A 58-year-old man with severe kyphoscoliosis (Cobb angle 85°) presents with progressive dyspnea on exertion and orthopnea. Spirometry shows FVC 2.1 L (55% predicted), FEV1 1.8 L (60% predicted), FEV1/FVC ratio 0.86, and total lung capacity (TLC) 2.8 L (62% predicted) by body plethysmography. His DLCO is 28 mL/min/mmHg (92% predicted). The pattern marked **A** in the diagram represents his spirometric findings. Which of the following is the MOST appropriate next diagnostic step to confirm the extra-pulmonary nature of his restriction?