Vitamins and Coenzymes — MCQs

A 30-year-old chronically malnourished male presented with weakness, lassitude, anorexia, poor appetite, abdominal discomfort, and diarrhea. He also exhibits apathy, lethargy, and mild disorientation. On examination, glossitis and dermatitis in sun-exposed areas are noted. What is the chromosome on which the gene for the metabolic disorder that mimics these findings is present?

Q47Easy

A raw beefy tongue is seen in the deficiency of which vitamin?

Q48Easy

Which of the following is a water-soluble vitamin?

Q49Easy

Hyperlipidemias are treated by administering which vitamin in megadoses?

Q50Easy

Exposure to ultraviolet light directly facilitates which of the following?

Vitamins and Coenzymes Indian Medical PG Practice Questions and MCQs

Question 41: What is the investigation of choice in Thiamine deficiency?

A. RBC-Thiamine levels
B. RBC - Transketolase levels (Correct Answer)
C. RBC - Glutathione reductase levels
D. WBC - Ascorbic acid levels

Explanation: ### Explanation **1. Why RBC Transketolase is the Correct Answer:** Thiamine (Vitamin B1) acts as a vital coenzyme in the form of **Thiamine Pyrophosphate (TPP)**. One of its key roles is serving as a cofactor for the enzyme **Transketolase** in the Pentose Phosphate Pathway (HMP Shunt). In clinical practice, measuring direct serum thiamine levels is often unreliable as it does not reflect intracellular stores. The **investigation of choice** is the measurement of **Erythrocyte (RBC) Transketolase activity**. The test is performed before and after the addition of exogenous TPP. An increase in enzyme activity (known as the "TPP effect") of >15–25% confirms a deficiency. **2. Analysis of Incorrect Options:** * **Option A (RBC-Thiamine levels):** While intracellular thiamine can be measured, it is technically difficult and less sensitive than functional enzymatic assays. * **Option C (RBC-Glutathione reductase levels):** This is the investigation of choice for **Riboflavin (Vitamin B2)** deficiency. Glutathione reductase requires FAD as a cofactor. * **Option D (WBC-Ascorbic acid levels):** This is used to assess **Vitamin C** status. WBC levels are a better reflection of body stores than plasma levels for Vitamin C. **3. High-Yield Clinical Pearls for NEET-PG:** * **Key TPP-dependent enzymes:** Pyruvate Dehydrogenase, $\alpha$-Ketoglutarate Dehydrogenase, and Transketolase. * **Clinical Triad of Wernicke’s Encephalopathy:** Ophthalmoplegia, Ataxia, and Confusion (Global dementia). * **Korsakoff Psychosis:** Characterized by anterograde amnesia and **confabulation** (filling memory gaps with fabricated stories). * **Golden Rule:** In a malnourished or alcoholic patient, **always administer Thiamine before Glucose** to prevent precipitating acute Wernicke’s encephalopathy.

Question 42: Which of the following is a consequence of Vitamin B12 deficiency?

A. Folate trap
B. Decreased methyl malonyl CoA (Correct Answer)
C. SACD
D. Megaloblastic anemia

Explanation: **Explanation:** Vitamin B12 (Cobalamin) acts as a coenzyme for two critical reactions in the body: the conversion of homocysteine to methionine and the conversion of **Methylmalonyl CoA to Succinyl CoA** (catalyzed by Methylmalonyl CoA mutase). **Why the Correct Answer is Right:** In Vitamin B12 deficiency, the enzyme Methylmalonyl CoA mutase cannot function. This leads to an **accumulation** (increase) of Methylmalonyl CoA in the blood and urine. Therefore, the option stating "Decreased methyl malonyl CoA" is physiologically incorrect regarding the consequence of the deficiency, making it the "except" or "incorrect consequence" often sought in such MCQ formats. (Note: In standard clinical pathology, B12 deficiency causes **increased** Methylmalonyl CoA). **Analysis of Other Options:** * **Folate Trap:** B12 is required to remove the methyl group from N5-methyl THF. Without B12, folate remains "trapped" in the methyl form, leading to a functional folate deficiency. * **SACD (Subacute Combined Degeneration of the Spinal Cord):** This is a classic neurological consequence of B12 deficiency caused by the accumulation of methylmalonic acid, which interferes with myelin synthesis in the dorsal columns and corticospinal tracts. * **Megaloblastic Anemia:** Due to the folate trap, DNA synthesis is impaired (specifically dTMP production), leading to macrocytic RBCs and hypersegmented neutrophils. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Marker:** Elevated **Methylmalonic Acid (MMA)** is the most sensitive and specific marker to distinguish B12 deficiency from pure Folate deficiency (MMA is normal in Folate deficiency). * **Schilling Test:** Historically used to determine the cause of B12 malabsorption (e.g., Pernicious anemia vs. dietary). * **Neurological Symptoms:** Unlike Folate deficiency, B12 deficiency presents with neurological deficits (paresthesia, loss of vibration/position sense).

Question 43: A patient consumes a strict vegetarian diet and does not take any animal products. He is at risk of deficiency of which vitamin?

A. Vitamin B1
B. Vitamin B3
C. Vitamin B4
D. Vitamin B12 (Correct Answer)

Explanation: **Explanation:** **Why Vitamin B12 is the correct answer:** Vitamin B12 (Cobalamin) is unique among vitamins because it is synthesized exclusively by microorganisms. It is found naturally only in **animal-derived foods** such as meat, fish, poultry, eggs, and dairy products. Plants do not require B12 and therefore do not synthesize or store it. Consequently, individuals following a strict vegan or vegetarian diet without supplementation are at a high risk of developing deficiency. Since the liver stores significant amounts of B12, clinical symptoms (like megaloblastic anemia or subacute combined degeneration of the spinal cord) may take 3–5 years to manifest after starting a strict vegan diet. **Why the other options are incorrect:** * **Vitamin B1 (Thiamine):** Found abundantly in whole grains, legumes, seeds, and nuts. Vegetarians usually get adequate amounts unless they consume a diet primarily of polished rice. * **Vitamin B3 (Niacin):** Found in legumes, nuts, and enriched cereals. It can also be synthesized in the body from the amino acid Tryptophan, which is present in plant proteins. * **Vitamin B4:** This is an obsolete term formerly used for adenine or choline; it is not considered a true vitamin in modern biochemistry and is not a standard NEET-PG focus. **High-Yield Clinical Pearls for NEET-PG:** * **Absorption:** Requires **Intrinsic Factor (IF)** secreted by gastric parietal cells; absorption occurs in the **terminal ileum**. * **Metabolic Role:** Acts as a coenzyme for **Methionine Synthase** (homocysteine to methionine) and **Methylmalonyl-CoA Mutase** (propionate metabolism). * **Diagnosis:** Deficiency leads to elevated levels of **Methylmalonic Acid (MMA)** and **Homocysteine**. * **Pernicious Anemia:** An autoimmune cause of B12 deficiency due to antibodies against parietal cells or IF, not diet-related.

Question 44: Which vitamin can be used for the treatment of hypercholesterolemia?

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

Explanation: **Explanation:** **Niacin (Vitamin B3)** is the correct answer because, in pharmacological doses (1.5–3 g/day), it acts as a potent lipid-lowering agent. It inhibits **lipolysis** in adipose tissue by acting on G-protein coupled receptors, leading to a decreased flux of free fatty acids to the liver. This results in reduced synthesis of VLDL and, consequently, LDL. Most importantly, Niacin is the most effective agent for **increasing HDL levels** by decreasing its fractional clearance. **Why other options are incorrect:** * **Thiamine (B1):** Functions as a coenzyme (TPP) for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase). Deficiency leads to Beriberi or Wernicke-Korsakoff syndrome, but it has no role in lipid metabolism. * **Pyridoxine (B6):** Primarily involved in amino acid metabolism (transamination and decarboxylation) and heme synthesis. It does not lower cholesterol. * **Vitamin B12 (Cobalamin):** Essential for DNA synthesis and myelin formation. While it helps lower homocysteine levels, it does not treat hypercholesterolemia. **High-Yield Clinical Pearls for NEET-PG:** * **Side Effect:** The most common side effect of Niacin is **cutaneous flushing**, mediated by Prostaglandin $D_2$. This can be prevented by taking **Aspirin** 30 minutes prior. * **Metabolic Effects:** Niacin can cause **hyperuricemia** (precipitating gout) and **hyperglycemia** (impaired glucose tolerance). * **Pellagra:** Deficiency of Niacin leads to the 3 Ds: Dermatitis, Diarrhea, and Dementia. * **Hartnup Disease:** A condition where tryptophan absorption is impaired, leading to secondary Niacin deficiency.

Question 45: Which vitamin is required for the hydroxylation of proline?

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

Explanation: **Explanation:** **Correct Option: C (Vitamin C / Ascorbic Acid)** Vitamin C is an essential cofactor 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**. * **Mechanism:** The hydroxylation reaction requires ferrous iron ($Fe^{2+}$). During the reaction, iron is oxidized to the ferric state ($Fe^{3+}$). Vitamin C acts as a reducing agent, converting iron back to its active ferrous state, thereby maintaining the enzyme's activity. * **Significance:** Hydroxyproline is critical for stabilizing the collagen triple helix via interchain hydrogen bonding. **Incorrect Options:** * **Vitamin A:** Primarily involved in vision (retinal), gene transcription (retinoic acid), and epithelial maintenance. * **Vitamin B:** This is a complex of vitamins. While B-vitamins act as coenzymes (e.g., B1 for decarboxylation, B6 for transamination), they do not participate in collagen hydroxylation. * **Vitamin D:** Primarily regulates calcium and phosphate homeostasis and bone mineralization. **Clinical Pearls for NEET-PG:** * **Scurvy:** Deficiency of Vitamin C leads to defective collagen synthesis. Clinical signs include "corkscrew hair," petechiae, easy bruising, and **swollen, bleeding gums** due to capillary fragility. * **Wound Healing:** Vitamin C is vital for wound healing because collagen is the primary structural protein of scar tissue. * **Localization:** Collagen hydroxylation occurs within the **Lumen of the Rough Endoplasmic Reticulum (RER)**.

Question 46: A 30-year-old chronically malnourished male presented with weakness, lassitude, anorexia, poor appetite, abdominal discomfort, and diarrhea. He also exhibits apathy, lethargy, and mild disorientation. On examination, glossitis and dermatitis in sun-exposed areas are noted. What is the chromosome on which the gene for the metabolic disorder that mimics these findings is present?

A. 5p (Correct Answer)
B. 21p
C. 6p
D. 8p

Explanation: **Explanation:** The clinical presentation of dermatitis (in sun-exposed areas), diarrhea, and dementia (apathy/disorientation), combined with glossitis, is the classic triad of **Pellagra**, caused by a deficiency of **Niacin (Vitamin B3)**. The question asks for the chromosome of a metabolic disorder that *mimics* these findings. This refers to **Hartnup disease**, an autosomal recessive disorder characterized by a defect in the transport of neutral amino acids (including **Tryptophan**) in the renal tubules and intestines. Since Tryptophan is a precursor for Niacin synthesis (60 mg Tryptophan = 1 mg Niacin), its deficiency leads to pellagra-like symptoms. The gene responsible for Hartnup disease is **SLC6A19**, which is located on **Chromosome 5p**. **Analysis of Options:** * **A. 5p (Correct):** Location of the SLC6A19 gene. Mutations here cause Hartnup disease, mimicking Niacin deficiency. * **B. 21p:** Associated with Down Syndrome (Trisomy 21) and certain amyloid proteins, but not neutral amino acid transport. * **C. 6p:** Contains the HLA complex (Major Histocompatibility Complex), linked to various autoimmune diseases but not pellagra-like symptoms. * **D. 8p:** Associated with conditions like Spherocytosis or certain neurodevelopmental disorders, but irrelevant to Tryptophan metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Pellagra 4 D’s:** Dermatitis (Casal’s necklace), Diarrhea, Dementia, and Death. * **Hartnup Disease Diagnosis:** High levels of neutral amino acids in urine (**Neutral Aminoaciduria**) but normal levels in blood. * **Niacin Synthesis:** Requires **Vitamin B6 (Pyridoxine)** as a cofactor (Kynureninase enzyme). Deficiency of B6 can also cause secondary pellagra. * **Dietary Link:** Pellagra is common in populations consuming **Maize (Corn)** as a staple, as the niacin in maize is bound (niacytin) and it is deficient in Tryptophan.

Question 47: A raw beefy tongue is seen in the deficiency of which vitamin?

A. Vitamin B1
B. Vitamin B2
C. Vitamin B3 (Correct Answer)
D. Vitamin B12

Explanation: **Explanation:** The correct answer is **Vitamin B3 (Niacin)**. **Why Vitamin B3 is correct:** A "raw beefy tongue" is a classic clinical sign of **Pellagra**, which is caused by a deficiency of Vitamin B3 (Niacin) or its precursor, the amino acid Tryptophan. In the early stages of deficiency, the tongue becomes red and swollen (glossitis) at the tip and margins. As the condition progresses, the entire tongue becomes intensely red, inflamed, and painful, resembling **raw beef**. This is often accompanied by the "3 Ds": Dermatitis (Casal’s necklace), Diarrhea, and Dementia. **Why other options are incorrect:** * **Vitamin B1 (Thiamine):** Deficiency primarily leads to Beriberi (Dry/Wet) or Wernicke-Korsakoff syndrome. It does not typically present with specific tongue changes. * **Vitamin B2 (Riboflavin):** Deficiency causes **Magenta tongue** (purplish-red), along with cheilosis, angular stomatitis, and corneal vascularization. * **Vitamin B12 (Cobalamin):** Deficiency leads to a **"Smooth, glazed, or Hunter’s glossitis."** The tongue appears pale and shiny due to the atrophy of lingual papillae, rather than the raw, beefy appearance of Pellagra. **High-Yield Clinical Pearls for NEET-PG:** * **Niacin Synthesis:** 60 mg of Tryptophan is required to synthesize 1 mg of Niacin (requires B6 as a cofactor). * **Hartnup Disease:** A genetic defect in neutral amino acid transport that leads to Pellagra-like symptoms due to Tryptophan loss. * **Carcinoid Syndrome:** Can lead to Niacin deficiency because Tryptophan is diverted to produce excessive Serotonin. * **Corn/Maize Diets:** Predispose to Pellagra because Niacin in corn is bound (Niacytin) and unavailable for absorption.

Question 48: Which of the following is a water-soluble vitamin?

A. Thiamine (Correct Answer)
B. Retinoic acid
C. Cholecalciferol
D. Tocopherol

Explanation: **Explanation:** Vitamins are classified into two broad categories based on their solubility: **Water-soluble** and **Fat-soluble**. This classification determines their absorption, transport, storage, and toxicity profiles. **1. Why Thiamine is Correct:** **Thiamine (Vitamin B1)** is a water-soluble vitamin belonging to the B-complex group. Water-soluble vitamins (B-complex and Vitamin C) are not stored in the body to any significant extent (except B12) and are excreted in the urine when in excess. Thiamine acts as a vital coenzyme (Thiamine Pyrophosphate - TPP) for decarboxylation reactions, such as the Pyruvate Dehydrogenase complex. **2. Why the other options are incorrect:** * **Retinoic acid (Vitamin A):** A fat-soluble vitamin essential for vision and epithelial integrity. * **Cholecalciferol (Vitamin D3):** A fat-soluble vitamin synthesized in the skin via UV light, crucial for calcium homeostasis. * **Tocopherol (Vitamin E):** A fat-soluble vitamin that acts as a potent antioxidant, protecting cell membranes from lipid peroxidation. **Clinical Pearls for NEET-PG:** * **Mnemonic for Fat-soluble vitamins:** **KADE** (Vitamins K, A, D, and E). * **Storage:** Fat-soluble vitamins are stored in the liver and adipose tissue; hence, they can lead to toxicity (Hypervitaminosis), whereas water-soluble vitamins rarely cause toxicity. * **Thiamine Deficiency:** Leads to **Beriberi** (Dry: neurological; Wet: cardiovascular) and **Wernicke-Korsakoff syndrome**, commonly seen in chronic alcoholics. * **Enzyme Marker:** Erythrocyte transketolase activity is used to clinically assess thiamine status.

Question 49: Hyperlipidemias are treated by administering which vitamin in megadoses?

A. Vitamin B12
B. Folate
C. Thiamine
D. Niacin (Correct Answer)

Explanation: **Explanation:** **Niacin (Vitamin B3)**, when administered in pharmacological doses (1.5–3 grams/day), acts as a potent lipid-lowering agent. Its primary mechanism involves the inhibition of **hormone-sensitive lipase** in adipose tissue. This reduces the lipolysis of triglycerides into free fatty acids (FFAs). Since FFAs are the primary substrate for hepatic triglyceride synthesis, their reduction leads to decreased production of **VLDL** and, subsequently, **LDL**. Furthermore, Niacin is the most effective agent for **increasing HDL levels** by decreasing the fractional clearance of apoA-I. **Why the other options are incorrect:** * **Vitamin B12 (Cobalamin) & Folate (B9):** These are essential for DNA synthesis and the conversion of homocysteine to methionine. While they help manage hyperhomocysteinemia (a cardiovascular risk factor), they do not directly lower serum cholesterol or triglycerides. * **Thiamine (B1):** Acts as a coenzyme for oxidative decarboxylation (e.g., Pyruvate Dehydrogenase). Its deficiency leads to Beriberi or Wernicke-Korsakoff syndrome, but it has no role in lipid metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Side Effects:** The most common side effect is **cutaneous flushing**, mediated by Prostaglandin $D_2$ and $E_2$. This can be mitigated by pre-treating with **Aspirin**. * **Metabolic Risks:** Niacin can cause **hyperuricemia** (precipitating gout) and **hyperglycemia** (impaired glucose tolerance). * **Lp(a) Reduction:** Niacin is one of the few drugs that significantly lowers **Lipoprotein(a)** levels.

Question 50: Exposure to ultraviolet light directly facilitates which of the following?

A. Conversion of 7-dehydrocholesterol to 25-hydroxycholecalciferol (Correct Answer)
B. Conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol
C. Transport of calcium into the extracellular fluid
D. Formation of calcium-binding protein

Explanation: ### Explanation **Correct Option: A (Conversion of 7-dehydrocholesterol to cholecalciferol/pre-vitamin D3)** The synthesis of Vitamin D begins in the skin (epidermis). **7-dehydrocholesterol** (an intermediate in cholesterol synthesis) acts as the precursor. When skin is exposed to **Ultraviolet B (UVB) radiation** (wavelength 290–315 nm), the B-ring of 7-dehydrocholesterol is photolytically cleaved to form **previtamin D3**, which spontaneously isomerizes to **cholecalciferol** (Vitamin D3). *Note: While the question mentions 25-hydroxycholecalciferol, in the context of NEET-PG, UV light is the specific trigger for the initial cutaneous step of the Vitamin D pathway.* **Why Incorrect Options are Wrong:** * **Option B:** The conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol (Calcitriol) occurs in the **kidneys** via the enzyme **1-alpha-hydroxylase**. This step is regulated by Parathyroid Hormone (PTH) and low serum phosphate, not UV light. * **Option C & D:** These are the **biological actions** of the active form of Vitamin D (Calcitriol). Calcitriol acts on the intestine to induce the synthesis of **Calbindin** (calcium-binding protein), which facilitates calcium absorption into the ECF. These are downstream metabolic effects, not direct photochemical reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The 1-alpha-hydroxylation in the kidney is the most strictly regulated step in Vitamin D activation. * **Storage form:** 25-hydroxycholecalciferol (Calcidiol) is the major circulating form and the best indicator of a patient's Vitamin D status. * **Active form:** 1,25-dihydroxycholecalciferol (Calcitriol) is the most potent metabolite. * **Deficiency:** Leads to **Rickets** in children (defective mineralization of osteoid) and **Osteomalacia** in adults (demineralization of pre-existing bone).

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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