Answer is A (Body Plethysmography): Body Plethysmography is the investigation of choice fir estimating lung volumes in the setting of COPD with bullous lesions (trapped air) Estimating Lung Volumes Gas Dil ion Technique (Helium / Nitrogen Dilution Method) Gas dilution methods are fairly accurate in measurement of lung volumes in restrictive lung diseases, but not so in obstructive lung diseases with trapped air in bullae. Gas dilution methods tend to under-estimate lung volumes in patients with emphysema with ballous lesions as they do not measure the volume of trapped gas or bullae. Body Plethysmography Body Plethysmography is fairly accurate in measurement of lung volumes in both restrictive lung diseases and obstructive lung diseases even with trapped air in bullae Body Plethysmography also measures the volume of any gas containing space in the thorax, including bullae. The difference between Lung volume measurements by Body Plethysmography and gas dilution technique reflects the amount of gas trapped, in bullae.