Erythema nodosum

An acute, self-limited nodular septal panniculitis characterized by tender, erythematous subcutaneous nodules, typically on the extensor surfaces of the lower legs, representing a hypersensitivity reaction to diverse triggers. Possible causes are infection (strep tonsillitis, tuberculosis, viral hepatitis, salmonella, campylobacter), inflammation (sarcoidosis, IBD), pregnancy, or medications (penicillin, sulfa drugs, birth control pills, TNF-α inhibitors).