A skin ulcer is an open sore in the skin. Pyoderma gangrenosum is an uncommon form of skin ulcers. It usually occurs on the lower legs, but can occur anywhere on the skin.

Side View of Skin Ulcer
Skin Ulcer small
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Pyoderma gangrenosum is thought to be caused by a problem with the immune system. The immune system finds and attacks foreign items in the body like viruses. Sometimes the immune system attacks the body's own tissue. In this case, the immune system attacks an area of the skin.