A 7-year-old boy has cramping abdominal pain and the rash shown mainly on the back of his legs and buttocks as well as on the extensor surfaces of his forearms. Laboratory analysis reveals proteinuria and micro-hematuria. In addition to his rash and abdominal pain, what other finding is he likely to have?

Henoch-Schonlein purpura (HSP), or anaphylactoid purpura, is a systemic IgA-mediated vasculitis. The rash of anaphylactoid purpura appears much like that in the picture of this child's buttocks and most often involves extensor surfaces of the extremities; the face, soles, palms, and trunk are less often affected. Other significant symptoms include edema, arthralgia or arthritis, colicky abdominal pain with GI bleeding, acute scrotal pain, and renal abnormalities ranging from microscopic hematuria to acute renal failure. As HSP is a systemic vasculitis, any organ system can be affected. The prognosis, however, is excellent, with only a small percentage of children going on to end-stage renal failure. Seizures are an uncommon complication of CNS involvement; more commonly, patients will complain of headache and will have behavioral changes. Unilateral lymphadenopathy and nonpurulent conjunctivitis are not typical of HSP (but are criteria for Kawasaki disease, another pediatric vasculitis).