A 62-year-old man is brought to the ED after suddenly developing double vision and difficulty walking. Examination shows the right eye is deviated "down and out" with marked right eyelid ptosis. The right pupil is dilated and minimally reactive to light. Motor strength is 5/5 in all extremities, but he has prominent intention tremor and dysmetria of the left upper extremity. MRI shows a small infarct in the rostral midbrain affecting a structure anterolateral to the cerebral aqueduct. Which of the following nuclei is most likely involved, explaining the pupillary finding?