**Core Concept**
Residual volume (RV) is the volume of air left in the lungs after a maximal exhalation. It is an important component of lung function testing, particularly in assessing patients with respiratory diseases such as chronic obstructive pulmonary disease (COPD).

**Why the Correct Answer is Right**
The correct answer is helium dilution technique, which involves having the patient inhale a known concentration of helium and then exhale maximally. The concentration of helium in the exhaled air is then measured, and the residual volume is calculated based on the difference in concentration. This method is considered the gold standard for measuring residual volume because it is not affected by airway resistance or lung compliance.

**Why Each Wrong Option is Incorrect**
**Option A:** Body plethysmography is a method that measures lung volume indirectly by measuring the change in volume of a sealed box when a person breathes in and out. While it is a useful method for measuring total lung capacity and other lung volumes, it is not as accurate for measuring residual volume.

**Option B:** Arterial blood gas (ABG) analysis measures the levels of oxygen and carbon dioxide in the blood and is not a direct measure of lung volume.

**Option C:** Spirometry measures the volume and flow of air in and out of the lungs during different phases of breathing, but it does not directly measure residual volume.

**Clinical Pearl / High-Yield Fact**
Remember that residual volume is a critical component of lung function testing, particularly in assessing patients with respiratory diseases. The helium dilution technique is a reliable and accurate method for measuring residual volume, but it requires careful technique and calibration to ensure accurate results.

**Correct Answer:** A. Helium dilution technique.