Henoch-Schonlein purpura. Purple rash seen on the thighs of a man caused by Henoch-Schonlein purpura (also known as anaphylactoid purpura). This disorder causes the inflammation of small blood vessels, causing leakage of blood into the skin, joints, kidneys and intestines. The disease is most common amongst young boys, although its precise cause is unknown. As well as causing a rash on the buttock and limbs, Henoch-Schonlein purpura may cause joint stiffness and blood in the urine and faeces. Most symptoms disappear within a month, although in some cases corticosteroid drugs may be required.