A 45-year-old woman presents to the emergency department with a 3-day history of progressive shortness of breath, dry cough, and fatigue. She has a history of systemic lupus erythematosus (SLE) and is currently on hydroxychloroquine and low-dose prednisone. She denies fever, chest pain, or recent travel. On examination, her temperature is 37.2°C, blood pressure is 110/70 mmHg, heart rate is 95 bpm, respiratory rate is 22 breaths per minute, and oxygen saturation is 89% on room air. Lung auscultation reveals bilateral basal crackles. Laboratory tests show a hemoglobin level of 10.5 g/dL, white blood cell count of 4,000/mm³, and platelets of 150,000/mm³. Arterial blood gas analysis reveals a pH of 7.45, PaCO2 of 35 mmHg, and PaO2 of 55 mmHg. A chest X-ray shows bilateral interstitial infiltrates. Which of the following is the most appropriate next step in management?