A 72-year-old woman post-abdominal surgery develops fever and cough on postoperative day 3. Chest X-ray shows a wedge-shaped opacity in the left lower lobe with the apex pointing toward the hilum, elevated left hemidiaphragm, and mediastinal shift toward the left. The clinical picture is suspicious for left lower lobe collapse. Which investigation would be most appropriate to confirm the diagnosis and assess for an obstructing lesion?