DOI: https://doi.org/10.36347/sjmcr.2026.v14i07.020

Pages: 1691-1695

Background: Acute infectious mononucleosis (IM) caused by Epstein–Barr virus (EBV) is a common cause of pharyngotonsillitis in adolescents and young adults and is clinically indistinguishable from streptococcal sore throat at the bedside. The development of a maculopapular eruption after exposure to an aminopenicillin is a classical, though now less frequent, clue to underlying EBV. Case presentation: A 24-year-old woman presented with fever, cervical lymphadenopathy and exudative tonsillitis on a background of incidental lymphocytosis. A rapid Group A Streptococcus (GAS) antigen test was positive and she was treated sequentially with a macrolide and then amoxicillin. Three days into amoxicillin she developed a widespread, pruritic maculopapular rash without angioedema. The aminopenicillin was stopped and EBV serology subsequently returned viral capsid antigen (VCA) IgM-positive, confirming acute EBV infection. The rash settled with antihistamine and a short course of corticosteroid. Conclusion: This case illustrates genuine diagnostic overlap between streptococcal and EBV pharyngitis, the limitations of a positive GAS antigen test against a background of asymptomatic carriage, and the evolving understanding of the aminopenicillin–EBV exanthem. It reinforces the value of considering EBV before prescribing an aminopenicillin for sore throat in this age group, and of labelling the reaction accurately rather than as a fixed penicillin allergy.