A 16 year old boy presents with rash on his buttocks and extensor surface following a sore throat. What is the most probable diagnosis?

The key is E. HSP. [Henoch-Schönlein purpura (HSP) is an acute immunoglobulin A (IgA)–mediated

disorder characterized by a generalized vasculitis involving the small vessels of the skin, the

gastrointestinal (GI) tract, the kidneys, the joints, and, rarely, the lungs and the central nervous system

(CNS). It is the most frequent vasculitis in childhood. In HSP rash typically found in buttocks, legs and

feets and may also appear on the arms, face and trunk. HSP usually follow a sorethroat].