**Core Concept**
A supracondylar fracture of the humerus is a common elbow injury in children, typically resulting from a fall onto a flexed elbow. This fracture involves the distal humerus, just above the elbow joint, and can be classified based on the direction of displacement (extension or flexion) and the degree of displacement.

**Why the Correct Answer is Right**
In a flexion-type, fully displaced supracondylar fracture, the posterior humeral artery is at risk of injury due to its proximity to the fracture site. The posterior humeral artery is a branch of the brachial artery, which also gives rise to the median nerve. The median nerve runs through the cubital tunnel and is closely associated with the brachial artery, making it susceptible to injury in cases of supracondylar fractures. The boy's complaint of finger numbness is likely related to median nerve injury.

**Why Each Wrong Option is Incorrect**
**Option A:** The ulnar nerve is less commonly injured in supracondylar fractures compared to the median nerve, as it is not as closely associated with the brachial artery.
**Option B:** The radial nerve is not typically injured in supracondylar fractures, as it is located more laterally and is not as closely associated with the fracture site.
**Option C:** The brachial artery itself is not typically injured in supracondylar fractures, as the posterior humeral artery is the more commonly affected branch.

**Clinical Pearl / High-Yield Fact**
In cases of supracondylar fractures, it is essential to carefully assess the median nerve for signs of injury, such as numbness, weakness, or paresthesia, as this can impact the patient's overall prognosis and treatment plan.

**Correct Answer: D. Median nerve**