## **Core Concept**
The volume of gas in the lungs at the end of a normal expiration is referred to as the **Functional Residual Capacity (FRC)**. FRC is the volume of air present in the lungs at the end of passive expiration, and it is the sum of the **Residual Volume (RV)** and the **Expiratory Reserve Volume (ERV)**.

## **Why the Correct Answer is Right**
The correct answer, Functional Residual Capacity (FRC), is the volume of air left in the lungs after a normal, relaxed expiration. This is the volume that remains after the elastic recoil of the lungs and chest wall reach equilibrium. FRC is crucial for maintaining gas exchange and is approximately 2300-3300 mL in a typical adult.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a known lung volume or capacity.
* **Option B:** This option is incorrect because it does not correspond to the definition of FRC or any other standard lung volume.
* **Option D:** This option is incorrect because, although tidal volume (TV) is a relevant lung volume, it refers to the amount of air inhaled or exhaled during normal breathing, not the volume at the end of expiration.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **FRC is increased in obstructive lung diseases** (like COPD) due to air trapping and is decreased in restrictive lung diseases. Understanding FRC and other lung volumes is critical for diagnosing and managing respiratory diseases.

## **Correct Answer:** .