A 52-year-old man with diabetes mellitus type 2 and hypertension presents with progressive fatigue and nocturia for 6 months. Serum creatinine is 2.8 mg/dL, eGFR 24 mL/min/1.73m², and urinalysis shows 2+ proteinuria. Which investigation is most appropriate to confirm the diagnosis and assess the degree of renal fibrosis in chronic kidney disease?