**Core Concept**
Osteoarthritis (OA) is a degenerative joint disease characterized by progressive loss of articular cartilage, leading to joint pain, stiffness, and limited mobility. It is the most common type of arthritis, affecting millions of people worldwide.

**Why the Correct Answer is Right**
OA primarily affects weight-bearing joints, such as the knees, hips, and spine, as well as joints that undergo repetitive stress, such as the hands and ankles. The degeneration of cartilage is caused by a combination of factors, including age-related wear and tear, genetic predisposition, and mechanical stress. In OA, the cartilage is gradually broken down, leading to exposure of the underlying bone, which can cause pain and inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because OA can affect any joint, including the temporomandibular joint (TMJ), which is responsible for opening and closing the mouth.
**Option B:** This option is incorrect because OA is a common cause of joint pain and stiffness, especially in older adults.
**Option C:** This option is incorrect because OA can affect the hands, especially the distal interphalangeal joints (DIP), leading to Heberden's nodes.

**Clinical Pearl / High-Yield Fact**
OA is a leading cause of disability worldwide, affecting not only the joints but also the surrounding muscles and other tissues, leading to a loss of function and quality of life.

**Correct Answer: D. The TMJ is not commonly affected by OA.