Infectious mononucleosis with ampicillin rash and a positive rapid streptococcal antigen test: a case report

Infectious mononucleosis (IM), caused by Epstein-Barr virus, often mimics group A streptococcal (GAS) pharyngitis and may result in an ampicillin rash when aminopenicillins are administered. We report a 22-year-old woman with fever, sore throat, lymphadenopathy, and a generalized erythematous rash after a 7-day course of amoxicillin-clavulanate. Although a rapid streptococcal antigen test was positive, non-tender lymphadenopathy and absence of left shift in the white blood cell count suggested viral infection, and the result was considered consistent with a carrier state rather than true GAS infection. No additional antibiotics were given ; corticosteroid and antihistamine treatment led to improvement. Serology confirmed acute Epstein-Barr virus infection, and throat culture revealed only alpha-hemolytic streptococci. This case underscores the diagnostic overlap between IM and streptococcal pharyngitis, the need for caution in interpreting rapid antigen tests, and the importance of avoiding unnecessary antibiotics in suspected viral illness.