Erythematosus plaques with macrophage infiltration as an initial manifestation of macrophage activation syndrome in a patient with systemic lupus erythematosus.

The diagnosis of macrophage activation syndrome (MAS) in patients with systemic lupus erythematosus (SLE) may be challenging as it can mimic the clinical features of the underlying disease or be confused with an infectious complication. In this report, a Japanese woman in her forties had diverse clinical features of MAS at initial presentation of SLE, where erythematosus plaques with histiocytic infiltrates focally surrounding degenerated collagen might be the earliest indicator of MAS.