Vitamins and Coenzymes Practice Questions

Q: Which vitamin deficiency may be observed in a patient on INH therapy?

Pyridoxine. **Explanation:** The correct answer is **Pyridoxine (Vitamin B6)**. **Mechanism of Interaction:** Isoniazid (INH), a primary drug for tuberculosis, is structurally similar to pyridoxine. It causes Vitamin B6 deficiency through two main mechanisms: 1. **Inhibition of Pyridoxal Kinase:** INH inhibits the enzyme that converts pyridoxine into its active form, Pyridoxal-5-Phosphate (PLP). 2. **Formation of Hydrazones:** INH reacts with PLP to form inactive isonicotinyl-hydrazone complexes, which are rapidly excreted in the urine. Since PLP is a vital cofactor for the synthesis of neurotransmitters (like GABA), its deficiency leads to **Peripheral Neuropathy**, characterized by numbness and "pins and needles" sensations. **Why other options are incorrect:** * **Thiamine (B1):** Deficiency (Wernicke-Korsakoff) is typically associated with chronic alcoholism, not INH. * **Folic acid (B9):** Deficiency is associated with drugs like Methotrexate or Phenytoin, leading to megaloblastic anemia. * **Cyanocobalamin (B12):** Deficiency is usually due to malabsorption (e.g., Pernicious anemia or Metformin use), not INH therapy. **High-Yield Clinical Pearls for NEET-PG:** * **Prophylaxis:** Patients on INH should be co-prescribed **10–50 mg/day of Pyridoxine** to prevent neuropathy. * **Sideroblastic Anemia:** B6 deficiency also impairs heme synthesis (ALA synthase is PLP-dependent), leading to microcytic hypochromic anemia with ringed sideroblasts. * **Risk Factors:** Slow acetylators, malnourished patients, and diabetics are at a higher risk of INH-induced B6 deficiency.

Q: Which dietary deficiency is most likely responsible for megaloblastic anemia, glossitis, pharyngeal ulcers, and impaired immunity?

Folate deficiency. **Explanation:** The clinical presentation of **megaloblastic anemia** combined with **glossitis** (inflammation of the tongue) and **mucosal ulcerations** is a classic hallmark of **Folate (Vitamin B9) deficiency**. **1. Why Folate Deficiency is Correct:** Folate is essential for **one-carbon metabolism**, specifically the synthesis of purines and thymidine required for DNA replication. * **Megaloblastic Anemia:** Impaired DNA synthesis leads to "nuclear-cytoplasmic asynchrony" in RBC precursors, where the nucleus matures slower than the cytoplasm, resulting in large, fragile macrocytes. * **Mucosal Lesions:** Rapidly dividing cells, such as those in the gastrointestinal tract and oropharynx, are highly sensitive to folate deficiency, leading to glossitis and pharyngeal ulcers. * **Impaired Immunity:** Folate is required for the proliferation of lymphocytes; deficiency leads to a blunted immune response. **2. Why Other Options are Incorrect:** * **Thiamine (B1):** Deficiency typically presents as Beriberi (cardiovascular or neurological symptoms) or Wernicke-Korsakoff syndrome, not megaloblastic anemia. * **Niacin (B3):** Deficiency causes **Pellagra**, characterized by the "3 Ds": Dermatitis, Diarrhea, and Dementia. * **Vitamin D:** Deficiency leads to Rickets (children) or Osteomalacia (adults) due to impaired calcium and phosphate metabolism. **NEET-PG High-Yield Pearls:** * **The "Folate Trap":** Vitamin B12 deficiency can cause a secondary folate deficiency by trapping folate in the N5-methyl THF form. * **Diagnostic Clue:** Both B12 and Folate deficiency show **hypersegmented neutrophils** (>5 lobes) on peripheral smear. * **Differentiation:** Unlike B12 deficiency, Folate deficiency **does not** cause neurological symptoms (subacute combined degeneration) and shows **normal** methylmalonic acid (MMA) levels.

Q: An example of a preventive antioxidant is:

Catalase. **Explanation:** Antioxidants are classified into three levels of defense based on their mechanism of action. **Preventive antioxidants** act as the first line of defense by preventing the formation of new reactive oxygen species (ROS) or by neutralizing precursors like hydrogen peroxide ($H_2O_2$) before they can form highly reactive hydroxyl radicals ($\cdot OH$). **Why Catalase is correct:** Catalase is a classic preventive antioxidant. It is a heme-containing enzyme located in peroxisomes that catalyzes the decomposition of hydrogen peroxide into water and molecular oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$). By clearing $H_2O_2$, it prevents the **Fenton reaction**, which would otherwise generate the extremely damaging hydroxyl radical. Other examples include Glutathione peroxidase and metal-binding proteins like Ceruloplasmin and Transferrin (which sequester pro-oxidant metals). **Why the other options are incorrect:** * **B. Tocopherol (Vitamin E):** This is a **chain-breaking antioxidant**. It acts as a scavenger that terminates the lipid peroxidation chain reaction in cell membranes. * **C. Superoxide dismutase (SOD):** While it acts early, it is often categorized as a **scavenging antioxidant** because it reacts with the superoxide radical ($O_2^{\cdot-}$) that has already been formed, converting it to $H_2O_2$. * **D. Urate:** Uric acid is a water-soluble **scavenging antioxidant** present in plasma that traps free radicals. **High-Yield Clinical Pearls for NEET-PG:** * **First line (Preventive):** Catalase, Glutathione peroxidase, Selenium, EDTA, Transferrin. * **Second line (Scavenging/Chain-breaking):** Vitamin E (most potent in membranes), Vitamin C, Carotenoids, Urate. * **Third line (Repair enzymes):** DNA repair enzymes, Proteases. * **Glutathione Peroxidase:** Requires **Selenium** as a cofactor; it is the primary protector against lipid peroxidation in the cytosol.

Q: Which of the following is the richest source of Vitamin C?

Amla. **Explanation:** Vitamin C (Ascorbic Acid) is a water-soluble vitamin essential for collagen synthesis, antioxidant defense, and iron absorption. While many fruits and vegetables contain Vitamin C, their concentrations vary significantly. **Why Amla is the Correct Answer:** **Amla (Indian Gooseberry)** is recognized as one of the richest natural sources of Vitamin C. It contains approximately **600–700 mg of Vitamin C per 100g** of edible portion. This concentration is significantly higher than that found in citrus fruits. Furthermore, the Vitamin C in Amla is uniquely stable even when subjected to heat or drying due to the presence of tannins. **Analysis of Incorrect Options:** * **Guava:** This is an excellent source (approx. 220 mg/100g) and is often considered the second richest source among common fruits, but it still falls short of Amla. * **Orange:** While synonymous with Vitamin C in popular culture, oranges contain only about **50 mg/100g**, which is nearly 12–14 times less than Amla. * **Cabbage:** Although a good vegetable source (approx. 30–40 mg/100g), it contains much lower levels compared to the fruits listed. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Function:** Acts as a co-factor for **prolyl and lysyl hydroxylase**, essential for the post-translational modification of collagen. * **Deficiency:** Leads to **Scurvy**, characterized by "corkscrew hair," petechiae, and swollen, bleeding gums due to defective collagen cross-linking. * **Iron Absorption:** Vitamin C facilitates the absorption of **non-heme iron** by reducing it from the ferric ($Fe^{3+}$) to the ferrous ($Fe^{2+}$) state in the stomach. * **Milk Fact:** Milk is a **poor source** of Vitamin C (and Iron), a common point of testing in pediatric nutrition questions.

Q: Which step in the HMP pathway requires Thiamine Pyrophosphate (TPP)?

Transketolase. ### Explanation **1. Why Transketolase is Correct:** The Hexose Monophosphate (HMP) Shunt consists of an irreversible oxidative phase and a reversible non-oxidative phase. **Transketolase** is a key enzyme in the non-oxidative phase that catalyzes the transfer of 2-carbon units between sugar phosphates. This enzyme requires **Thiamine Pyrophosphate (TPP)**, a derivative of Vitamin B1, as a mandatory cofactor. It serves as a bridge between the HMP shunt and glycolysis by recycling pentose phosphates back into glycolytic intermediates (Glyceraldehyde-3-P and Fructose-6-P). **2. Why Other Options are Incorrect:** * **Options A & B (G6PD and 6-Phosphogluconate dehydrogenase):** These are the regulatory enzymes of the *oxidative* phase. They require **NADP+** as a cofactor to produce NADPH; they do not require TPP. * **Option D (Transaldolase):** While this enzyme also functions in the non-oxidative phase (transferring 3-carbon units), it does **not** require any cofactor. It utilizes a Schiff base mechanism involving a lysine residue at its active site. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Utility:** Erythrocyte transketolase activity is the **most sensitive biochemical marker** to diagnose Thiamine (B1) deficiency. An increase in enzyme activity after adding TPP in vitro confirms the deficiency. * **Wernicke-Korsakoff Syndrome:** Some individuals have a genetic variation in transketolase that reduces its affinity for TPP, making them more susceptible to neuropsychiatric symptoms during thiamine deficiency (often seen in chronic alcoholism). * **Mnemonic:** TPP is required by "The **A**lpha-**K**eto **T**eam": **A**lpha-ketoglutarate dehydrogenase, **K**eto-acid dehydrogenase (branched-chain), **T**ransketolase, and Pyruvate dehydrogenase.

Q: Which of the following is NOT a dietary source of vitamin B12?

Soya bean. **Explanation:** The core concept tested here is the unique biological origin of **Vitamin B12 (Cobalamin)**. Unlike most other vitamins, B12 is synthesized exclusively by microorganisms (bacteria and archaea). These microbes are found in the soil and the digestive tracts of animals. Consequently, Vitamin B12 is found **naturally only in foods of animal origin**. * **Why Soya bean is the correct answer:** Soya bean is a plant-based legume. Plants do not require B12 for their metabolism and do not possess the machinery to synthesize it. Therefore, natural plant products are devoid of Vitamin B12 unless they are artificially fortified or contaminated with soil bacteria. * **Why Fish, Meat, and Liver are incorrect:** These are all animal tissues. Animals ingest B12-producing bacteria or consume other animals in the food chain, storing the vitamin in their tissues. The **liver** is the primary storage site for B12 in the body, making it the richest dietary source. **NEET-PG High-Yield Clinical Pearls:** 1. **Strict Vegans:** Individuals who consume no animal products (vegans) are at high risk for B12 deficiency and must rely on fortified foods or supplements. 2. **Absorption:** B12 requires **Intrinsic Factor (IF)**, secreted by gastric parietal cells, for absorption in the **terminal ileum**. 3. **Storage:** Unlike other water-soluble vitamins, the liver stores significant amounts of B12 (enough for 3–5 years), which is why deficiency symptoms take years to manifest after stopping intake. 4. **Diagnostic Marker:** In B12 deficiency, both **Homocysteine** and **Methylmalonic Acid (MMA)** levels are elevated (distinguishing it from Folate deficiency, where only Homocysteine is raised).

Q: Rickets is due to which of the following?

Loss of phosphate in urine. **Explanation:** The pathophysiology of Rickets is primarily centered on a failure of mineralization of the osteoid matrix. While Vitamin D deficiency is the most common cause, the biochemical hallmark that leads to the clinical manifestation of Rickets is **hypophosphatemia**. **Why "Loss of phosphate in urine" is correct:** In Vitamin D deficiency, there is decreased intestinal absorption of calcium, leading to transient hypocalcemia. This triggers the parathyroid glands to secrete **Parathyroid Hormone (PTH)** (Secondary Hyperparathyroidism). PTH acts on the proximal renal tubules to **inhibit phosphate reabsorption**, leading to significant phosphaturia (loss of phosphate in urine). Low serum phosphate levels are the critical limiting factor that prevents the formation of hydroxyapatite crystals, directly causing the softening of bones seen in Rickets. **Analysis of Incorrect Options:** * **A & C:** While decreased absorption of calcium occurs, the body’s compensatory rise in PTH prevents severe hypocalcemia at the expense of phosphate. It is the low phosphate, not just low calcium, that prevents mineralization. * **B:** Increased mobilization of calcium from bone is an *effect* of high PTH levels to maintain serum calcium, but this process (resorption) is what leads to bone demineralization, not the primary cause of the rickets pathology itself. **High-Yield Clinical Pearls for NEET-PG:** * **Biochemical Profile:** Low/Normal Calcium, **Low Phosphate**, **High Alkaline Phosphatase (ALP)**, and High PTH. * **Radiological Signs:** Cupping, splaying, and fraying of metaphyses (most prominent at the wrist and knee). * **Key Feature:** The most sensitive biochemical marker for rickets activity is an **elevated serum ALP**. * **Vitamin D Resistant Rickets:** Usually due to X-linked hypophosphatemia (mutation in PHEX gene), where the primary defect is also renal phosphate wasting.

Q: What is the Vitamin A activity of beta-carotene compared to that of retinol?

1:6. **Explanation** The correct answer is **1:6**. This ratio represents the biological conversion efficiency of dietary beta-carotene into active Vitamin A (retinol) in the human body. **1. Why 1:6 is Correct:** Vitamin A exists in two forms: **Preformed Vitamin A** (Retinol, found in animal sources) and **Provitamin A** (Carotenoids like beta-carotene, found in plant sources). Biochemically, one molecule of beta-carotene is symmetrically cleaved by the enzyme **$\beta$-carotene 15,15'-dioxygenase** in the intestinal mucosa to yield two molecules of retinal. However, due to poor intestinal absorption and inefficient conversion, the biological activity is significantly lower. According to the WHO and traditional nutritional standards, **6 $\mu$g of beta-carotene is required to produce 1 $\mu$g of retinol**, making the activity ratio 1:6. **2. Analysis of Incorrect Options:** * **A (1:2):** This is a theoretical chemical ratio. While one molecule of beta-carotene can theoretically yield two molecules of retinol, the physiological conversion efficiency is much lower. * **B (1:4):** This ratio does not correspond to any standard nutritional conversion factor for Vitamin A. * **D (1:8):** While some modern studies suggest conversion might be even less efficient (e.g., 1:12 for mixed diets), the classic textbook and exam-standard value remains 1:6. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Retinol Equivalent (RE):** 1 RE = 1 $\mu$g of Retinol = 6 $\mu$g of Beta-carotene = 12 $\mu$g of other Provitamin A carotenoids. * **International Units (IU):** 1 IU of Vitamin A = 0.3 $\mu$g of Retinol. * **Storage:** Vitamin A is stored in the liver as **retinyl palmitate** within **Ito cells** (Stellate cells). * **Toxicity:** Excessive intake of preformed Vitamin A is toxic (Hypervitaminosis A), but excessive beta-carotene only causes **carotenemia** (yellowish skin) and is not toxic because the conversion to retinol is rate-limited.

Question 1

Which vitamin deficiency may be observed in a patient on INH therapy?

Accepted Answer

Pyridoxine

Answer

Thiamine

Answer

Folic acid

Answer

Cyanocobalamin

Question 2

What is the primary mechanism by which Vitamin A deficiency increases the risk of respiratory tract infections?

Accepted Answer

Dysfunction of the epithelial layer

Answer

Keratinization of upper epithelial cells

Answer

Lack of antibody production

Answer

Defective chemotaxis

Question 3

Which dietary deficiency is most likely responsible for megaloblastic anemia, glossitis, pharyngeal ulcers, and impaired immunity?

Accepted Answer

Folate deficiency

Answer

Thiamine deficiency

Answer

Niacin deficiency

Answer

Vitamin D deficiency

Question 4

A 43-year-old woman with chronic alcoholism presents with shortness of breath and edema. On examination, her blood pressure is 100/60 mm Hg, pulse 110/min, JVP is 8 cm, the cardiac apex is displaced and enlarged, there are bilateral inspiratory crackles, and there is pedal edema. For this patient with a potential vitamin deficiency, what is the most likely diagnosis?

Accepted Answer

Thiamine deficiency

Answer

Niacin deficiency

Answer

Pyridoxine deficiency

Answer

Vitamin C deficiency

Question 5

An example of a preventive antioxidant is:

Accepted Answer

Catalase

Answer

Tocopherol

Answer

Superoxide dismutase

Answer

Urate

Question 6

Which of the following is the richest source of Vitamin C?

Accepted Answer

Amla

Answer

Orange

Answer

Guava

Answer

Cabbage

Question 7

Which step in the HMP pathway requires Thiamine Pyrophosphate (TPP)?

Accepted Answer

Transketolase

Answer

Glucose-6-phosphate dehydrogenase (G6PD)

Answer

6-Phosphogluconate dehydrogenase

Answer

Transaldolase

Question 8

Which of the following is NOT a dietary source of vitamin B12?

Accepted Answer

Soya bean

Answer

Fish

Answer

Meat

Answer

Liver

Question 9

Rickets is due to which of the following?

Accepted Answer

Loss of phosphate in urine

Answer

Loss of calcium in urine

Answer

Increased mobilization of calcium from bone

Answer

Decreased absorption of calcium

Question 10

What is the Vitamin A activity of beta-carotene compared to that of retinol?

Accepted Answer

1:6

Answer

1:2

Answer

1:4

Answer

1:8

Vitamins and Coenzymes — MCQs

Vitamins and Coenzymes — MCQs

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