ABIM IM Practice Test 12

A 41-year-old woman is evaluated for 2 months of double vision and drooping of the left eyelid that worsen in the evening, and recent difficulty chewing tough foods toward the end of meals. She has no limb weakness or dyspnea. She takes no medications. Vital signs are normal. Examination shows left ptosis that worsens after 60 seconds of sustained upward gaze and improves after an ice pack is applied, and variable diplopia on lateral gaze. Pupils are equal and reactive. Limb strength, reflexes and sensation are normal, and forced vital capacity is normal. Serum acetylcholine receptor binding antibodies are positive at a high titer. Which of the following is the most appropriate next diagnostic test?