A 42-year-old woman with a 2-month history of progressive dyspnea, hemoptysis, and sinusitis presents with a serum creatinine of 2.8 mg/dL (baseline 0.9 mg/dL) and active urinary sediment (RBC casts, proteinuria 2.5 g/day). Chest X-ray shows bilateral pulmonary infiltrates. ANCA testing is pending. What is the most appropriate immediate next step in management?