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': 'Not a second messenger: \nSelect and explain. \nA. cAMP, B. cGMP, C. Ca, D. None', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]