A 44-year-old renal transplant patient develops severe cough and shortness of breath on exertion. On examination, he appears dyspneic, respirations 24/min, pulse 110/min, and oxygen saturation 88%. His lungs are clear on auscultation and heart sounds are normal. CXR shows bilateral diffuse perihilar infiltrates. Bronchoscopy and bronchial brushings show clusters of cysts that stain with methenamine silver. Which of the following is the most appropriate next step in management?