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
Neostigmine is not able to cross blood-brain barrier because of its: 
Select and explain. 
A. Primary structure, B. Secondary structrure, C. Tertiary structure, D. Quarternary structure
D: Quarternary structure
Answer: D: Quarternary structure 
Explanation: Quarternary ammonium compounds are water soluble and thus cannot cross the blood-brain barrier. Neostigmine is a quarternary derivative whereas physostigmine is a tertiary amine.