This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Introduction to Central Nervous System, Subject name: Physiology
The perivascular space (Virchow-Robin space) in the brain is formed between the wall of small penetrating vessels and which structure? 
Select and explain. 
A. Dura mater, B. Arachnoid membrane, C. Pia mater, D. Choroid plexus
C: Pia mater
Answer: C: Pia mater 
Explanation: The perivascular space (Virchow- Robin space) is formed between the outer wall of small vessels penetrating into the brain and the pia mater, which lines the outer surface of the brain and is only loosely attached to the brain. It is an extension of the subarachnoid space which separates penetrating vessels from brain tissue.