<prompt> Which of the following is longest acting cholinesterase inhibitor used for myasthenia gravis?PyridostigmineEdrophoniumNeostigminePhysostigmine  <response>: Both neostigmine and pyridostigmine are used in myasthenia gravis. Pyridostigmine is longer acting than neostigmine. These drugs act by inhibiting acetylcholinesterase. Edrophonium is very sho acting anti-cholinesterase and is preferred drug for diagnosis of myasthenia gravis. Physostigmine is a lipid soluble anticholinesterase used in glaucoma and atropine poisoning. 