Granulomatosis with Polyangiitis (GPA) with Diffuse Alveolar Hemorrhage

A 48-year-old man presents with a 3-month history of recurrent epistaxis, chronic nasal crusting with saddle-nose deformity, hemoptysis, and palpable purpura. Laboratory investigations reveal C-ANCA positivity with anti-PR3 antibodies at high titer, rising creatinine (2.1 mg/dL), microscopic hematuria with red cell casts, and renal biopsy showing pauci-immune necrotizing crescentic glomerulonephritis. HRCT chest demonstrates cavitary lung lesions and tracheal-wall thickening. Pulmonary function testing is performed. The flow-volume loop shows the pattern marked **A** — a flattened inspiratory limb with reduced FEV1/FVC ratio of 0.63. Which of the following best explains the pathophysiological basis for this spirometric finding?