toxicity causes painful hyperostosis (excessive bone growth) and premature closure of epiphyses in children. 3. **Acute vs. Chronic:** * *Acute:* Nausea, vomiting, vertigo, blurred vision. * *Chronic:* Alopecia, hepatomegaly, bone pain, and hyperlipidemia. 4. **Carotenemia:** Excessive intake of Vitamin A precursors (Beta-carotene from carrots) causes yellow-orange skin but **does not** cause toxicity or yellowing of the sclera (unlike jaundice).

Question 299: Which dietary deficiency of a vitamin can cause Scurvy?

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

Explanation: **Explanation:** **Vitamin C (Ascorbic Acid)** is the correct answer. It serves 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 in collagen synthesis. Hydroxyproline is essential for stabilizing the collagen triple helix via hydrogen bonding. A deficiency leads to the production of defective, unstable collagen, resulting in **Scurvy**, characterized by capillary fragility (petechiae, ecchymosis), "corkscrew" hairs, swollen bleeding gums, and impaired wound healing. **Incorrect Options:** * **Niacin (Vitamin B3):** Deficiency leads to **Pellagra**, characterized by the "3 Ds": Dermatitis (Casal’s necklace), Diarrhea, and Dementia. * **Vitamin D:** Deficiency causes **Rickets** in children (defective mineralization of osteoid) and **Osteomalacia** in adults. * **Biotin (Vitamin B7):** Acts as a co-enzyme for carboxylation reactions (e.g., Pyruvate carboxylase). Deficiency is rare but can cause dermatitis, alopecia, and neurological symptoms, often linked to excessive consumption of raw egg whites (avidin binding). **NEET-PG High-Yield Pearls:** * **Microcytic Anemia:** Scurvy often presents with anemia because Vitamin C is required to maintain iron in the **ferrous (Fe²⁺) state**, which is essential for intestinal absorption. * **Osteoid Matrix:** In Scurvy, the osteoid matrix formation is defective, whereas in Rickets, the mineralization of the matrix is defective. * **Infantile Scurvy (Barlow’s Disease):** Look for subperiosteal hemorrhage and a "frog-leg" position in clinical vignettes.

Question 300: All of the following have antioxidant action except:

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

Explanation: **Explanation:** The correct answer is **Vitamin D**. Antioxidants are substances that neutralize reactive oxygen species (ROS) and free radicals, preventing cellular damage. While Vitamins A, C, E, and certain minerals are well-known for this role, Vitamin D primarily functions as a hormone involved in calcium and phosphorus homeostasis and bone mineralization. It does not possess a significant direct antioxidant mechanism. **Analysis of Options:** * **Vitamin E (Tocopherol):** The most powerful natural antioxidant. It acts as a chain-breaking antioxidant in cell membranes, protecting polyunsaturated fatty acids (PUFAs) from lipid peroxidation. * **Vitamin A (and Beta-carotene):** Carotenoids are effective antioxidants that quench singlet oxygen and neutralize free radicals, particularly in the skin and retina. * **Selenium:** An essential trace element that acts as a cofactor for **Glutathione Peroxidase**, an enzyme that neutralizes hydrogen peroxide, thereby protecting cells from oxidative stress. * **Vitamin D:** Its primary role is increasing intestinal calcium absorption. While some recent research suggests indirect anti-inflammatory effects, it is not classified as a classic antioxidant in standard biochemical curricula. **High-Yield Clinical Pearls for NEET-PG:** * **ACE of Antioxidants:** Remember **A**, **C**, and **E** as the primary antioxidant vitamins. * **Synergy:** Vitamin C helps regenerate the reduced (active) form of Vitamin E. * **Enzymatic Antioxidants:** Superoxide Dismutase (requires Zn, Cu, Mn), Catalase (requires Heme/Fe), and Glutathione Peroxidase (requires Selenium). * **Lipid Peroxidation:** Vitamin E is the first line of defense against peroxidation of membrane lipids.

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