Which virus is most likely responsible for the acute infectious illness described in the 11-year-old given EBV IgM positivity?

• A. CMV
• B. EBV
• C. Adenovirus
• D. Parvovirus

Answer: (B)

Epstein-Barr virus infection is indicated by IgM antibodies to EBV, which appear early in an acute EBV infection. In an 11-year-old with an acute illness, this serology points to infectious mononucleosis caused by EBV as the most likely cause. EBV is the classic etiology of mono in school-age children and adolescents, presenting with fever, sore throat, and lymphadenopathy, often with fatigue and sometimes hepatosplenomegaly.

Why this fits best: the presence of EBV-specific IgM shows a current EBV infection, which directly identifies the virus responsible. While CMV can cause mono-like illness, it would not produce EBV IgM. Adenovirus or Parvovirus infections can cause febrile illnesses or rash but would not account for EBV IgM positivity.