This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous, 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: Ans. (A) Neostigmine(Ref: KDT 6th/e p101, 455)Neostigmine is an inhibitor of acetylcholinesterase and thus acts like a cholinergic drug. Therefore, it can produce diarrhea (not constipation).Atropine is an anti-cholinergic drug, thus can cause constipation.Morphine and fentanyl are opioids. These can also result in constipation.