**Core Concept**
Spirometry is a pulmonary function test used to assess lung function, measuring the volume and flow of air inhaled and exhaled by the lungs. It provides valuable information about obstructive and restrictive lung diseases.

**Why the Correct Answer is Right**
Spirometry is unable to estimate lung volumes, such as residual volume (RV) and functional residual capacity (FRC), as it only measures the volume of air that can be exhaled after a maximal inhalation (forced expiratory volume in 1 second, FEV1, and forced vital capacity, FVC). This limitation is due to the inability of spirometry to measure the volume of air remaining in the lungs after a maximal exhalation.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect as spirometry can estimate total lung capacity (TLC) indirectly by measuring the sum of FVC and RV. However, the RV itself cannot be directly estimated.
**Option B:** This choice is incorrect as spirometry can estimate the diffusing capacity of the lungs for carbon monoxide (DLCO) using the single-breath method, which measures the rate of gas exchange between the lungs and the bloodstream.
**Option C:** This choice is incorrect as spirometry can estimate the bronchodilator response, which is an important indicator of airway reversibility, by measuring the change in FEV1 and FVC after administering a bronchodilator.

**Clinical Pearl / High-Yield Fact**
A key limitation of spirometry is its inability to differentiate between obstructive and restrictive lung diseases based on lung volumes alone, as both types of diseases can present with reduced lung volumes.

**Correct Answer: A. Residual volume (RV) and functional residual capacity (FRC) cannot be estimated by routine spirometry.**