MRCP2-3604
A 75-year-old man presents to MAU with worsening dyspnea. He has a history of locally advanced lung cancer of the left upper lobe treated with chemo and radiotherapy. He reports a recent decline in his breathing over the past 48 hours, accompanied by a dry cough and minor hemoptysis. Although he has not felt well since completing his treatment several weeks ago, he has not felt worse until now. His wife recently received antibiotics for a cold. On examination, his temperature is 37.7°C, BP 130/85 mmHg, pulse 120 bpm regular, respiratory rate 18 bpm, and oxygen saturations 92% on 24% oxygen. Chest examination reveals dullness and reduced air entry over the left upper zone. Heart sound auscultation is normal, and abdominal examination is unremarkable. What is the most effective diagnostic test to determine the underlying cause of his worsening dyspnea?