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
[{'content': 'Which of the following anticholinesterase is derived from a natural source? \nSelect and explain. \nA. Physostigmine, B. Neostigmine, C. Pyridostigmine, D. Tacrine', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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', 'role': 'assistant'}]