In Order to Determine If a New Drug Will Cross

In order to determine if a new drug will cross the blood-brain barrier, a research scientist tags it with a dye and then injects it into the vascular system of a rat. Examination of the sectioned brain indicates that the dye was not detected in the brain tissue. What is the most likely explanation of this observation?

A) Tight junctions between capillary endothelium
B) Fenestrated junctions between capillary endothelium
C) Sinusoidal junctions between capillary endothelium
D) Tight junctions between venous endothelium
E) Fenestrated junctions between venous endothelium