Elsa, a 25-year-old teacher who is pregnant, complains of double vision and difficulty speaking after prolonged speaking. She has no past medical history, allergies, or past medication except for those taken during her pregnancy. After a physical examination, Elsa appears tired with drooping eyelids, and her eye movements are limited when looking to the right and left. She also needs to use her hands to stand up after 5 squats. What is the diagnosis?