This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Which of the following anticholinesterase is derived from natural source ? 
Select and explain. 
A. Physostigmine, B. Neostigmine, C. Pyridostigmine, D. Tacrine
A: Physostigmine
Answer: A: Physostigmine 
Explanation: None