The patient presented with a sore throat and was diagnosed by the original physician as suffering from strep throat. He was prescribed Amoxicillin, a penicillin antibiotic.

Hypersensitivity reactions to Amoxicillin have been reported in about 10% of patients and side effects of Amoxicillin treatment include some that are serious such as:

• anaphylaxis
• erythematous macropapular rash
• erythema multiforme
• hypotension
• exfoliative dermatitis
• toxic epidermal necrosis
• angioedema (1)
• Stevens-Johnson syndrome
• acute exanthematous pustulosis

This is a case of allergy to penicillin antibiotics. When tested at the hospital after the onset of rash, the patient was negative for strep throat, a bacterial infection. He, however, tested positive for infectious mononucleosis, a viral infection caused by Epstein-Barr Virus.

Treatment of acute infectious mononucleosis patients with Amoxicillin results in severe rash in about a third of patients.

Infectious mononucleosis should be suspected in patients under 30 who present with a sore throat, fatigue, palatal petechiae or adenopathy. Because of the high incidence of major side effects in these patients treated with Amoxicliin or ampicillin, a rapid strep test should be carried out and they should be treated with anti-bacterial antibiotics only if the strep test is positive. If the patient is strep positive, Amoxicillin and ampicillin should not be used.

(1) A vascular reaction involving the dermis or subcutaneous or submucal tissues that involves local edema resulting from dilatation and increased permeability of the capillaries. It is characterized by development of large wheals.