A 25-year-old woman presents with symptoms of fatigue and weakness. On examination, there is diplopia, ptosis, weakness, and fatigability of muscles on repeated use. Which of the following is the most likely diagnosis?

The distribution of muscle weakness is characteristic with early involvement of the cranial nerves, especially the lids and extraocular muscles. Women are more frequently affected than men (3:2 ratio), and the age for peak incidence in women is in the third or fourth decade.