Shortest acting anticholinesterase is -

Ans. is 'a' i.e., Edrophonium o Edrophonium is the shortest acting anti-ChE.o Why is it so, lets see :o When carbamates (other than edrophonium) and organophophates react with cholinesterase, they form covalent bond at esteratic site, which is considerably resistant to hydrolysis,o In contrast, edrophonium binds electrostatically and by hydrogen bonds at anionic site (not at esteratic site) of cholinesterase. The enzyme inhibitor complex does not involve a covalent bond and is correspondingly short lived (2-10 minutes).o So edrophonium is also called as noncovalent inhibitor.