Q. A 35-year-old woman presents with a chief complaint of feeling "wiped out" at the end of the day. She has had generalized weakness for the past several months, which is worst late in the day but that she feels refreshed after taking a nap and feels her best first thing in the morning. She has difficulty raising her arms up to reach the cabinets and to put on a sweater and has noticed some double vision, especially late in the day. Exam demonstrates eyelid droop (ptosis) and shoulder weakness and trouble getting up from a chair. Which of the following is a commonly used diagnostic test for this condition?