Over the course of the last half a year, a 22-year old patient has been complaining of weakness in the afternoon and after physical exertion and double vision in her eyes. In the morning she feels satisfactory. Objectively, she presents with ptosis of the eyelids on both sides, divergent strabismus, hypomimia, and nasal voice. She has the full range of active movements in her limbs. Her skeletal muscle strength is reduced to 4 points. What is the most likely diagnosis in this case?