Which of the following is a prerequisite condition for barotrauma?

Barotrauma occurs when there is a difference in pressure between the inside of a gas-filled space and the surrounding environment. This condition is predominantly seen in areas of the body that contain gas, such as the lungs, ears, and sinuses. A gas-filled space is essential because when external pressure increases (such as during a descent in diving), it can lead to compression of the gas within these spaces, potentially causing injury if the pressure difference exceeds the ability of the body to equalize the pressure.

In contrast, other choices lack the inherent requirement for barotrauma. Flexible walls might aid in equalization but are not a prerequisite for barotrauma itself. Fluid in the inner ear does not involve gas, hence does not relate directly to the mechanics of barotrauma. External exposure alone does not contribute to barotrauma without the presence of gas-filled spaces to create a pressure differential. Therefore, the presence of a gas-filled space is fundamental to the occurrence of barotrauma.