We continued to track the etiology of nephrotic syndrome and performed a renal biopsy, showing dsDNA 1:10 positivity, low C3, low platelets and hemoglobin, anticardiolipin IgM 12 U/ml, anti-β2-glycoprotein I (β2GPI) 223 U/ml; renal pathology suggested lupus nephritis (LN), and the patient was ultimately diagnosed with SLE, secondary APS and LN. Here, C3 is linked to lupus nephritis.