Why Is It So Difficult for Many Toxins and Poisons

Multiple Choice

Why is it so difficult for many toxins and poisons to penetrate the blood-brain barrier?

A) Because the human immune system has killer t-cells that attack such foreign agents.
B) Because those agents are too large to get out of the human blood stream.
C) Because the presence of such invaders triggers a fever, which kills them.
D) Because those agents are not soluble in fat.