This 23-year-old woman presented with a four-week history of balance impairment, short-term memory impairment, and irritability. Six weeks prior to her evaluation, she developed Epstein–Barr virus (EBV)-associated infectious mononucleosis, presenting with severe cervical lymphadenopathy and tonsillitis. She was treated with amoxicillin-clavulanate and subsequently prednisone (60 mg a day for 3 days, followed by a 10-day downward titration and discontinuation). Two weeks later, she noted imbalance and a tendency to fall toward the right, along with bilateral loss of hand dexterity. Except for occasional word-finding difficulties, she denied language changes. About the same time, her family noticed she would forget what she had recently been told, particularly lists, and had difficulty driving, damaging three car wheels. Both she and her family reported that her mood was uncharacteristically labile, and she would become easily upset over trivial issues. All of her symptoms peaked at two weeks from onset of imbalance and, since then, had been slowly improving over the subsequent two weeks.