An 11-year-old with fever and sore throat has EBV IgM positive, CMV IgM negative, heterophile test negative. Which diagnosis is most likely?

• A. Acute CMV infection
• B. Acute EBV infection
• C. Chronic CMV infection
• D. Chronic EBV infection

Answer: (B)

Acute EBV infection is indicated by the presence of EBV-specific IgM antibodies, which appear early in the immune response to a new EBV infection and signal current activity. The absence of CMV IgM makes acute CMV unlikely. The heterophile (Monospot) test can be negative in EBV infections, especially in some patients or early in illness, so a negative result does not exclude EBV when EBV IgM is positive. Chronic EBV infection would not show EBV IgM, as IgM wanes and IgG/EBNA patterns suggest past infection. Putting these serology patterns together points to acute EBV infection as the most likely diagnosis.