42-year-old woman comes to clinic due to double vision and difficulty speaking and breathing. O/E there is bilateral ptosis, which is worse on the right. An ice pack is applied to her right eyelid, and the ptosis improves after several minutes. She reports having had a few episodes of double vision during the last 2 months, usually at the end of a long day. Which of the following is the most likely cause of this patient’s symptoms?