**Core Concept**
Humeral supracondylar fractures are a common injury in children, typically occurring in the distal humerus just above the elbow joint. The surrounding soft tissues, including nerves, are at risk of injury due to the force and displacement of the fracture.

**Why the Correct Answer is Right**
The most commonly injured nerve in humeral supracondylar fractures is the **median nerve**. This is because the median nerve is closely related to the medial epicondyle of the humerus, which is often involved in the fracture. The median nerve arises from the brachial plexus, specifically from the C5-T1 roots, and courses down the arm through the cubital fossa, where it is vulnerable to injury from the fracture fragments.

**Why Each Wrong Option is Incorrect**
**Option A:** The **ulnar nerve** is also at risk in elbow injuries, but it is less commonly injured in humeral supracondylar fractures. This is because the ulnar nerve is more lateral and is not as closely related to the medial epicondyle.
**Option B:** The **radial nerve** is not typically injured in humeral supracondylar fractures, as it is more lateral and is not as closely related to the fracture site.
**Option C:** The **axillary nerve** is not commonly injured in humeral supracondylar fractures, as it is more proximal and is not as closely related to the fracture site.

**Clinical Pearl / High-Yield Fact**
Remember that the **median nerve** is the most commonly injured nerve in humeral supracondylar fractures, and a thorough examination of the median nerve function is essential in the assessment of these injuries.

**Correct Answer:** A. Median nerve.