Single-Breath Nitrogen Test - Closing Volume and Phase III Slope

A 40-year-old woman with 20 pack-years of cigarette smoking presents with persistent cough and reduced exercise tolerance. Her spirometry is normal (FEV₁ 92% predicted, FEV₁/FVC 0.78). A single-breath nitrogen washout test is performed: after exhaling to residual volume, she inhales a vital-capacity breath of 100% oxygen to total lung capacity, then exhales slowly while a nitrogen analyzer measures expired N₂ concentration against expired volume. The resulting curve marked **C** shows four characteristic phases. The Phase III slope in this patient is 4% N₂ per liter (normal <2%), and closing volume is elevated at 22% of vital capacity (normal <10% at age 40). Which of the following best explains the abnormal Phase III slope and elevated closing volume despite normal spirometry?