Even though it is in the midbrain, it is unlikely that a PCA occlusion would injure CN IV.Remember, there are three peculiar things about CN IV, the trochlear nerve. It is the only cranial nerve which exits the brainstem dorsally (seen here exiting from the dorsal midbrain, just caudal to the colliculi). It is the only cranial nerve that crosses sides, and, unlike the others, is not prone to injury from vascular occlusions. What does cause trochlear palsies are traumatic injuries and congenital abnormalities. Consider another nerve or return to the case to try again.