Progressive Massive Fibrosis - Complicated Silicosis

A 61-year-old retired sandblaster with 35 years of cumulative silica exposure presents with severe progressive dyspnoea (MMRC grade 4), chronic productive cough with black-tinged sputum, and significant weight loss. On examination, he is cachectic with central cyanosis, respiratory rate 28/min, SpO₂ 86% on room air. Chest X-ray demonstrates large confluent opacities in both upper lobes with upper lobe contraction, diagnostic of progressive massive fibrosis (complicated silicosis). The spirometry pattern marked **B** in the diagram is observed in this patient. Which of the following best describes the physiological basis for the pattern marked **B**?