## **Core Concept**
The question tests understanding of the body plethysmograph technique used to measure lung volumes, specifically Functional Residual Capacity (FRC). The body plethysmograph is a sealed box that measures changes in volume within the chest cavity. When a person inspires against a closed glottis, the diaphragm and intercostal muscles increase intrathoracic volume.

## **Why the Correct Answer is Right**
When the man inspires against a closed glottis, the muscles of inspiration (diaphragm and intercostal muscles) contract. This increases the volume of the thoracic cavity. Since the glottis is closed, no air enters or leaves the lungs, but the increase in thoracic volume results in a decrease in intrathoracic pressure. According to Boyle's Law, at a constant temperature, the volume of a gas is inversely proportional to the pressure. Therefore, the decrease in intrathoracic pressure (increase in volume) causes air to flow out of the box (since the box is sealed around the person), which is detected by the plethysmograph. This maneuver allows for the calculation of FRC because the change in box volume (which reflects the change in thoracic volume) is equal to the change in lung volume.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately describe the physiological changes occurring during inspiration against a closed glottis in the context of body plethysmography.
- **Option B:** This option is incorrect as it does not accurately reflect the changes in pressure and volume that occur during the maneuver.
- **Option C:** Without specific details on what this option states, we can infer based on the context that it does not correctly describe the relationship between thoracic volume changes, intrathoracic pressure, and the measurement of FRC via body plethysmography.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that body plethysmography can measure volumes that are not in direct communication with the atmosphere (like FRC), which cannot be measured by spirometry alone. This technique is particularly useful in assessing lung volumes in patients with obstructive lung diseases.

## **Correct Answer:** . Boyle's law is applied.