Review Question - QID 214379

A 56-year-old man presents to the emergency department with eyelid drooping and a hoarse voice. His symptoms began a few hours prior to presentation and have progressively worsened. He denies any unilateral motor weakness or sensory deficits but does endorse experiencing some shortness of breath. He occasionally experiences double vision while reading a book or climbing the stairs which typically occurs towards the end of the day, but did not seek medical attention since his symptoms improved after taking a short nap. Medical history is significant for hypertension and hyperlipidemia. He has smoked 1 pack of cigarettes daily for the past 30 years. On physical examination, the patient has ptosis of the right eyelid. His pupillary reflexes are present bilaterally to light, and his extraocular movements are intact. The rest of his neurological examination is unremarkable. Which of the following is most likely the cause of this patient's symptoms?