Eosinophilic Granulomatosis with Polyangiitis (Churg-Strauss / EGPA)

A 47-year-old woman with a 7-year history of adult-onset steroid-dependent asthma and chronic rhinosinusitis with nasal polyps presents with mononeuritis multiplex (wrist drop), palpable purpura on the legs, and migratory pulmonary infiltrates. CBC shows eosinophilia of 28% (absolute count 7,200/microliter). Spirometry shows the pattern marked **A** in the diagram: FEV1 58% predicted with 18% improvement (+340 mL) after bronchodilator administration. p-ANCA/MPO-ANCA is positive at moderate titer. Which of the following best explains the spirometric finding marked **A** in the context of this patient's diagnosis?