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
Effectiveness of blood brain barrier is by: 
Select and explain. 
A. Tight endothelial function, B. Microglial cell, C. Thick basement membrane, D. Tight arrangement of astrocyfes
A: Tight endothelial function
Answer: A: Tight endothelial function 
Explanation: A i.e. Tight endothelial function