## **Core Concept**
Osteoarthritis (OA) is a degenerative joint disease characterized by progressive damage to articular cartilage, appositional new bone formation in the subchondral areas, and a variable degree of inflammation. It commonly affects weight-bearing joints and joints with high mobility.

## **Why the Correct Answer is Right**
The correct answer, **Metacarpophalangeal joint of index finger**, is not typically the most commonly affected; however, among the given options, joints like the **Metacarpophalangeal (MCP) and Metatarsophalangeal (MTP) joints** are relatively spared or less commonly involved in comparison to others like the DIP, PIP, hips, knees, and spine.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option might represent a commonly affected joint; however, the exact joint isn't specified. Generally, DIP and PIP joints of the hands are commonly involved in OA.
- **Option B:** Similarly, another commonly affected joint isn't specified but could imply joints frequently involved in OA.
- **Option D:** This might suggest another commonly affected area; specifics are needed for a detailed explanation, but typically, weight-bearing joints and certain hand joints are involved.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while OA can affect any joint, it commonly involves the **DIP and PIP joints of the hands**, the **hip**, the **knee**, and the **lumbar and cervical spine**. Joints that are usually spared or less commonly involved include the **MCP joints** and **wrist**. This pattern helps in differentiating OA from other types of arthritis.

## **Correct Answer: C. Metacarpophalangeal joint of index finger**