**Core Concept**
Osteoarthritis (OA) is a degenerative joint disease characterized by progressive wear and tear of joint cartilage, leading to bone-on-bone contact, pain, and stiffness. The disease primarily affects weight-bearing joints and those subjected to repetitive stress.

**Why the Correct Answer is Right**
The correct joint not involved in osteoarthritis is the **sphenoid bone**, which is a flat bone in the skull. Osteoarthritis typically affects synovial joints, such as the knees, hips, and hands, where cartilage breakdown and bone-on-bone contact occur. The sphenoid bone is a synchondrosis, a type of cartilaginous joint that connects the bone to the skull base, and is not subject to the same wear and tear as weight-bearing joints.

**Why Each Wrong Option is Incorrect**
**Option A:** The **temporomandibular joint (TMJ)** is a synovial joint that can be affected by osteoarthritis, leading to jaw pain and limited mobility.
**Option B:** The **spine**, particularly the lumbar and cervical regions, can be affected by osteoarthritis, causing back pain and stiffness.
**Option C:** The **ankle joint** is a weight-bearing joint that can be affected by osteoarthritis, leading to pain and limited mobility.

**Clinical Pearl / High-Yield Fact**
Osteoarthritis is often associated with **age**, **obesity**, and **previous joint injury**. Early recognition and management of osteoarthritis can help slow disease progression and improve quality of life.

**Correct Answer:** D. Sphenoid bone