DefinitionErythema nodosum is an inflammatory disorder that is characterized by tender, red nodules under the skin.Causes, incidence, and risk factorsIn about half of cases, the exact cause of erythema nodosum is unknown. Some cases may be associated with the following:Infections. Some of the more common ones are: Streptococcus (most common)Cat scratch diseaseChlamydiaCoccidioidomycosisHepatitis BHistoplasmosisLeptospirosisMononucleosis(EBV)MycobacteriaMycoplasmaPsittacosisSyphilisTuberculosisTularemiaYersiniaPregnancySensitivity to certain medications, including: Amoxicillin and other penicillinsSulfonamidesSulfonesOral contraceptivesOther antibioticsProgestinOther associated disorders include leukemia, lymphoma, sarcoidosis, rheumatic fever, Bechet's disease, and ulcerative colitis.The c