A 60-year-old man is brought to the emergency department because of new symptoms of double vision and discomfort in his left eye. He reports no other neurologic symptoms. On physical examination, there is ptosis of the left eyelid, the eye is rotated down and out, and the pupil is 3 mm and reactive to light. The right eye is normal, and there are no other focal neurological deficits. Which of the following is the most likely diagnosis?