A middle aged woman was referred to renal clinic for evaluation of her renal disorder. She had intermittent pre-tibial swelling for almost 3 months. She has mild hypertension, but no diabetes. Neither viral hepatitis B nor C history. Laboratory tests were: albumin 3.1 g/dL, creatinine 1.7 mg/dL, Both serum complement levels (C3 and C4) are low, with a particularly depressed C4 compared with C3. Which of the following should be considered ?