Spirometry — Helium Dilution vs Body Plethysmography Discrepancy in COPD (Trapped Gas)

A 62-year-old man with a 40-pack-year smoking history presents with progressive dyspnea. Pulmonary function testing is performed. Body plethysmography reports a total lung capacity (TLC) of 7.2 L (140% predicted), while helium dilution reports a TLC of 5.8 L (110% predicted). Chest X-ray confirms the presence of large bullae in the bilateral upper lobes. The discrepancy between these two measurements is explained by the mechanism illustrated at **A** in the diagram. Which of the following best describes why body plethysmography is the gold standard for lung volume measurement in this patient's condition?