## **Core Concept**
Functional residual capacity (FRC) is a lung volume parameter that represents the volume of air present in the lungs at the end of passive expiration. It is the sum of the **expiratory reserve volume (ERV)** and the **residual volume (RV)**. FRC is crucial for maintaining lung inflation and facilitating gas exchange.

## **Why the Correct Answer is Right**
The correct answer, , signifies that FRC is the volume of air remaining in the lungs after a passive expiration. At this point, the **elastic recoil of the lungs** and the **chest wall** are equal in magnitude but opposite in direction, and there is no airflow. This balance allows for a continuous exchange of gases without the need for active respiratory effort.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe FRC.
* **Option B:** This option is incorrect because FRC is specifically related to the volume after passive expiration, not after forced expiration.
* **Option C:** This option might seem plausible but is incorrect because FRC includes both ERV and RV, representing the air left after a passive expiration, not just at the end of a normal expiration.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that FRC helps in understanding lung mechanics and is affected by factors like lung disease (obstructive or restrictive), posture, and the presence of conditions that alter lung volumes. For example, in **obstructive lung diseases** like COPD, FRC is often increased due to air trapping.

## **Correct Answer:** . At the end of passive expiration.