**Core Concept**
Nysten's rule is a clinical principle used in the assessment of the extent of joint involvement in patients with rheumatoid arthritis. It is based on the observation that the joint involvement in rheumatoid arthritis follows a specific pattern, which can be used to estimate the severity of the disease.

**Why the Correct Answer is Right**
Nysten's rule states that the joint involvement in rheumatoid arthritis progresses in a predictable manner, starting from the proximal joints and moving distally. This rule is based on the fact that the synovial fluid in the proximal joints is under increased pressure, which leads to the erosion of the joint cartilage and the subsequent involvement of the distal joints. This pattern of joint involvement is due to the inflammatory process in rheumatoid arthritis, which involves the release of various cytokines and chemokines that stimulate the synovial cells to produce inflammatory mediators.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Nysten's rule is not related to the assessment of joint deformities in rheumatoid arthritis.

**Option B:** This option is incorrect because Nysten's rule is not based on the presence of specific antibodies in the serum of patients with rheumatoid arthritis.

**Option C:** This option is incorrect because Nysten's rule is not related to the treatment of rheumatoid arthritis with non-steroidal anti-inflammatory drugs (NSAIDs).

**Clinical Pearl / High-Yield Fact**
Nysten's rule is a useful clinical tool for assessing the severity of rheumatoid arthritis, but it should be used in conjunction with other diagnostic criteria, such as the presence of specific antibodies and the clinical manifestations of the disease.

**Correct Answer: C. Nysten's rule is regarding the extent of joint involvement in rheumatoid arthritis.**