Decompression sickness occurs when:

Decompression sickness, also known as "the bends," occurs due to the rapid formation of nitrogen bubbles in the body when a diver ascends too quickly after being at depth. Understanding the mechanics behind this condition involves several key factors related to how nitrogen behaves in the body during and after a dive.

When a diver is underwater, they breathe compressed air, which increases the amount of nitrogen absorbed into their body tissues. If they then ascend too quickly, the reduced pressure allows nitrogen to shift out of the tissues and form bubbles in the bloodstream and around body tissues. This is exacerbated when divers spend extended time at depth, leading to an increased absorption of nitrogen.

The concept of nitrogen tolerance is also critical here. Each individual has a limit to how much nitrogen their tissues can tolerate before bubbles form. If this gradient is exceeded during ascent, it can lead to symptoms of decompression sickness as the gas escapes too rapidly.

Given this understanding, each of the scenarios listed is a factor that contributes to the risk of decompression sickness, thereby making the comprehensive choice the most accurate representation of the cause of this condition.