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
Blood brain barrier is absent in all of the following areas except 
Select and explain. 
A. Subfornical region, B. Habenuclear trigone, C. Area posterma, D. Neurohypophysis
B: Habenuclear trigone
Answer: B: Habenuclear trigone 
Explanation: None