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
Constipation is caused by all of the following drugs EXCEPT : 
Select and explain. 
A. Neostigmine, B. Atropine, C. Morphine, D. Fentanyl
A: Neostigmine
Answer: A: Neostigmine 
Explanation: Neostigmine is an inhibitor of acetylcholinesterase and thus acts like a cholinergic drug. Therefore, it can produce diarrhea (not constipation).
Atropine is an anticholinergic drug, thus can cause constipation.
Morphine and fentanyl are opioids. These can also result in constipation.