**Core Concept**
A Baker's cyst, also known as a popliteal cyst, is a fluid-filled swelling that causes a bulge and a sense of tightness behind the knee. It is often associated with conditions like osteoarthritis, meniscal tears, or ligament sprains in the knee joint.

**Why the Correct Answer is Right**
A Baker's cyst forms when fluid from the knee joint leaks through a tear in the joint's lining, called the synovial membrane. This fluid accumulation in the popliteal fossa creates a cyst. The condition often presents with symptoms like knee pain, swelling, and stiffness, especially after activities that stress the knee joint. The cyst can be asymptomatic or cause discomfort and limited mobility.

**Why Each Wrong Option is Incorrect**
**Option A:** A ganglion cyst is a different type of fluid-filled swelling that typically arises from a joint or tendon sheath but is not directly related to the knee joint's synovial fluid.
**Option B:** A popliteal aneurysm is an abnormal dilation of the popliteal artery, which does not involve fluid accumulation or cyst formation.
**Option C:** A Baker's cyst is not a type of ligament sprain or meniscal tear but rather a symptom that can arise from these conditions.

**Clinical Pearl / High-Yield Fact**
A Baker's cyst often resolves with conservative management like physical therapy, bracing, and pain control. However, if the underlying cause is not addressed, the cyst may recur or cause complications like joint stiffness or limited range of motion.

**Correct Answer:** D.