A 62-year-old male smoker presents with progressive dyspnea and cough. Spirometry shows FEV1/FVC = 0.65 with FEV1 = 45% predicted. The flow-volume loop demonstrates a concave expiratory limb and reduced PEFR. Post-bronchodilator testing (400 μg salbutamol, repeated after 15 minutes) shows an increase in FEV1 of 8% and 150 mL. The feature marked **D** in the diagram — absence of significant bronchodilator reversibility — is most consistent with which diagnosis?