Erythema nodosum.

Erythema nodosum represents an inflammatory process in the septa between the subcutaneous fat lobules. The hallmark lesions are exquisitely tender, erythematous nodules that are distributed symmetrically on the extensor surfaces of the lower extremities. Erythema nodosum is considered an immunologic response, and a wide variety of clinically important antigens have been implicated. It may serve as a cutaneous marker for systemic diseases, such as tuberculosis, sarcoidosis, inflammatory bowel disease and lymphoma. The disease is diagnosed by deep elliptical biopsy and pathologic evaluation. The natural history is resolution with treatment of the underlying disorder or spontaneous regression of idiopathic disease.