Bronchiolitis Obliterans Syndrome (Post-BMT)

A 45-year-old man undergoes allogeneic hematopoietic stem cell transplantation (HSCT) for acute myeloid leukemia. At 8 months post-transplant, he presents with progressive dyspnea and dry cough. Spirometry shows FEV1/FVC ratio of 0.68, FEV1 decline of 14% from baseline, and minimal response to bronchodilators (3% improvement). High-resolution expiratory CT demonstrates mosaic attenuation and air trapping. Respiratory cultures and bronchoscopy are negative for infection. The pattern marked **A** in the diagram represents the pulmonary physiology findings consistent with this clinical presentation. Which of the following best describes the underlying pathophysiological mechanism of this condition?

A 32-year-old woman is 2 years post-allogeneic hematopoietic stem cell transplant for acute myeloid leukemia. She presents with insidious dyspnea on exertion, dry cough, and wheezing. She has chronic graft-versus-host disease affecting skin, mouth, and ocular surfaces. Pulmonary function testing shows FEV1 42% predicted, FVC 78% predicted, FEV1/FVC ratio 0.48 with no significant bronchodilator reversibility. High-resolution CT demonstrates patchy mosaic attenuation with air trapping on expiratory images, bronchial wall thickening, and a tree-in-bud pattern. The flow-volume loop is shown in the diagram. Which of the labeled flow-volume loop patterns, marked **C**, best represents the pulmonary physiology in bronchiolitis obliterans syndrome (BOS) post-HSCT?