A 58-year-old male smoker with a 30-year history of cigarette use presents with progressive dyspnea and chronic productive cough. Spirometry shows FEV1/FVC ratio of 0.62 (below LLN, indicating obstruction). However, his FVC is also reduced at 68% predicted. The pattern marked **D** in the diagram represents examples of combined ventilatory defects. To confirm a true mixed obstructive-restrictive pattern and rule out incomplete exhalation as the cause of reduced FVC, which of the following investigations is MOST appropriate?