**Core Concept:** Supracondylar fractures are a type of elbow fracture. They involve the humerus and can lead to nerve injury due to compression or shearing forces. The most commonly injured nerve in this context is the ulnar nerve.

**Why the Correct Answer is Right:** In a supracondylar fracture of the humerus, the ulnar nerve is often at risk due to its close proximity to the fracture line. The ulnar nerve runs along the medial aspect of the arm, passing through the cubital tunnel and branching into the ulnar nerve terminal branches. When a supracondylar fracture occurs, the forces applied to the arm can compress or shear the nerve, leading to injury.

**Why Each Wrong Option is Incorrect:**

A. The radial nerve is not the most commonly injured nerve in supracondylar fracture of the humerus. The ulnar nerve is the primary target due to its anatomy and proximity to the fracture site.
B. The radial nerve is also not the most commonly injured nerve in this context. It is more susceptible to injury in certain circumstances, such as posterior extension injuries, but not in general supracondylar fractures.
C. The median nerve is less likely to be injured in supracondylar fracture of the humerus compared to the ulnar and radial nerves. Its anatomy and location make it less vulnerable to the forces involved in these fractures.
D. This option is incorrect because the posterior interosseous nerve is a variant of the radial nerve and is less commonly injured in supracondylar fracture of the humerus compared to the ulnar and radial nerves.

**Clinical Pearl:** The ulnar nerve is the most commonly injured nerve in supracondylar fracture of the humerus, as it has a high risk of compression or shearing injury due to its anatomy and proximity to the fracture site. Prompt recognition and management of ulnar nerve injury is crucial to ensure optimal patient outcomes.