**Core Concept**
Posterior elbow dislocation occurs when the ulna and radius bones are displaced posteriorly relative to the humerus bone. This injury typically results from a forceful extension of the elbow joint, causing the radius and ulna to move backward in relation to the humerus. The radiographic appearance of posterior elbow dislocation is a key diagnostic feature.

**Why the Correct Answer is Right**
In a posterior elbow dislocation, the ulna and radius bones are displaced posteriorly, which can lead to a characteristic "cubitus varus" deformity. On an x-ray, the radial head may be hidden behind the capitellum of the humerus, and the ulna may overlap the distal humerus. This overlap can cause the appearance of a "tear drop" or "crescent" sign on the x-ray, indicating the posterior displacement of the ulna and radius. The correct answer is related to this specific radiographic feature.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe the characteristic radiographic appearance of posterior elbow dislocation.

**Option B:** This option is incorrect because it describes a different type of elbow injury, such as a fracture or a dislocation in a different direction.

**Option C:** This option is incorrect because it does not accurately describe the radiographic appearance of posterior elbow dislocation.

**Clinical Pearl / High-Yield Fact**
In posterior elbow dislocation, the radial head may be hidden behind the capitellum of the humerus, making it difficult to visualize on x-ray. This can lead to a delay in diagnosis if the clinician is not aware of this characteristic radiographic feature.

**Correct Answer: D. Tear drop or crescent sign on x-ray.**