Case 6: Rash and Fever in a 9-month-old Girl

Presentation

A 9-month-old girl who was born at term presents with rash for 11 days and low-grade fevers (100.4°F–101.3°F [38.0°C–38.5°C]) for 5 days. When febrile, she is fussy and her rash seems more prominent, but otherwise she has been at her baseline and playful. She has had no cough, rhinorrhea, congestion, vomiting, diarrhea, conjunctivitis, mucosal changes, change in appetite, evidence of pain or swelling of her extremities, or weight loss. Her parents deny new exposures or recent travel. She has received no medications and is up to date with her immunizations.

Physical examination reveals a well-developed, well-appearing infant. She is febrile (100.6°F [38.1°C]) and tachycardic (heart rate, 170 beats/min), but the rest of her vital signs are normal. Her rash is noted to be raised, erythematous, and blanching, in some areas coalescing to cover large areas of skin (Fig). It is present throughout her trunk and extremities, while sparing her mucosa, palms, and soles. She is not noted to have swelling or pain with manipulation of her joints. There is no lymphadenopathy or hepatosplenomegaly.

Initial laboratory results include a white blood cell count of 25,800/μL (25.8×10⁹/L) with a normal differential count, mild normocytic anemia (hemoglobin level of 10.3 g/dL [103 g/L]), and a platelet count of 495×10^{3 …}