Visual Diagnosis: Vesicular Rash in a Neonate

Presentation

A 3-day-old girl presents to the emergency department with a vesicular rash on her left leg. She was born at 39 weeks' gestation via an uncomplicated vaginal delivery and went home after 24 hours. No skin findings or any other physical abnormalities were noted at birth. Her mother received appropriate prenatal care and was started on valacyclovir prophylaxis at 36 weeks' gestation after testing positive for herpes simplex virus (HSV) serologies. The infant’s mother has no history of genital or oral ulcers, and no lesions were noted at the time of delivery. The mother took no other medications during pregnancy, and she reports no history of alcohol, tobacco, or illicit drug use. The infant’s rash developed on the morning of presentation. She has otherwise been feeding well and has had no fevers. There has been no exposure to any plants, animals, or lotions.

On physical examination, she appears well and is afebrile, with vital signs within age-appropriate limits. Growth parameters include a weight of 3,200 g (38th percentile), length of 52 cm (84th percentile), and head circumference of 34 cm (45th percentile). Skin examination is positive for crusted vesicular lesions arranged in a linear pattern on the posterior left leg, extending from the gluteal fold to the Achilles tendon (Figs 1 and 2). The remainder of her physical examination findings are normal.

A full sepsis evaluation is performed. Laboratory tests reveal a normal …