Picture This

Rash, Reaction, or Red Flag?

Many disorders begin with a red patch on the skin. Some diagnoses are easy to determine, based on clinical presentation and history alone. Others are more elusive, possibly leaving the patient with chronic disease and long-term consequences.


1. The patient had just recovered from a sore throat and noticed discrete red nodules, which eventually coalesced into a single large edematous plaque over the right anterior tibia. The deep intradermal and subdermal edema is exquisitely tender to touch, considerably warmer than the surrounding skin, and highly blanchable.

Diagnosis: Erythema nodosum is a reactive form of septal panniculitis with many potential triggers. Notable triggers include Crohn disease flares and use of drugs such as sulfa, gold salts, and oral contraceptives. Several infections have been identified as triggers, including strep, mycoplasma, and campylobacter, as well as deep fungal infections (histoplasmosis, blastomycosis, coccidioidomycosis, and sporotrichosis). More unusual causes include pregnancy and diseases such as sarcoidosis, tuberculosis, Behçet disease, and leukemia/lymphoma.

For more information, see “Painful Lesion Hasn’t Responded to Antibiotics.” Clin Rev. 2015;25(11):10,12.

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