Sarcoidosis Stage II Parenchymal Restrictive Spirometry

A 45-year-old African American woman presents with 6 months of progressive exertional dyspnea, dry cough, and bilateral fine end-inspiratory crackles. Chest X-ray shows bilateral hilar lymphadenopathy with perihilar reticulonodular infiltrates (Scadding Stage II). HRCT demonstrates bilateral perilymphatic micronodules in an upper-mid zone predominant distribution. Transbronchial lung biopsy reveals non-caseating granulomas, and BAL shows lymphocytosis with CD4/CD8 ratio >3.5. The pulmonary function pattern marked **A** in the diagram shows FEV1 55% predicted, FVC 52% predicted, FEV1/FVC 0.85, TLC 58% predicted, and DLCO 45% predicted. Which of the following best explains the disproportionate reduction in DLCO relative to TLC reduction in this patient's restrictive pattern?