A 25-year-old woman presents to the clinic complaining of symptoms of fatigue and weakness. The symptoms are worse near the end of the day and she finds it difficult to continue working as a bank teller. She has no prior health issues and is not taking any medications. Her sleep patterns are unchanged and she feels refreshed in the morning.On examination, there is diplopia and ptosis noted in the extraocular muscle with prolonged upward gaze. In addition, repetitive muscle exercises of her hands with open and close maneuvers causes hand grip weakness. The remaining neurological examination is normal. Which of the following is the most likely diagnosis?