A 25 years old woman presents with recurrent slurring of speech that worsens when she continues to talk. She has trouble swallowing, which deteriorates when she continues to eat, and has double vision that gets worse when sewing, reading, or watching TV. She reports that her head is heavy and hard to hold up. Her symptoms have progressively deteriorated over the past 6 months. She has intermittent weakness in her legs and arms, and is fearful of falling due to her legs giving out. She reports a feeling of generalized fatigue and is occasionally short of breath. What is the most likely diagnosis?