A 28-year-old male smoker presents with hemoptysis, dyspnea, and acute kidney injury (creatinine 4.2 mg/dL). Chest X-ray shows bilateral alveolar infiltrates. Kidney biopsy shows crescentic glomerulonephritis on light microscopy. Direct immunofluorescence reveals the pattern marked **A** in the diagram. Serum anti-GBM antibodies are positive. Which of the following is the most appropriate FIRST-LINE intervention to prevent irreversible renal failure in this patient?