A 28-day-old, previously healthy, full-term girl presents to the emergency department after 2 days of irritability and 1 day of a rash. She has had no fevers, hypothermia, cough, congestion, increased work of breathing, easy bleeding or bruising, jaundice, or other symptoms. She was born via a scheduled repeat cesarean delivery at 38 weeks’ gestation after an uncomplicated pregnancy. Her mother received adequate prenatal care and had no serologic evidence of prenatal infections. After 3 days in the well-infant nursery, the child was discharged. No one at home is currently ill; however, the patient’s 8-year-old sister had a “cold sore” ∼2 weeks before admission.

On approximately day 10 after birth, the patient’s mother had an outbreak of painful “itchy blisters” that started on her lower back, then spread to the inguinal region and abdomen, appearing most prominently over her cesarean delivery scar. She does not remember whether the rash was...

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