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: Physiology
[{'content': 'Effectiveness of blood brain barrier is by: \nSelect and explain. \nA. Tight endothelial function, B. Microglial cell, C. Thick basement membrane, D. Tight arrangement of astrocyfes', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Tight endothelial function', 'role': 'assistant'}]