A Case of Erythema Nodosum Associated with Mycoplasma pneumoniae Infection: Pathologic Findings and a Presumed Pathogenesis

Erythema nodosum (EN) is a painful skin disease characterized by erythematous tender nodules located predominantly over the extensor aspects of the legs. Various etiological factors, including infection, drug administration, and systemic illness have been implicated as causes of EN. Mycoplasma pneumoniae is one of rare infectious agents to cause EN in children. We report a case of a 7-year-old boy with context of respiratory illness and skin lesions with arthralgia. From stepwise approaches, IgM antibody against M. pneumoniae was positive with titers of 12.18, consistent with respiratory infection of M. pneumoniae and histopathology showed findings of septal and lobular inflammation without vasculitis consistent with EN. In addition, we reviewed the pathogenesis of this disease based on our case and the previous reports.