According to the 2019 EULAR/ACR classification criteria (3), the patient fulfills four clinical criteria (malar rash, leukopenia, pleural effusion, and lupus nephritis) and two immunological criteria (low complement C3 levels and a positive anti-Smith antibody) with a total score of 33, consistent with the diagnosis of SLE. Here, C3 is linked to systemic lupus erythematosus.