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
Which of the following is longest acting cholinesterase inhibitor used for myasthenia gravis? 
Select and explain. 
A. Pyridostigmine, B. Edrophonium, C. Neostigmine, D. Physostigmine
A: Pyridostigmine
Answer: A: Pyridostigmine 
Explanation: 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.