**Core Concept**
Caisson's disease, also known as decompression sickness (DCS), occurs when rapid changes in pressure cause gases dissolved in the bloodstream to form bubbles in the body's tissues and bloodstream. This condition is a result of the body's inability to eliminate excess gases during rapid decompression.

**Why the Correct Answer is Right**
The correct answer is **nitrogen**. Nitrogen is a gas that is present in the air we breathe and is dissolved in the bloodstream. When a diver or a person in a pressurized environment rapidly ascends to a lower pressure, the excess nitrogen in the bloodstream forms bubbles, leading to DCS. This condition can cause symptoms ranging from mild joint pain to life-threatening neurological problems.

**Why Each Wrong Option is Incorrect**
**Option A:** **Oxygen** - While oxygen is essential for life, it is not the primary gas responsible for Caisson's disease. Oxygen toxicity can occur at high partial pressures, but it is not related to decompression sickness.
**Option B:** **Carbon dioxide** - Carbon dioxide is a byproduct of respiration and is not typically associated with decompression sickness.
**Option C:** **Helium** - Helium is a gas used in deep-sea diving to mitigate the effects of nitrogen narcosis, but it is not directly related to the development of Caisson's disease.

**Clinical Pearl / High-Yield Fact**
To avoid Caisson's disease, divers and individuals working in pressurized environments must follow established protocols for gradual decompression, including staged ascents and use of decompression chambers.

**Correct Answer:** C. Nitrogen.