This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pathology
Which of the following amino acid in a protein is involved in urea cycle and is required for synthesis of Nitric Oxide? 
Select and explain. 
A. Arginine, B. Histidine, C. Tryptophan, D. Lysine
A: Arginine
Answer: A: Arginine 
Explanation: .