## Core Concept
Spirometry is a common pulmonary function test that measures lung function, specifically the volume and/or flow of air that can be inhaled and exhaled. It assesses the functional capacity of the lungs and is crucial in diagnosing and managing diseases like asthma and chronic obstructive pulmonary disease (COPD). The test provides several key metrics, including forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and the FEV1/FVC ratio.

## Why the Correct Answer is Right
The correct answer, **Residual Volume (RV)**, represents the volume of air remaining in the lungs after maximum exhalation. Spirometry cannot directly measure residual volume because it only measures the air that can be exhaled or inhaled through forced breathing maneuvers. The residual volume, by definition, is the air left in the lungs after a maximal exhalation and cannot be expelled through spirometry. Specialized tests like helium dilution, nitrogen washout, or body plethysmography are required to measure RV.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, spirometry can measure Forced Vital Capacity (FVC) and Forced Expiratory Volume in one second (FEV1), among other parameters.
- **Option B:** This option is also not provided, but spirometry can assess parameters like Peak Expiratory Flow Rate (PEFR) and the FEV1/FVC ratio, which are crucial in diagnosing obstructive and restrictive lung diseases.
- **Option C:** Similarly, this option is missing, but spirometry is adept at measuring various lung volumes and capacities that involve exhaled air.
- **Option D:** This option is not specified, but spirometry can indeed measure parameters related to airflow and lung volumes that are exhaled.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that spirometry is invaluable in differentiating between obstructive and restrictive lung diseases. For instance, in obstructive lung diseases like COPD, the FEV1/FVC ratio is decreased, indicating an obstruction to airflow. In contrast, restrictive lung diseases show a reduced FVC but a normal or increased FEV1/FVC ratio. Remember, spirometry cannot measure residual volume or total lung capacity directly, which often requires additional testing.

**Correct Answer: D. Residual Volume.**