**Core Concept**
Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by inflammation and destruction of synovial joints, primarily affecting the hands and feet. The disease process involves the activation of immune cells, leading to the production of pro-inflammatory cytokines and the degradation of joint cartilage and bone.

**Why the Correct Answer is Right**
The joint that is typically spared in RA is the DIP (distal interphalangeal) joint. This is because the DIP joint is a non-synovial joint, meaning it does not have a synovial lining that can become inflamed and damaged. The DIP joint is also not as mobile as other joints, which may reduce the mechanical stress and strain that can contribute to joint damage in RA. As a result, the DIP joint is often spared from the destructive process of RA, unlike other joints such as the MCP (metacarpophalangeal) and PIP (proximal interphalangeal) joints.

**Why Each Wrong Option is Incorrect**
**Option A:** The carpometacarpal joint of the thumb is often involved in RA, leading to pain and stiffness in this joint.
**Option B:** The elbow joint can be involved in RA, particularly in more severe cases, leading to pain, stiffness, and limited mobility.
**Option C:** The ankle joint can also be involved in RA, leading to pain, stiffness, and limited mobility in the foot and ankle.

**Clinical Pearl / High-Yield Fact**
In RA, the presence of a "sausage digit" (a swollen, sausage-shaped finger) is a classic clinical sign that suggests involvement of the DIP joint.

**Correct Answer: D. DIP joint**