Which one of the following is synthesised in human body that dilates blood vessels and increases blood flow?

Answer: A. Nitric oxide (NO) is the body's natural vasodilator. Nitric oxide is a free-radical signalling molecule produced in the body by endothelial cells lining blood vessels. It diffuses to the smooth muscle around vessels and triggers relaxation, causing vessels to dilate and blood flow to increase. The discovery of NO as a signalling molecule won the 1998 Nobel Prize in Physiology or Medicine (Furchgott, Ignarro, Murad). Drugs like nitroglycerin work by releasing NO to relieve angina. Distractors fail: (B) Nitrous oxide N2O is laughing gas, an anaesthetic, not a natural vasodilator. (C) Nitrogen dioxide NO2 is a toxic air pollutant. (D) Nitrogen pentoxide N2O5 is an unstable industrial intermediate, not biologically synthesised. Source: NCERT Class 11 Biology / Nobel Prize 1998 citation.