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: Pharmacology
[{'content': 'Constipation is caused by all of the following drugs EXCEPT : \nSelect and explain. \nA. Neostigmine, B. Atropine, C. Morphine, D. Fentanyl', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Neostigmine is an inhibitor of acetylcholinesterase and thus acts like a cholinergic drug. Therefore, it can produce diarrhea (not constipation).\nAtropine is an anticholinergic drug, thus can cause constipation.\nMorphine and fentanyl are opioids. These can also result in constipation.', 'role': 'assistant'}]