Spirometry — Cystic Fibrosis Severe Obstruction

A 16-year-old boy with cystic fibrosis presents to the CF clinic with progressive dyspnea. Spirometry shows FEV1 of 28% predicted with a markedly concave expiratory limb and hyperinflation pattern (RV/TLC ratio elevated). His sputum cultures grow Pseudomonas aeruginosa and he has had three pulmonary exacerbations requiring hospitalization in the past 12 months. He is currently on standard CF therapy including airway clearance, dornase alfa, hypertonic saline, inhaled tobramycin, and azithromycin. The structure marked **D** in the diagram indicates the clinical threshold at which specific therapeutic interventions become mandatory. Which of the following best represents the PRIMARY indication for intervention at this stage of disease severity?