**Core Concept**
A Baker's cyst, also known as a popliteal cyst, is a fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee. It is a common condition that occurs due to the accumulation of synovial fluid from the knee joint into a cystic structure, often as a result of joint inflammation or degenerative changes.

**Why the Correct Answer is Right**
The correct answer involves understanding the pathophysiology of Baker's cyst. A Baker's cyst typically arises from a communication between the joint space and the cyst, allowing synovial fluid to leak into the cyst. This fluid accumulation causes the cyst to swell, leading to the characteristic bulge behind the knee. The cyst is often associated with conditions such as osteoarthritis, meniscal tears, or ligament injuries that cause joint inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the pathophysiology of Baker's cyst. While fluid accumulation is involved, the specific mechanism of joint-synovial fluid communication is not accurately represented.
**Option B:** This option is incorrect because it does not specify the location of the cyst. Baker's cysts are specifically located in the popliteal fossa, behind the knee.
**Option C:** This option is incorrect because it does not describe the underlying condition that often causes Baker's cyst. While fluid accumulation is a key feature, the associated joint inflammation or degenerative changes are not mentioned.

**Clinical Pearl / High-Yield Fact**
A Baker's cyst is often associated with knee joint conditions such as osteoarthritis, meniscal tears, or ligament injuries. It is essential to investigate and manage the underlying joint condition to effectively treat the Baker's cyst.

**Correct Answer: D. Baker's cyst is a fluid-filled swelling behind the knee, caused by joint inflammation or degenerative changes, and often associated with conditions like osteoarthritis or meniscal tears.