## **Core Concept**
The question tests the understanding of lung volume measurements in patients with emphysema, particularly those with a "hollow" or air-filled lesion, likely referring to a bulla. Emphysema is a condition characterized by destruction of lung tissue, leading to loss of alveolar surface area and elastic recoil. Accurate measurement of lung volumes is crucial for assessing disease severity and guiding management.

## **Why the Correct Answer is Right**
The correct answer, **D. Body Plethysmography**, is the best investigation for measuring lung volumes in emphysematous patients, especially those with bullae. Body plethysmography measures the total volume of air in the lungs, including air trapped in bullae or poorly communicating airspaces, which may not be accurately measured by other methods like spirometry or gas dilution techniques. This method involves the patient sitting in a sealed box (plethysmograph) and making respiratory efforts against a closed glottis, allowing for the calculation of lung volumes based on changes in box volume and pressure.

## **Why Each Wrong Option is Incorrect**
- **Option A: Spirometry** is incorrect because, although it is a fundamental pulmonary function test, it may underestimate lung volumes in patients with emphysema and large air-filled spaces that do not communicate well with the airways.
- **Option B: Helium Dilution Technique** is incorrect because it measures volumes of air that communicate with the airways, potentially missing air trapped in bullae or poorly ventilated areas.
- **Option C: Nitrogen Washout** is incorrect for similar reasons as helium dilution; it measures lung volumes based on the dilution or washout of gases and may not accurately quantify air trapped in poorly ventilated spaces.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that body plethysmography is particularly useful in patients with obstructive lung diseases like emphysema, especially when there is suspicion of air trapping or large bullae. This method can accurately measure total lung capacity (TLC) and residual volume (RV), providing valuable information on the extent of air trapping.

## **Correct Answer: D. Body Plethysmography**