What will happen to an untreated pneumothorax during ascent?

When examining what happens to an untreated pneumothorax during ascent, it's crucial to understand the principles of gas behavior under pressure changes. As altitude increases, atmospheric pressure decreases. Because the pneumothorax consists of air trapped in the pleural space, the air volume will behave according to Boyle's Law, which states that the volume of a gas is inversely proportional to the pressure exerted on it, provided the temperature remains constant.

As a consequence of decreasing external pressure during ascent, the air within the pneumothorax will expand. This occurs because there is less external pressure to counterbalance the pressure inside the pneumothorax, allowing it to increase in volume. If the pneumothorax is untreated, this expansion can lead to significant complications, including an increased risk of lung collapse or rupture, which makes it a critical condition requiring close monitoring and intervention.

In contrast, the other options do not accurately reflect the physical behavior of gas within a pneumothorax during altitude changes. The scenario of remaining unchanged is not viable due to the dynamics of pressure change, while shrinking would imply a reduction in volume contrary to the principles of gas expansion with decreasing pressure. Rupturing, although a potential severe consequence, is not the immediate