A 52-year-old woman from Mumbai with a 2-month history of progressive dyspnea, chest pain, and hemoptysis presents to the emergency department. She has a blood pressure of 158/92 mmHg, bilateral crackles on auscultation, and a serum creatinine of 3.2 mg/dL with active urinary sediment (RBC casts, proteinuria 2.8 g/day). Chest X-ray shows bilateral pulmonary infiltrates. Serum c-ANCA is positive with anti-PR3 antibodies. What is the most appropriate immediate next step in management?