A 52-year-old man with chronic obstructive pulmonary disease (COPD) on long-term oral corticosteroids presents with a 6-week history of productive cough, hemoptysis, and low-grade fever. Chest X-ray shows a cavitary lesion in the right upper lobe with a crescent-shaped radiolucency within the cavity (air-crescent sign). Sputum smear is negative for acid-fast bacilli. What is the most appropriate next step in management?