A boy with purpura on the legs

A boy in his mid teens presented to the dermatology department with a one week history of non-pruritic rash on his legs, and abdominal and bilateral knee pain. On examination, palpable purple papules were present on both legs, extending from the ankles to the thighs (fig 1). Other examination findings were unremarkable. The boy reported no history of upper respiratory tract infection in the four weeks before symptom onset. Table 1 shows the results of relevant blood and urine investigations.

Several conditions were considered unlikely: meningococcal sepsis, because of the absence of fever, neurological symptoms, and biomarkers of bacterial infection; thrombocytopenia, because the platelet count was within normal range; and coagulopathy, …