**Core Concept**
Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by inflammation of the joints, leading to cartilage and bone destruction. The immune system mistakenly attacks the lining of the joints (synovium), causing pain, swelling, and stiffness. Specific autoantibodies are present in RA patients, indicating an autoimmune etiology.

**Why the Correct Answer is Right**
The most specific antibody seen in RA is Rheumatoid Factor (RF). RF is an autoantibody directed against the Fc portion of IgG antibodies. It is produced by B cells in response to an unknown antigen, leading to the formation of immune complexes that deposit in the joints, triggering inflammation. RF is present in approximately 70-80% of RA patients and is used as a diagnostic marker.

**Why Each Wrong Option is Incorrect**
**Option A:** Anti-CCP (anti-citrullinated protein antibodies) are also present in RA patients but are less specific than RF. They are directed against citrullinated proteins and are present in approximately 60-70% of RA patients.
**Option B:** Anti-SSA (Sjögren's syndrome A) and anti-SSB (Sjögren's syndrome B) antibodies are associated with Sjögren's syndrome, an autoimmune disorder characterized by exocrine gland inflammation.
**Option C:** Anti-Sm (Smith) antibodies are associated with systemic lupus erythematosus (SLE), a chronic autoimmune disorder affecting multiple organ systems.

**Clinical Pearl / High-Yield Fact**
RF is not the only autoantibody present in RA patients; other antibodies, such as anti-CCP and anti-TRAF3IP2, have also been identified. However, RF remains the most specific marker for RA.

**Correct Answer: D. Rheumatoid Factor**