<prompt> A 20-year-old female presents with a 5-day history of fatigue, lowgrade fever, and sore throat. Physical examination reveals bilateral enlarged, tender cervical lymph nodes, an exudative tonsillitis, and an enlarged spleen. A complete blood cell count reveals the hemoglobin and platelet counts to be within normal limits. The total white blood cell count is increased to 9200 cells per uL. Examination of the peripheral blood reveals the presence of atypical mononuclear cells with abundant cytoplasm. These cells have peripheral condensation of the cytoplasm, which gives them a ballerina ski appearance. Which one of the listed findings is most likely to be present in this individual?Aggregates of mononuclear cells with cytoplasmic Birbeck granules in the liverElevated levels of d-ALA in the urineGroup A streptococcus on culture of the tonsillar exudateHeterophil antibodies in the serum  <response>: Infectious mononucleosis is a benign lymphoproliferative disorder caused by infection with the Epstein-Barr virus EBV. It typically occurs in young adults and presents with systemic symptoms, lymphadenopathy, and pharyngitis. Hepatosplenomegaly may be present. Peripheral blood shows an absolute lymphocytosis, and many lymphocytes are atypical with irregular nuclei and abundant basophilic vacuolated cytoplasm. These represent CD8 T killer cells induced by EBV-transformed B lymphocytes. These atypical lymphocytes are usually adequate for diagnosis, along with a positive heterophil or monospot test increased sheep red cell agglutinin. Administration of ampicillin for a mistaken diagnosis of streptococcal pharyngitis results in a rash in many patients. Reference: Robbins Cotran Pathologic Basis of Disease, 9edition. 