Which of the following is longest acting cholinesterase inhibitor used for myasthenia gravis?

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.