Maculopapular Rash in a 19-Year-Old on Antibiotics for Sore Throat

A 19-year-old female university student presented to the urgent care clinic with a widespread, itchy rash. She reported a sore throat for the past 5 days. Three days ago, her general practitioner prescribed amoxicillin 500 mg three times daily for a suspected bacterial pharyngitis. She also takes a combined oral contraceptive pill (ethinylestradiol 30 µg/levonorgestrel 150 µg) daily, and she took two doses of ibuprofen 400 mg for headache on the day before presentation. On the afternoon of the second day of amoxicillin treatment, she noticed a pink, flat rash on her trunk, which spread to her arms and legs overnight. It was more prominent on pressure areas but also involved her face. She had no mucosal involvement, no fever, and no breathing difficulties. She stopped taking amoxicillin that morning, and the rash had already begun to fade. She had taken amoxicillin previously three years earlier without any problems. Her medical history included mild asthma and a previous episode of hay fever. She had no known drug allergies. On examination, she had a temperature of 37.8 °C, pulse 96 beats per minute, blood pressure 118/75 mmHg, and respiratory rate 16 breaths per minute. There was a widespread maculopapular rash, non-blanching on diascopy in some areas, with distinct erythematous macules and papules on the trunk, arms, and legs. No vesicles, bullae, or target lesions were present. There was no conjunctival or oral involvement. She had tender posterior cervical and axillary lymphadenopathy, and the liver and spleen were not palpable. Laboratory investigations revealed a white cell count of 12.5 × 10^9/L with 60% lymphocytes and 15% atypical lymphocytes. A heterophile antibody test was positive. Liver function tests showed a mild elevation of aspartate aminotransferase at 55 U/L and alanine aminotransferase at 70 U/L. The rash improved significantly with cool compresses, and she was given oral antihistamines.