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: Pharmacology
The primary endogenous substrate for nitric oxide synthase (NOS) is : 
Select and explain. 
A. Citrulline, B. Arginine, C. Heme, D. Methionine
B: Arginine
Answer: B: Arginine 
Explanation: None