This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: A.N.S., 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: Ans. (D) Quarternary structure(Ref: KDT 8th/e p116)Quarternary ammonium compounds are water soluble and thus cannot cross blood brain barrier. Neostigmine is a quarternary derivative whereas physostigmine is a tertiary amine.', 'role': 'assistant'}]