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: Unknown
Nitric oxide is requires which of the following AA 
Select and explain. 
A. Arginine, B. Serine, C. Threonine, D. Lysine
A: Arginine
Answer: A: Arginine 
Explanation: Ref. Guyton. 13th edition. Page. 208
Synthesis of NO
Nitric oxide is produced by a group of enzymes called nitric oxide synthases.
These enzymes convert arginine into citrulline, producing NO in the process.

Oxygen and NADPH are necessary co-factors.
There are three isoforms of nitric oxide synthase (NOS)
The isoforms of NOS are neuronal NOS (or nNOS), endothelial NOS (or eNOS) and inducible NOS (or iNOS).