**Core Concept**
Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by inflammation and destruction of joints. X-ray findings in RA include erosions, joint space narrowing, and subluxations. These changes reflect the underlying inflammatory process and joint damage.

**Why the Correct Answer is Right**
RA is typically associated with symmetrical joint involvement, particularly in the hands and feet, as well as the presence of erosions, which are a hallmark of the disease. Erosions in RA occur due to the destruction of bone and cartilage by inflammatory cells, such as neutrophils and macrophages, which secrete enzymes like matrix metalloproteinases (MMPs) and cathepsins. Joint space narrowing is also seen in RA due to cartilage destruction.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because joint space narrowing is indeed a common X-ray finding in RA.
* **Option B:** This option is incorrect because subluxations, or partial dislocations, are a possible X-ray finding in RA due to joint instability.
* **Option C:** This option is incorrect because erosions are a characteristic feature of RA, reflecting the underlying inflammatory and destructive process.

**Clinical Pearl / High-Yield Fact**
A useful mnemonic for remembering the common X-ray findings in RA is "E-S-S": erosions, subluxations, and joint space narrowing.

**Correct Answer: A. Joint space expansion**