**Diagnostic Sign of a Fracture**

### Core Concept
A fracture is a break in a bone, and identifying it is crucial for proper management. The diagnostic sign of a fracture is an important aspect of radiology and emergency medicine.

### Why the Correct Answer is Right
The diagnostic sign of a fracture is a displacement of bone fragments, which can be seen on imaging studies such as X-rays. This displacement can be due to the bone fragment being pulled in a direction away from its original position, often due to muscle pull or external force. The correct answer is related to this concept.

### Why Each Wrong Option is Incorrect
**Option A:** does not accurately describe a diagnostic sign of a fracture. While swelling and pain are common symptoms of a fracture, they are not diagnostic signs.
**Option B:** may be related to a fracture, but it is not a diagnostic sign. A joint effusion can be a sign of various conditions, including fractures, but it is not specific to fractures.
**Option C:** is a sign of a different condition. A subluxation is a partial dislocation of a joint, which is distinct from a fracture.

### Clinical Pearl / High-Yield Fact
When evaluating a patient for a possible fracture, remember to look for displacement of bone fragments on imaging studies, as this is a key diagnostic sign.

### Correct Answer: D. Displacement of bone fragments