**Core Concept**
Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by inflammation and symmetrical joint involvement. The diagnosis of polyarticular rheumatoid arthritis is based on the number of joints affected, with a specific threshold distinguishing it from other forms of arthritis.

**Why the Correct Answer is Right**
Polyarticular rheumatoid arthritis is diagnosed when at least 5 joints are involved, with at least one joint being from the small joints of the hands or feet. This definition is crucial for distinguishing RA from other forms of arthritis, such as osteoarthritis or psoriatic arthritis. The involvement of multiple joints, particularly those in the hands and feet, is a hallmark of RA, reflecting the autoimmune nature of the disease. The presence of rheumatoid factor or anti-CCP antibodies, along with characteristic radiographic findings, further supports the diagnosis of polyarticular RA.

**Why Each Wrong Option is Incorrect**
**Option A:** Involvement of only one joint is a characteristic of monoarticular arthritis, which can be caused by various conditions, including osteoarthritis, septic arthritis, or trauma.
**Option B:** Involvement of two joints is not sufficient to diagnose polyarticular RA, as this can still be consistent with other forms of arthritis, such as osteoarthritis or psoriatic arthritis.

**Clinical Pearl / High-Yield Fact**
The American College of Rheumatology (ACR) criteria for the diagnosis of rheumatoid arthritis include the involvement of at least 5 joints, with at least one joint being from the small joints of the hands or feet. This criterion is essential for distinguishing RA from other forms of arthritis and guides the management and treatment of patients with RA.

**Correct Answer:**
✓ Correct Answer: D. Five