Erythema nodosum.

Erythema nodosum (EN) is the most common form of panniculitis. It is characterized by the presence of rounded or oval, slightly raised, nonulcerative painful red nodules in the skin and subcutaneous fatty tissue, 1 to 6 cm in diameter, sometimes coalescing. They tend to be symmetrical in distribution and are usually located bilaterally on the lower extremities, particularly on the anterior tibial surface, although they may also involve the ankles, the lower parts of the thighs, and the forearms.