A 16-year-old female developed ptosis 9 days after a cold, without hyperhidrosis or tremors. 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 weakness in orbicularis oculi, masseter, and orbicularis oris muscles. Normal limb muscle strength. No muscle atrophy, reflex abnormalities, or sensory deficits. Mild thyroid enlargement. A specific test transiently resolved all neurological symptoms. Which condition is most likely?