**Core Concept**
The surgical neck of the humerus is a common site for fractures, particularly in older adults. The surrounding musculature and nerves are at risk of injury in such fractures. The most common nerve involvement is due to the anatomical relationship between the nerves and the humerus.

**Why the Correct Answer is Right**
The axillary nerve is most commonly involved in fractures of the surgical neck of the humerus. This is because the axillary nerve runs close to the surgical neck, and fractures in this area can easily damage the nerve. The axillary nerve is a branch of the posterior cord of the brachial plexus and supplies the deltoid muscle and the skin over the shoulder. Damage to the axillary nerve can lead to weakness of the deltoid muscle and sensory loss over the lateral aspect of the arm.

**Why Each Wrong Option is Incorrect**
**Option A:** The radial nerve is not commonly involved in fractures of the surgical neck of the humerus. While it is a branch of the posterior cord of the brachial plexus, its course is more lateral and is not as closely associated with the surgical neck as the axillary nerve.

**Option B:** The musculocutaneous nerve is not typically involved in fractures of the surgical neck of the humerus. It is a branch of the lateral cord of the brachial plexus and supplies the biceps brachii, brachialis, and coracobrachialis muscles.

**Option C:** The median nerve is not commonly involved in fractures of the surgical neck of the humerus. It is a branch of the medial cord of the brachial plexus and supplies the muscles of the anterior compartment of the arm, as well as the skin over the palmar surface of the hand.

**Clinical Pearl / High-Yield Fact**
In cases of suspected axillary nerve injury, it is essential to examine the patient for deltoid weakness and sensory loss over the lateral aspect of the arm.

**Correct Answer:** A. The axillary nerve.