**Core Concept**
The fallen fragment sign is a radiological finding associated with a specific type of femoral neck fracture. It is an important diagnostic clue in orthopedic trauma.

**Why the Correct Answer is Right**
The fallen fragment sign is seen in femoral neck fractures, particularly in the context of a femoral neck impaction fracture. This type of fracture occurs when a fragment of bone is displaced into the joint capsule due to a fall onto a flexed hip. The fragment becomes "impacted" into the surrounding bone, leading to a characteristic appearance on radiographs. The fallen fragment sign is a result of the radiolucent line between the impacted fragment and the surrounding bone, which can be seen on an anteroposterior (AP) or oblique radiograph of the hip.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the fallen fragment sign is not typically associated with hip dislocations. While hip dislocations can result in similar-looking radiographs, the mechanism and appearance of the fracture are distinct.
* **Option B:** This option is incorrect because the fallen fragment sign is not a feature of osteoarthritis. While osteoarthritis can lead to joint space narrowing and bone spurs, it does not typically result in a fallen fragment sign.
* **Option C:** This option is incorrect because the fallen fragment sign is not a characteristic finding in hip fractures that are not associated with an impaction mechanism. Other types of hip fractures, such as those caused by a direct blow to the hip, may have a different appearance on radiographs.

**Clinical Pearl / High-Yield Fact**
The fallen fragment sign is an important diagnostic clue in femoral neck fractures, particularly in patients who present with a history of a fall onto a flexed hip. It is essential to consider this sign in the differential diagnosis of hip pain and to obtain appropriate radiographs to confirm the diagnosis.

**Correct Answer:** D.