A 32-year-old man with newly diagnosed HIV (CD4 count 85/μL) presents with a 3-week history of progressive exertional dyspnea, non-productive cough, and low-grade fever. Chest X-ray shows bilateral interstitial opacities. Induced sputum is sent for staining. On GMS silver staining, the structure marked **A** in the diagram (cup/disc-shaped cyst, 5–8 μm) is visualized within foamy alveolar exudate. Which of the following is the most appropriate next step in management?