**Core Concept**
The joint cavity is a fluid-filled space within synovial joints that allows for smooth movement and reduced friction between bones. This cavity is lined by synovial membrane, which produces synovial fluid that lubricates and nourishes the joint.

**Why the Correct Answer is Right**
Synovial fluid is a clear, viscous liquid that contains hyaluronic acid, glycoproteins, and other nutrients. It is produced by the synovial membrane and helps to reduce friction between cartilage and bone surfaces. The fluid also provides nutrients to the cartilage and helps to remove waste products. This lubrication is essential for smooth joint movement.

**Why Each Wrong Option is Incorrect**
**Option A:** The joint cavity is not a bursa, which is a fluid-filled sac that reduces friction between tendons or ligaments and surrounding bone.
**Option B:** The joint cavity does not contain lymphatic vessels, which are responsible for draining excess fluid from the interstitial space.
**Option C:** The joint cavity is not a tendon sheath, which is a fibrous tube that surrounds a tendon and reduces friction as it moves through the surrounding tissue.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the synovial fluid is a key component of joint health, and its production can be affected by various factors, including joint disease, trauma, and systemic conditions. Inflammation or infection of the joint cavity can lead to changes in synovial fluid composition, which can be used as a diagnostic marker.

**Correct Answer: D. Synovial fluid**