A 27-year-old woman complains of double vision and fatigue at the end of the day. Further history reveals difficulty in chewing food, and some weakness in climbing stairs. She has stopped running because of easy leg fatigue and leg weakness. The symptoms improve with resting. On examination, there is weakness of the eyelids, masticatory muscles, and thigh flexors. Her handgrip decreases with repetitive action. There is no sensory abnormality, and reflexes are normal. Which of the following is the most likely diagnosis?