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
NO is synthesized by all except - 
Select and explain. 
A. Endothelium, B. Neuron, C. Macrophages, D. Platelets
D: Platelets
Answer: D: Platelets 
Explanation: Platelets
o Three isoforms of NO synthase have been identified Neuronal NOS (nNOS or NOS-1); (ii) Macrophage or inducible NOS (iNOS orNOS-2); and (Hi) Endothelial NOS (eNOS or NOS-3). NOS-1 and NOS-3 are constitutive enzymes, whereas NOS2 is inducible in inflammatory' conditions.
o NOS-1 and NOS-3 are activated by Ca +2Q
o NO synthase is a very complex enzyme, employing five redox cofactors: NADPH, FAD, FMN, heme, and tetrahydrobiopterin Q .