This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Autonomic nervous system, Subject name: Pharmacology
which of the following drug binds only with the aniotic site of cholinesterase? 
Select and explain. 
A. Physostigmine, B. Neostigmine, C. Edrophonium, D. Pyridostigmine
C: Edrophonium
Answer: C: Edrophonium 
Explanation: Ref:KD Tripathi pharmacology 7th edition (page.no: 105) Edrophonium and Tacrine attach only to the anionic site and don't form a covalent bond with the enzyme. so, edrophonium is a very sho-acting drug. While organophosphates attach only to the esteratic site forming a covalent bond. Carbamates (physostigmine, pyridostigmine, neostigmine) carbamylate the esteratic site of the enzyme