Erythema nodosum.

Erythema nodosum is the most frequent clinicopathologic variant of panniculitis. The process is a cutaneous reaction that may be associated with a wide variety of disorders, including infections, sarcoidosis, rheumatologic diseases, inflammatory bowel diseases, medications, autoimmune disorders, pregnancy, and malignancies. Histopathologically, erythema nodosum is the stereotypical example of a mostly septal panniculitis with no vasculitis. The composition of the inflammatory infiltrate in the septa varies with age of the lesion. Treatment of erythema nodosum should be directed to the underlying associated condition, if identified.