This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
Nitric acid in synthesized from 
Select and explain. 
A. L-arginine, B. L-citrulline, C. Lysine, D. Trptophan
A: L-arginine
Answer: A: L-arginine 
Explanation: Ref-Sathyanarayana 4/e p387 Dietary supplements containing L-arginine have been marketed with the purpose of increasing vasodilatation, and thus, blood and oxygen supply to the exercising muscle. The present study evaluated the acute effect of L-arginine supplementation on indicators of NO production, nitrite (NO2-) + nitrate (NO3-) (NOx), in healthy subjects.