## **Core Concept**
Routine spirometry measures lung volumes and capacities that involve air moving in and out of the lungs during breathing. It directly measures volumes that can be exhaled or inhaled with maximum effort. However, not all lung volumes can be directly measured with spirometry.

## **Why the Correct Answer is Right**
Spirometry directly measures:
- **Forced Vital Capacity (FVC)**: The total amount of air exhaled after maximum inhalation.
- **Forced Expiratory Volume in 1 second (FEV1)**: The volume exhaled in the first second of forced exhalation.
- **Tidal Volume (TV)**: The volume of air inhaled or exhaled during normal breathing.
- **Inspiratory Capacity (IC)**: The maximum volume of air that can be inhaled after a normal exhalation.

However, spirometry cannot directly measure:
- **Residual Volume (RV)**: The volume of air remaining in the lungs after maximum exhalation.
- **Functional Residual Capacity (FRC)**: The volume of air present in the lungs at the end of passive exhalation.

## **Why Each Wrong Option is Incorrect**
- **Option B: VC (Vital Capacity)** can be estimated by spirometry as it is the maximum amount of air a person can expel from the lungs after a maximum inhalation.
- **Option D: ERV (Expiratory Reserve Volume)** can be measured by spirometry as it is the additional volume of air that can be forcefully exhaled after the end of a normal exhalation.
- **Option E: FEV1 (Forced Expiratory Volume in 1 second)** is directly measured by spirometry.

## **Why the Correct Answer Includes a, c**
- **Option A: FRC (Functional Residual Capacity)** and
- **Option C: RV (Residual Volume)**
cannot be directly measured by routine spirometry. FRC and RV require additional techniques like helium dilution, nitrogen washout, or body plethysmography for measurement.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that spirometry is crucial for diagnosing and monitoring obstructive lung diseases (like asthma and COPD) but might not give a complete picture of restrictive lung diseases without additional lung volume measurements.

## **Correct Answer: C. RV.**