Derm Dx: Small red spots on the lower extremities with knee pain

A 7-year old boy presents with a rash on his lower extremities and knee pain. The rash started two days prior as small red spots and progressed to raised maculopapular areas mixed with uticarial wheals.

The boy was diagnosed and treated for streptococcal pharyngitis two weeks prior to rash onset.

Pain in the knee joint started one day after the rash’s appearance ...