Which cranial nerve is known to exit the brainstem dorsally?

• A. CN III
• B. CN IV
• C. CN V
• D. CN VI

Answer: (B)

The nerve that exits the brainstem dorsally is the one that innervates the superior oblique muscle. It has a unique path: it originates in the dorsal midbrain and emerges from the posterior surface of the brainstem, just below the inferior colliculus, then curves around the brainstem to travel anteriorly. This dorsal exit is a distinctive feature that sets it apart from the others, which all exit from ventral or ventrolateral surfaces of the brainstem. The oculomotor nerve leaves ventrally from the midbrain, the trigeminal nerve emerges laterally from the pons, and the abducens exits ventrally at the pontomedullary junction. Because only this nerve exits dorsally, it is identified by that unique exit point and its role in moving the eye via the superior oblique muscle.