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
[{'content': 'In urea synthesis, the amino acid functioning solely as an enzyme activator: \nSelect and explain. \nA. N-acetyl glutamate, B. Ornithine, C. Citrulline, D. Arginine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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.\nReference: Vasudevan 7th\xa0 ed, pg 205', 'role': 'assistant'}]