A 19-year-old college student develops a severe sore throat, cervical lymphadenopathy, and atypical lymphocytes on blood film. A heterophil antibody test is positive.For the above patient with a hematologic abnormality, select the most likely diagnosis.

Heterophil antibodies react against sheep red cells and are not absorbed by the guinea pig kidney. They are positive in patients with infectious mononucleosis. In 90% of cases, liver enzymes are elevated. Examination of the blood film reveals a lymphocytosis with atypical lymphocytes.