A 58-year-old man with a 6-month history of nephrotic syndrome (proteinuria 5.2 g/day, serum albumin 2.3 g/dL) and progressive renal insufficiency (serum creatinine 2.8 mg/dL, eGFR 22 mL/min) undergoes renal biopsy. Light microscopy shows thickened glomerular basement membrane with a 'spike and dome' appearance on electron microscopy. Immunofluorescence shows granular IgG and C3 deposits along the capillary wall. Which additional investigation is most appropriate to identify the underlying etiology?