**Core Concept**
A supracondylar fracture of the humerus is a type of fracture that occurs just above the elbow joint in children. It is a common injury in pediatric orthopedics, often resulting from a fall onto an outstretched hand. The fracture can be classified into extension and flexion types based on the direction of the displacement.

**Why the Correct Answer is Right**
The ulnar nerve is not the most commonly involved nerve in supracondylar fractures. The most commonly affected nerve is the median nerve, which can be compressed due to the swelling and displacement of the fracture fragments. The median nerve passes close to the fracture site and can be compromised, leading to symptoms such as numbness, tingling, and weakness in the hand. The ulnar nerve, on the other hand, is less commonly involved in these fractures.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is correct. Supracondylar fractures are indeed uncommon after the age of 15 years, as they typically occur in children between the ages of 4 and 7 years.
**Option B:** This statement is also correct. Extension type fractures are more common than flexion type fractures, accounting for approximately 98% of all supracondylar fractures.
**Option C:** This statement is correct. Cubitus varus deformity is a common complication of supracondylar fractures, particularly if the fracture is not properly reduced and immobilized. It is characterized by a varus (inward) angulation of the elbow.

**Clinical Pearl / High-Yield Fact**
A key aspect of managing supracondylar fractures is early recognition and prompt reduction of the fracture to prevent complications such as cubitus varus deformity and nerve damage. It is essential to maintain a high index of suspicion for this type of fracture in children who present with elbow pain and swelling.

✓ Correct Answer: D. Ulnar nerve is most commonly involved