This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Parasympathetic System & Glaucoma, Subject name: Pharmacology
A drug X reverses the actions produced by parasympathetic nervous system in the body but is not able to reverse the actions of exogenously administered acetylcholine in in-vitro experiments. The mechanism of drug X can be 
Select and explain. 
A. Acetylcholinesterase inhibitor, B. Muscarinic receptor antagonist, C. Muscarinic receptor agonsit, D. Inhibitor of release of Ach from synaptic vesicles
D: Inhibitor of release of Ach from synaptic vesicles
Answer: D: Inhibitor of release of Ach from synaptic vesicles 
Explanation: Muscarinic receptor antagonist and agonists acts on receptor. Acetylcholine esterase inhibitor inhibits acetylcholinesterase enzyme leading to increase in the levels of acetylcholine. Inhibitor of release of acetylcholine will inhibit the release of endogenous acetylcholine from synaptic vesicles. This will not interfere with exogenously administered acetylcholine.