Rapidly Enlarging “Bruise” on the Back of an Infant

A previously healthy infant younger than 3 months presented with sudden development of a “bruise” on her back. The mother denied any preceding trauma, and the baby was otherwise well. Physical examination revealed a nonblanching, blotchy, violaceous patch extending from the right flank to the lower back (Figure 1). Histologic and immunohistochemical findings are illustrated in Figure 2. Initial serum studies revealed a thrombocytopenia (platelet count as low as 8 × 10³/μL), and the patient was subsequently admitted. Within 2 days, the lesion had become progressively more indurated and purpuric, and the patient developed worsening thrombocytopenia and consumptive coagulopathy despite multiple platelet transfusions. Magnetic resonance imaging was performed, which showed a large retroperitoneal mass with extensive infiltration into the right paraspinous muscles and anterior displacement of the right kidney and aorta. A deep incisional biopsy was performed.