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: Biochemistry
In urea synthesis, the amino acid functioning solely as an enzyme activator: 
Select and explain. 
A. N-acetyl glutamate, B. Ornithine, C. Citrulline, D. Arginine
A: N-acetyl glutamate
Answer: A: N-acetyl glutamate 
Explanation: The major regulatory step is catalyzed by CPS-I where the positive effector is N-acetyl glutamate (NAG). It is formed from glutamate and acetyl CoA. Arginine is an activator of NAG synthase.
Reference: Vasudevan 7th  ed, pg 205