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
Neostigmine is not able to cross blood brain barrier because of its 
Select and explain. 
A. Primary structure, B. Secondary structure, C. Teritiary structure, D. Quaternary structure
D: Quaternary structure
Answer: D: Quaternary structure 
Explanation: Quaternary ammonium compounds are water soluble and thus cannot cross blood brain barrier neostigmine is a quaternary derivatives where as physostigmine is a teritiary amine Ref-KDT 6/e p101