A 38-year-old woman with severe kyphoscoliosis diagnosed in adolescence is referred to pulmonology for evaluation of progressive exertional dyspnea over 2 years. She has never smoked and has no occupational exposures. She reports no cough. On examination, she has a pronounced thoracic spinal curvature. Lung auscultation reveals clear breath sounds bilaterally without crackles. Chest radiograph demonstrates small lung volumes and no parenchymal infiltrates. Pulmonary function testing reveals:
• FVC: 58% predicted
• FEV₁: 62% predicted
• FEV₁/FVC ratio: 0.88
• TLC: 62% predicted
• DLCO: 86% predicted
Which of the following findings most specifically indicates that this patient's restrictive lung disease is extrapulmonary rather than parenchymal in origin?