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 a natural source? 
Select and explain. 
A. Physostigmine, B. Neostigmine, C. Pyridostigmine, D. Tacrine
A: Physostigmine
Answer: A: Physostigmine 
Explanation: Ref: KD Tripathi pharmacology 7th edition (page.no: 108) Physostigmine is a Natural alkaloid from Physostigma venenosum (Calabar bean) and All others are synthetic Physostigmine is a highly toxic parasympathomimetic alkaloid, specifically, a reversible cholinesterase inhibitor