Multiple-Breath Nitrogen Washout - Lung Clearance Index

A 45-year-old woman with a 15 pack-year smoking history presents with chronic productive cough. Her spirometry shows FEV₁ 92% predicted and FEV₁/FVC 0.78 (within lower limit of normal), with mildly reduced DLCO at 78% predicted. Suspecting early small airway disease, she undergoes multiple-breath nitrogen washout testing by breathing 100% oxygen through a mouthpiece with nose clip during tidal breathing. Expired gas is analyzed for nitrogen concentration and flow. The pattern marked **B** in the diagram shows a prolonged, slow nitrogen decay curve with an elevated Lung Clearance Index (LCI) of 9.2 (normal <7). Which of the following best explains the mechanism underlying this abnormal result?