Concurrent Lupus Nephritis and Lupus Carditis in a Young Woman With Systemic Lupus Erythematosus: A Case Report With an Incidental Arachnoid Cyst.

Systemic lupus erythematosus (SLE) is a complex autoimmune disorder with highly variable clinical presentations. We report the case of a 32-year-old woman who presented with fever, polyarthritis, rash, and renal and cardiac involvement. Laboratory and immunological investigations confirmed SLE with Class IV lupus nephritis and lupus carditis, while brain imaging incidentally revealed an arachnoid cyst. The patient received high-dose corticosteroids, cyclophosphamide, and hydroxychloroquine, resulting in marked clinical and laboratory improvement, with the stabilization of renal function and the resolution of cardiac inflammation. This case highlights the importance of early recognition of severe multi-organ involvement in SLE, the application of standardized diagnostic criteria, and the benefits of a multidisciplinary treatment approach. It also emphasizes the need for careful interpretation of incidental findings to avoid misattribution, providing valuable teaching points for clinicians managing complex autoimmune disease presentations.