A 52-year-old woman with a 2-month history of progressive dyspnea, hemoptysis, and constitutional symptoms is found to have pulmonary infiltrates on chest X-ray and a serum creatinine of 2.1 mg/dL. Urinalysis reveals dysmorphic RBCs and RBC casts. ANCA serology shows positive c-ANCA with anti-PR3 antibodies. Which investigation is most appropriate to confirm the diagnosis and assess disease extent?