Microscopic Polyangiitis Alveolar Hemorrhage Spirometry

A 55-year-old male presents to the emergency department with hemoptysis, progressive dyspnea, palpable purpura on lower extremities, and dark urine. Laboratory investigations reveal severe anemia (Hb 6.8 g/dL), elevated creatinine (4.2 mg/dL), microscopic hematuria with dysmorphic RBCs and RBC casts, and positive P-ANCA with MPO specificity. Chest imaging shows diffuse bilateral alveolar infiltrates, and bronchoscopy with BAL confirms diffuse alveolar hemorrhage with hemosiderin-laden macrophages. Spirometry is performed and demonstrates the pattern marked **A** in the diagram. Which of the following best explains the elevated DLCO finding in this patient's spirometry?