This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
Which of the following aminoacid in a protein is involved in urea cycle and is required for synthesis of NO? 
Select and explain. 
A. Arginine, B. Histidine, C. Tryptophan, D. Lysine
A: Arginine
Answer: A: Arginine 
Explanation: Ans. A. ArginineFallowing are required for Nitric Oxide (NO) synthesis: Arginine, NO synthase, Ca++_and NADPH.Urea cycle: