**Core Concept**
Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by inflammation and degeneration of synovial joints, leading to pain, stiffness, and decreased mobility. The disease process involves the interplay of genetic, environmental, and hormonal factors, ultimately resulting in the activation of immune cells and the production of pro-inflammatory cytokines.

**Why the Correct Answer is Right**
The correct answer is hands, specifically the proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints, as well as the wrists and knees. This is because RA often affects the small joints of the hands, causing pain, swelling, and deformity, particularly in the PIP and MCP joints. The activation of immune cells, such as T cells and macrophages, and the production of pro-inflammatory cytokines, including tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1β), contribute to the inflammation and joint destruction characteristic of RA.

**Why Each Wrong Option is Incorrect**
**Option A:** While osteoarthritis (OA) also affects the joints, it is a degenerative disorder characterized by the wear and tear of cartilage, rather than an autoimmune process like RA.
**Option B:** The spine is not a typical site of RA, although it can be affected in advanced cases or in patients with a specific subtype of RA known as rheumatoid spondylitis.
**Option C:** The feet are not commonly affected by RA, although they can be involved in some cases, particularly in the presence of other autoimmune disorders.

**Clinical Pearl / High-Yield Fact**
It's essential to note that RA can often be differentiated from OA by the presence of morning stiffness, which typically lasts more than an hour in RA, and the presence of systemic symptoms, such as fever and weight loss.

**Correct Answer:** C.