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 is naturally occurring tertiary amine and lipid-soluble reversible anticholinesterases: 
Select and explain. 
A. Physostigmine., B. Neostigmine., C. Pyridostigmine., D. All of the above.
A: Physostigmine.
Answer: A: Physostigmine. 
Explanation: All other reversible anticholinesterases are synthetic quaternary compounds and are lipid insoluble.