**Core Concept**
Pulled elbow, also known as radial head subluxation, is a common injury that occurs in young children when the radius bone partially dislocates from the annular ligament that holds it in place. This injury typically occurs due to a sudden pull on the extended arm, causing the radius to slip out of its normal position.

**Why the Correct Answer is Right**
The correct answer, B. Subluxation of head of radius, is the result of a sudden force applied to the extended arm, causing the annular ligament to be stretched or torn, and the radius to partially dislocate. The radial head subluxation is a common injury in young children because their bones are still developing, and their muscles are not yet strong enough to provide adequate support to the joint. The radial head subluxation can be reduced by flexing the elbow and supinating the forearm, which helps to reposition the radius bone.

**Why Each Wrong Option is Incorrect**
**Option A:** Fracture of head of radius - While a fracture of the radial head can occur, it is not the same as a pulled elbow, which involves a subluxation rather than a complete break in the bone.
**Option C:** Fracture dislocation of elbow - A fracture dislocation of the elbow is a more severe injury that involves a break in one or more bones and a disruption of the joint, which is not the same as a pulled elbow.
**Option D:** Fracture ulna - A fracture of the ulna can occur separately from a pulled elbow, and is not directly related to the subluxation of the radial head.

**Clinical Pearl / High-Yield Fact**
A pulled elbow is a common injury in young children, and can be reduced by flexing the elbow and supinating the forearm. It is essential to diagnose and treat this injury promptly to prevent further complications, such as radial head dislocation.

**✓ Correct Answer: B. Subluxation of head of radius**