**Core Concept**
Osteoarthritis (OA) is a degenerative joint disease characterized by progressive damage to joint cartilage and underlying bone. It is the most common type of arthritis and a leading cause of disability worldwide. The radiological features of OA are critical for diagnosis and monitoring disease progression.

**Why the Correct Answer is Right**
The correct answer is related to the characteristic radiological features of osteoarthritis. OA is typically associated with joint space narrowing, osteophytes (bone spurs), subchondral sclerosis, and cysts. These changes are due to the loss of articular cartilage and underlying bone remodeling. The subchondral bone becomes thicker and more dense, and cysts form in the subchondral bone as a result of bone marrow edema.

**Why Each Wrong Option is Incorrect**
* **Option A:** Joint effusion is actually a common radiological feature of osteoarthritis, which can be seen as a fluid collection around the joint on imaging studies.
* **Option B:** Subchondral sclerosis is a characteristic feature of osteoarthritis, representing a thickening of the subchondral bone in response to cartilage loss and increased bone remodeling.
* **Option C:** Bone marrow edema is also a feature of osteoarthritis, often seen as a fluid collection in the subchondral bone on imaging studies.
* **Option D:** Osteophyte formation is a hallmark of osteoarthritis, representing a bony growth at the edge of the joint.

**Clinical Pearl / High-Yield Fact**
When evaluating radiological features of osteoarthritis, it's essential to consider the joint-space narrowing and osteophyte formation as key indicators of disease progression.

**Correct Answer: D. Osteophyte formation is not a radiological feature of osteoarthritis.**