**Core Concept**
Inspiration involves a decrease in alveolar pressure, driven by negative intrapleural pressure. Intrapleural pressure is the pressure within the pleural cavity, which is always negative during inspiration and remains below atmospheric pressure due to the elastic recoil of the lungs and the chest wall.

**Why the Correct Answer is Right**
During inspiration, the diaphragm contracts and the chest cavity expands, leading to a drop in intrapleural pressure. This negative pressure is essential for lung expansion, as it creates a pressure gradient that pulls the lungs outward. The intrapleural pressure is the key determinant of lung inflation and is directly responsible for maintaining alveolar expansion. It is distinct from alveolar or transpulmonary pressures, which are derived from it.

**Why Each Wrong Option is Incorrect**
Option B: Transpulmonary pressure is the difference between alveolar and intrapleural pressure, not the pressure of inspiration. It reflects lung compliance but is not the pressure itself.
Option C: Transchest wall pressure is not a standard physiological term and does not represent inspiration pressure.
Option D: Alveolar pressure rises during inspiration (becomes slightly negative), but it is not the pressure of inspiration; it is the pressure within the alveoli, which is different from intrapleural pressure.

**Clinical Pearl / High-Yield Fact**
Intrapleural pressure is always negative during inspiration and is the driving force behind lung expansion. If this pressure becomes positive (e.g., in pneumothorax), the lung collapses due to loss of negative pressure.

✓ Correct Answer: A. Intrapleuarl