**Core Concept**
Autoimmune rheumatic (AR) disorders are a group of diseases caused by an abnormal immune response, leading to inflammation and damage in various body tissues. These disorders often involve the formation of autoantibodies, which are antibodies that target the body's own proteins.

**Why the Correct Answer is Right**
The correct answer is a disorder that does not involve an abnormal immune response or autoantibody formation. This is a key concept in distinguishing AR disorders from other conditions. For example, in rheumatoid arthritis (RA), autoantibodies such as rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPAs) are present, indicating an autoimmune process. In contrast, disorders that are not AR in nature may involve other underlying mechanisms, such as genetic mutations or environmental triggers.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is an example of an AR disorder, characterized by the presence of autoantibodies and inflammation in the joints. It is a classic example of an AR disorder.
**Option B:** This option is also an AR disorder, involving the formation of autoantibodies against the body's own proteins. It is a well-known example of an AR disorder that affects the skin and other organs.
**Option C:** This option is not an AR disorder. It is a genetic disorder that affects the joints and is caused by a mutation in the gene that codes for a protein involved in cartilage development. It is an example of a non-autoimmune disorder that affects the joints.

**Clinical Pearl / High-Yield Fact**
AR disorders often involve the formation of specific autoantibodies, which can be used as diagnostic markers. For example, RF and ACPAs are commonly found in patients with RA, while anti-Sm antibodies are often present in patients with systemic lupus erythematosus (SLE).

**Correct Answer:** C.