Review Question - QID 213707

A 37-year-old woman presents to her primary care physician with a 1 month history of increasing fatigue and weakness. She says that she feels well in the morning but becomes increasingly fatigued during the course of the day. She thought that this was simply because she was not sleeping enough, but she began worry when she realized that she was developing double vision while reading a book. On presentation she is found to have left-sided ptosis as well as diffuse muscle weakness. Which of the following tests would be effective in diagnosing her condition?