Learn more about the 9 most common food allergies. A rash resembling large purple patches or dark bruises can indicate a serious condition, such as vasculitis and deep vein thrombosis. A medical ...

Henoch-Schonlein purpura (HSP) produces a distinctive rash on a child's arms, legs, and buttocks that looks like bruises (purpura). It is a type of vasculitis, or inflammation of blood vessels.