A Caucasian gentleman with end stage renal disease on hemodialysis presents with the wounds shown above. He recently had minor trauma to this area but the skin breakdown and lack of healing is out of proportion to the trauma. Note the ischemic necrosis of dermis and subcutaneous fat. Radiologic studies show something odd - his vessels have become radio-opaque on X-ray and CT.
Challenge: What's the diagnosis?
Image shown under fair use.