A 62-year-old man is brought to the emergency department after sudden onset of double vision and drooping of the right eyelid. Examination shows the right eye is deviated “down and out,” with marked ptosis and a dilated right pupil that is poorly reactive to light. Strength and sensation in the limbs are intact. MRI reveals a small infarct in the rostral midbrain involving a region just anterolateral to the cerebral aqueduct and anterior to a parasympathetic nucleus.

Which of the following structures is most directly damaged?