What is the primary cause of decompression sickness?

The primary cause of decompression sickness, commonly known as "the bends," relates specifically to rapid ascent from high pressure to low pressure. When a pilot or diver ascends quickly, the external pressure surrounding them decreases rapidly. As a result, dissolved gases—primarily nitrogen, which is absorbed by the body under higher pressures—start to come out of solution and form bubbles.

During slower ascents, the body has time to eliminate the excess nitrogen through respiration, which helps to prevent the formation of bubbles. However, during a rapid ascent, the nitrogen gas leaves the tissues too quickly, leading to bubble formation in the bloodstream and tissues. These bubbles can cause various symptoms, ranging from joint pain and rashes to more severe neurological or cardiovascular issues if they block blood vessels or form in critical areas.

In the context of the other options, prolonged exposure to high altitude and sudden changes in cabin pressure are associated with altitude-related illnesses but do not directly cause the rapid bubble formation characteristic of decompression sickness. Rapid descent affects the pressure experienced but does not lead to the same physiological changes that trigger decompression sickness. Therefore, the correct answer highlights the crucial role of managing ascent rates to mitigate the risks of this condition.