## **Core Concept**
Osteoarthritis (OA) is a degenerative joint disease characterized by progressive damage to the articular cartilage, subchondral bone, and surrounding tissues. It primarily affects weight-bearing joints and joints with high mobility. The disease involves biochemical, morphological, and mechanical changes.

## **Why the Correct Answer is Right**
The correct answer involves understanding which part of the joint or body is typically spared in osteoarthritis. Osteoarthritis primarily affects the articular cartilage, subchondral bone, and the joint capsule. It does not typically involve the **synovium** in the early stages or as a primary feature, unlike inflammatory arthritides such as rheumatoid arthritis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because osteoarthritis does involve the **articular cartilage**, which undergoes degeneration and loss, leading to joint space narrowing.
- **Option B:** This option is incorrect as OA does affect the **subchondral bone**, leading to changes such as sclerosis and cyst formation.
- **Option D:** This option is incorrect because the **joint capsule** is indeed involved in osteoarthritis, often leading to capsular thickening and fibrosis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while osteoarthritis is primarily a non-inflammatory condition, secondary synovitis can occur, especially in more advanced disease. However, the primary pathology does not originate in the synovium.

## **Correct Answer:** C. Synovium.