**Core Concept**
Nursemaid's elbow, also known as radial head subluxation, is a common injury in young children, typically occurring between 1-4 years of age. It involves a partial dislocation of the radial head from the annular ligament, leading to pain and limited mobility in the affected elbow.

**Why the Correct Answer is Right**
The injury often occurs when a child's arm is forcibly pronated and extended, causing the radial head to subluxate from the annular ligament. The annular ligament is a fibrous ring that surrounds the head of the radius, providing stability to the elbow joint. In nursemaid's elbow, the ligament becomes caught around the neck of the radius, resulting in the characteristic deformity and pain. The normal X-ray in this case is due to the fact that the injury is a subluxation, not a fracture.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not match the clinical presentation of nursemaid's elbow. Osteomyelitis would typically present with fever, swelling, and pain in the affected limb.

**Option B:** This option is incorrect because it does not accurately describe the mechanism of injury. A fracture would typically require a more significant force, such as a fall onto an outstretched hand.

**Option C:** This option is incorrect because it does not match the age and presentation of the child. Juvenile idiopathic arthritis typically presents with chronic joint pain and swelling, not acute pain after a specific injury.

**Clinical Pearl / High-Yield Fact**
In young children, a painful, pronated, and extended elbow with a normal X-ray is highly suggestive of nursemaid's elbow. Treatment typically involves reduction of the subluxation, which can be achieved by supination of the forearm and flexion of the elbow.

**Correct Answer: D. Nursemaid's elbow**