A 23-year-old man complains of intermittent double vision and fatigue. The symptoms are worse at the end of the day, and he has stopped working as a courier because of easy leg fatigue and weakness. He finds that the symptoms improve with resting. On physical examination, there is weakness of the eyelids, masticatory muscles, and thigh flexors. Having him look up at the ceiling for a long time brings on his double vision. There is no sensory abnormality, and reflexes are normal. Which of the following treatments is contraindicated in this patient?