A 62-year-old man presents to the emergency department 1 hour after sudden-onset vertigo and unsteady gait. He also reports that his left eyelid feels “droopy.” On exam, he has ptosis and miosis of the left eye. Motor strength is 5/5 in all extremities. Vibration and joint-position sense are intact bilaterally. Pain and temperature sensation is decreased over the right arm and right leg. A brainstem stroke is suspected.

Based on the Rules of 4 (brainstem-ventral view), which structure is most likely affected by this lesion?