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 anticholinesterase is derived from natural source 
Select and explain. 
A. Physostigmine, B. Pyridostigmine, C. Neostigmine, D. Tacrine
A: Physostigmine
Answer: A: Physostigmine 
Explanation: Refer kDT 6/e p 102 Physostigmine is a highly toxic parasympathomimetic alkaloid, specifically, a reversible cholinesterase inhibitor. It occurs naturally in the Calabar bean and the Manchineel tree. The chemical was synthesized for the first time in 1935 by Percy Lavon Julian and Josef Pikl.