Central black necrosis with red, tender surrounding skin. It is due to infection lodging in the capillaries and causing infection, thrombosis and infarction.
Pseudomonas leads the list, but other bacteria, yeasts and moulds can cause it.
Depends on the organism.
If you see it, biopsy for culture and gram stain. Right away.
ICD9 Codes (Soon to be supplanted by ICD10)
Ecthyma 686.8 gangrenosum 686.09.