A 26-year-old man is evaluated after a motorcycle collision. CT head is negative for hemorrhage, but MRI brainstem shows a small focal lesion in the dorsal tegmentum at the level of the inferior colliculus, immediately adjacent to the cerebral aqueduct. On exam, he has vertical diplopia that worsens when looking down and in (e.g., while reading or walking downstairs). The affected eye is slightly elevated at rest. Which cranial nerve is most likely involved, and what is unique about its course as it exits the brainstem?