Nitric oxide (NO) is synthesized in the human body by the enzyme nitric oxide synthase from the amino acid arginine. It plays a crucial role in dilating blood vessels, which increases blood flow and reduces blood pressure. This function helps in regulating cardiovascular health. _Why the other options are incorrect:_ B. Nitrous oxide: Used as an anesthetic, not a vasodilator. C. Nitrogen dioxide: A toxic air pollutant, not involved in vasodilation. D. Nitrogen pentoxide: A reactive compound, not related to blood vessel dilation.