Volume of lung after maximal forceful expiration ?
**Core Concept**
The question is testing the concept of lung volumes and capacities, specifically the residual volume (RV) and the total lung capacity (TLC). The residual volume is the volume of air left in the lungs after a maximal forceful expiration.
**Why the Correct Answer is Right**
The residual volume is the volume of air that remains in the lungs after a maximal forceful expiration. This is because the diaphragm and intercostal muscles are contracting maximally, but the intrapleural pressure becomes more negative, and the lungs are still able to expand slightly due to the elastic recoil of the lung tissue and the chest wall. The residual volume is approximately 1200 mL in an average adult.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a recognized lung volume or capacity. It is a distractor that may seem plausible but is not a valid measurement.
**Option B:** This option is not the correct answer, as the residual volume is not the same as the functional residual capacity (FRC). The FRC is the volume of air in the lungs at the end of a passive expiration, which is approximately 2400 mL.
**Option C:** This option is not the correct answer, as the residual volume is not the same as the total lung capacity (TLC). The TLC is the total volume of air in the lungs after a maximal inhalation, which is approximately 6000 mL.
**Option D:** This option is not the correct answer, as the residual volume is not the same as the vital capacity (VC). The VC is the maximum amount of air that can be expelled from the lungs after a maximal inhalation, which is approximately 4800 mL.
**Clinical Pearl / High-Yield Fact**
Residual volume is an important concept in pulmonary function testing, as it can be used to diagnose conditions such as chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis.
**Correct Answer:** C. 1200 mL.