**Core Concept**
Caisson's disease, also known as decompression sickness, occurs when rapid changes in pressure cause gas bubbles to form in the bloodstream and tissues. In the context of this question, joint pain is a common symptom of caisson's disease.

**Why the Correct Answer is Right**
The joint pain in caisson's disease is primarily due to the formation of gas bubbles in the synovial fluid and surrounding tissues of the joints. This occurs when nitrogen, dissolved in the bloodstream, is released as the pressure decreases during ascent. The gas bubbles can cause inflammation, pain, and swelling in the affected joints. The knee, elbow, and shoulder are common sites for joint pain in caisson's disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because joint pain in caisson's disease is not primarily due to hypoxia. While hypoxia can occur in caisson's disease, it is not the direct cause of joint pain.

**Option B:** This option is incorrect because joint pain in caisson's disease is not primarily due to an immune response. While inflammation may occur in response to the gas bubbles, it is not the primary cause of joint pain.

**Option C:** This option is incorrect because joint pain in caisson's disease is not primarily due to a mechanical issue such as a ligament sprain or tendon rupture. While mechanical issues can cause joint pain, they are not the primary cause of joint pain in caisson's disease.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that caisson's disease can occur at any altitude or depth, and even in situations where rapid changes in pressure are not immediately apparent. This includes situations such as air travel, where the pressure changes can be significant.

**Correct Answer: D. Nitrogen release.**