A Rare Presentation of Cytomegalovirus Mononucleosis in a Nine-Year-Old Girl: A Case Report.

Epstein-Barr virus (EBV) and cytomegalovirus (CMV) are common viruses from the Herpesviridae family. While EBV is the classic cause of infectious mononucleosis (IMN), characterized by fever, pharyngitis, cervical lymphadenopathy, and atypical lymphocytes, CMV can also cause a similar mononucleosis syndrome, though it is less frequently reported. Mononucleosis caused by both EBV and CMV is more common in preadolescent children or young adults. Here, we present a case of a nine-year-old girl presenting with a two-week history of sore throat, fever, cervical lymphadenopathy, and cough. Additional findings included periorbital swelling and splenomegaly. Peripheral blood smear revealed atypical lymphocytosis, prompting serologic evaluation for EBV and CMV. IgM antibodies for EBV were negative, whereas both IgM and IgG antibodies for CMV tested positive, thus indicating an active infection, which is either a primary (first-time) infection or a reactivation of a previous infection. She recovered with conservative management gradually over eight days. This case highlights that although uncommon, CMV mononucleosis should be considered in patients presenting with fever, sore throat, and cervical lymphadenopathy. Additional findings may include periorbital edema and splenomegaly. Diagnosis relies on atypical lymphocytosis and serologic testing, and management is generally supportive. Antiviral therapy and corticosteroids are reserved for severe disease in immunocompromised patients.