UPSC Prelims 2024— Question 63

The correct answer is option (A). Nitric oxide is produced by endothelial cells and acts as a signalling molecule that relaxes vascular smooth muscle, producing vasodilation and increasing blood flow. Nitrous oxide is an anaesthetic gas, nitrogen dioxide is a toxic air pollutant, and nitrogen pentoxide is not the relevant physiological molecule. The key to this question is to distinguish the exact constitutional provision, factual association, scientific principle, or current-affairs detail being tested from statements that are partly true but incomplete or incorrectly framed. UPSC often makes the distractors plausible by changing one institutional role, date, classification, geographic association, or causal relationship. Option A — “Nitric oxide” is correct. It matches the established fact described above and gives the complete combination or identification required by the question. Option B — “Nitrous oxide” is incorrect. It does not match the established fact or contains the incorrect combination, institution, relationship, classification, or proposition identified in the explanation above. Choosing it would therefore introduce a factual error or omit a necessary element. Option C — “Nitrogen dioxide” is incorrect. It does not match the established fact or contains the incorrect combination, institution, relationship, classification, or proposition identified in the explanation above. Choosing it would therefore introduce a factual error or omit a necessary element. Option D — “Nitrogen pentoxide” is incorrect. It does not match the established fact or contains the incorrect combination, institution, relationship, classification, or proposition identified in the explanation above. Choosing it would therefore introduce a factual error or omit a necessary element. For examination purposes, the safest approach is to verify each statement independently before looking at the answer-code combination. This is particularly important in statement-based questions, because one false clause makes an entire combination wrong. In factual questions, compare the defining characteristic asked in the stem with each alternative rather than relying on superficial familiarity. The answer recorded here follows the verified Set C key used for this database.