A febrile 23-year-old college student presents with fatigue and difficulty swallowing. Physical exam reveals exudative tonsillitis, palatal petechiae, cervical lymphadenopathy, and tender hepatosplenomegaly. A complete blood count reveals mild anemia, lymphocytosis with about 30% of the lymphocytes exhibiting atypical features, and a mild thrombocytopenia. Coombs' test is positive. Which of the following is the most likely complication of this syndrome?