Bronchiectasis CT and Spirometry

A 54-year-old non-smoking female with a 6-year history of productive cough (60 mL mucopurulent sputum daily), recurrent hemoptysis, and progressive exertional dyspnea presents for pulmonary function assessment. HRCT chest shows bilateral lower-lobe cylindrical bronchiectasis with signet-ring sign and tram-track appearance. The flow-volume loop marked **B** in the diagram demonstrates a distinctly scooped expiratory limb with reduced peak expiratory flow (4.2 L/s) and slow tail. Pulmonary function testing shows FEV1 62% predicted, FVC 78% predicted, and FEV1/FVC 0.58. Which of the following best explains the pathophysiologic mechanism underlying the flow-volume loop pattern marked **B**?