Henoch-Schonlein purpura:

It is also referred to as anaphylactoid purpura, is a distinct systemic vasculitis syndrome that is characterized by palpable purpura (most commonly distributed over the buttocks and lower extremities), arthralgias, gastrointestinal signs and symptoms, and glomerulonephritis. It is a small vessel vasculitis. Most patients recover completely, and do not require therapy.