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
[{'content': 'Neostigmine is not able to cross blood-brain barrier because of its: \nSelect and explain. \nA. Primary structure, B. Secondary structrure, C. Tertiary structure, D. Quarternary structure', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 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.', 'role': 'assistant'}]