A 16-year-old female developed ptosis 9 days after a cold, without hyperhidrosis or tremor. Examination: Normal BP. Pupils round and equal in size, pupillary light reflex present. Moderate impairment of right eye abduction and adduction, mild impairment of upward and downward gaze. Left eye shows only mild adduction impairment. Mild muscle strength reduction in orbicularis oculi, masseter, and orbicularis oris muscles. Normal limb muscle strength. No muscle atrophy, reflex abnormalities, or sensory deficits. Mild thyroid enlargement. A transient complete resolution of neurological symptoms was observed after a specific test. Which ancillary test is most closely related to confirming this diagnosis?