**Core Concept**
The intrapleural pressure is a critical component of the respiratory system, playing a key role in maintaining lung expansion and facilitating breathing. Intrapleural pressure is the pressure within the pleural cavity, the space between the lungs and the chest wall. It is a negative pressure that helps to keep the lungs inflated.

**Why the Correct Answer is Right**
At the start of inspiration, the intrapleural pressure becomes even more negative, allowing the lungs to expand and draw in air. This increase in negative pressure is due to the contraction of the diaphragm and the intercostal muscles, which decrease the volume of the thoracic cavity. As a result, the intrapleural pressure drops, creating a pressure gradient that pulls air into the lungs. The normal intrapleural pressure at the start of inspiration is around -4 to -6 cm of H2O.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the intrapleural pressure is not typically positive at the start of inspiration. A positive pressure would push the lungs out of the chest cavity, rather than drawing them in.

**Option B:** Incorrect because the intrapleural pressure at the start of inspiration is not around 0 cm of H2O. This would indicate no pressure gradient, and breathing would not be possible.

**Option C:** Incorrect because the intrapleural pressure at the start of inspiration is not around -10 cm of H2O. While the pressure does become more negative, -10 cm of H2O is an excessively low value.

**Clinical Pearl / High-Yield Fact**
A key concept to remember is that the intrapleural pressure is always negative, and its magnitude changes during inspiration and expiration. Understanding the relationship between intrapleural pressure and lung expansion is crucial for diagnosing and managing respiratory disorders.

**Correct Answer:** . -4 cm of H2O