Boy with Leg Rash, Knee Pain, and Abdominal Pain

This photograph was taken of a 6-year-old boy who presented with a rash on his legs. Two weeks earlier, he had an upper respiratory infection. His father said that he had 2 weeks of fever and headache before the rash began. The boy also reported knee pain and diffuse abdominal pain. On physical examination, purpuric papules were apparent on his lower extremities.

View the image taken (Figure 1) and consider what your diagnosis would be.