**Question:** In an emphysema patient with bullous lesions, which is the investigation to measure lung volume?

**Core Concept:** Emphysema is a chronic lung disease characterized by the destruction of the alveolar walls, leading to the formation of bullae (air-filled spaces) and impaired lung function. Measuring lung volume is essential in assessing the severity and progression of emphysema.

**Why the Correct Answer is Right:**

The correct answer is **D.** Spirometry. Spirometry is a non-invasive pulmonary function test that measures the volume and flow of air in the lungs. In emphysema, lung volumes are typically reduced due to the destruction of alveolar walls and the formation of bullae. Spirometry helps to evaluate lung volumes, specifically FEV1 (Forced Expiratory Volume in the first second) and FVC (Forced Vital Capacity).

**Why Each Wrong Option is Incorrect:**

A. **X-ray:** While X-rays can show the bullae in emphysema, they are not accurate in measuring lung volumes. X-rays only provide visual information about the distribution and severity of emphysema but do not directly measure lung function.

B. **CT scan:** Computed tomography (CT) scans are more detailed than X-rays but are also not used to directly measure lung volumes. CT scans are valuable in assessing the distribution and severity of emphysema but do not provide functional data.

C. **Barium swallow test:** This test is performed to evaluate swallowing disorders and not lung volumes. It is not relevant for assessing lung function in emphysema.

**Clinical Pearl:**

In clinical practice, measuring lung volumes alongside other pulmonary function tests is essential for assessing the severity and progression of emphysema. Spirometry, specifically FEV1 and FVC, provide valuable information for diagnosing and monitoring emphysema patients.

**Correct Answer:** **D.** Spirometry.