**Core Concept**
A diagnostic sign of a fracture refers to a clinical finding or radiographic feature that indicates the presence of a bone fracture. This concept is crucial in trauma medicine, as timely identification of fractures is essential for prompt management and prevention of complications.

**Why the Correct Answer is Right**
The correct answer is based on the principle of **crepitus**, which is a grating or crunching sensation felt upon palpation of a fractured bone. This occurs due to the rough edges of the fractured bone fragments rubbing against each other, causing a characteristic grating sound or sensation. Crepitus is often accompanied by pain and limited mobility of the affected joint or limb. The presence of crepitus is a strong indicator of a fracture, particularly in cases where radiographic findings are not immediately available.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect as it does not describe a diagnostic sign of a fracture. Common signs of a fracture include pain, swelling, deformity, and limited mobility.
* **Option B:** This option is incorrect as it describes a symptom of a fracture rather than a diagnostic sign. Pain is a common symptom of a fracture, but it is not a specific diagnostic sign.
* **Option C:** This option is incorrect as it describes a condition that can be caused by a variety of factors, including osteoarthritis, rheumatoid arthritis, and other inflammatory conditions. While crepitus can be a feature of osteoarthritis, it is not a diagnostic sign of a fracture.

**Clinical Pearl / High-Yield Fact**
Crepitus is a critical diagnostic sign of a fracture that should prompt immediate radiographic evaluation and further management. It is essential to differentiate crepitus from other causes of joint or limb pain, such as osteoarthritis or soft tissue injuries.

**Correct Answer:** C. Crepitus