Q. A 68-year-old woman with a 50-pack-year smoking history presents to the primary care clinic with a recurrent cough, unexplained weight loss, and episodes of hemoptysis over the past year. She reports three instances of pneumonia in the same duration, which seemed to resolve with antibiotics, but the cough and other symptoms persist. On physical examination, the physician noted decreased breath sounds, dullness to percussion, and clubbing of the fingers, all localized to the right lower lobe. A chest x-ray reveals consolidation in the same area, which remains unchanged compared to an x-ray from 6 months ago. Given her smoking history, clinical presentation, and recurrent pneumonia episodes, a high-resolution CT scan of the chest and bronchoscopy are advised. What is the most likely underlying cause of this patient's recurrent pneumonia and persistent symptoms?