This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Nitric oxide is derived from which amino acid? 
Select and explain. 
A. Histidine, B. Lysine, C. Methionine, D. Arginine
D: Arginine
Answer: D: Arginine 
Explanation: Ans:D.)Arginine.NITRIC OXIDE (NO)/ENDOTHELIUM DERIVED RELAXING FACTOR (EDRF)Nitric oxide, a vasodilator is formed by the action of nitric oxide synthase on Arginine. Synthesis:NO is formed by the action of Cytosolic enzyme NO synthase in endothelial & neuronal cells.Substrate: Arginine, O 2 , NADPHCoenzyme: NADPH, FAD, FMN, Heme, tetrahydrobiopterinProduct: Citrulline, NOFeaturesReacts with oxygen & superoxide ? peroxynitrite which decomposes to form highly reactive OH-radicalIt stimulates guanyl cyclase (cGMP)FunctionsRelaxes smooth muscles Prevents platelet aggregationFunctions as neurotransmitter in brainMediates tumoricidal, bactericidal actions of macrophagesInvolved in penile erection; (sildenafil affects this process by inhibiting a cGMP dependent phosphodiesterase)