A 28-year-old man presents with sudden onset haematuria, mild periorbital oedema, and hypertension (BP 148/92 mmHg) for 3 days. Urine dipstick shows 3+ protein and 4+ blood. Serum creatinine is 1.2 mg/dL (baseline 0.8 mg/dL). Complement C3 levels are low. Which investigation is most specific for confirming the diagnosis of post-infectious glomerulonephritis?