**Core Concept**
Nursemaid's elbow, also known as radial head subluxation, occurs when the radius bone in the forearm partially dislocates from the elbow joint, commonly seen in young children.

**Why the Correct Answer is Right**
This condition typically occurs when a child is swung by the forearm, causing the radial head to subluxate out of its normal position. The mechanism of injury involves a sudden pull on the arm, resulting in stretching and tearing of the annular ligament that holds the radial head in place. As a result, the radial head becomes partially dislocated, leading to pain, swelling, and limited mobility in the affected elbow.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fracture of the ulna would not typically result from a forearm swing, and the presentation would likely include more significant pain and instability.
* **Option B:** Supracondylar fracture of the humerus is a more severe injury typically seen in older children, often resulting from a fall onto an outstretched hand.
* **Option D:** Elbow dislocation would be more likely to result from a high-energy trauma or a direct blow to the elbow, rather than a forearm swing.

**Clinical Pearl / High-Yield Fact**
Nursemaid's elbow is a common pediatric injury that can be diagnosed clinically by the characteristic "nurse's elbow" position, where the child holds the affected arm in a flexed and pronated position, with the forearm rotated inward. A simple reduction maneuver by a healthcare provider can often relieve the subluxation and restore normal elbow function.

**Correct Answer:** C.