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