## **Core Concept**
The question tests knowledge of the anatomy and function of the shoulder joint, specifically the muscles and nerves involved in arm movement. The neck of the humerus is a common site for fractures, and the injury can affect the surrounding muscles and nerves. The **suprascapular nerve** and **axillary nerve** are at risk due to their proximity to the shoulder joint.

## **Why the Correct Answer is Right**
The correct answer, , involves the **axillary nerve**. The axillary nerve arises from the posterior cord of the brachial plexus and wraps around the neck of the humerus, supplying the **deltoid muscle** and **teres minor**. The deltoid muscle is primarily responsible for **abduction of the arm**, which is the action the boy cannot perform. The axillary nerve's involvement in fractures of the neck of the humerus can lead to weakness or paralysis of the deltoid muscle, making it difficult to raise the arm.

## **Why Each Wrong Option is Incorrect**
* **Option A:** The **musculocutaneous nerve** primarily supplies the biceps brachii, brachialis, and coracobrachialis muscles, which are involved in elbow flexion and forearm supination, not arm abduction.
* **Option B:** The **median nerve** primarily controls muscles involved in wrist and finger flexion and supplies sensation to the palmar surface of the thumb, index, and middle fingers, not arm abduction.
* **Option D:** The **radial nerve** primarily controls extension of the wrist and fingers and supplies sensation to the back of the arm and forearm, not arm abduction.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **axillary nerve** is commonly injured in fractures of the neck of the humerus, leading to difficulty in **abduction of the arm**. Clinically, this can be assessed by asking the patient to abduct their arm against resistance. A useful mnemonic is that the axillary nerve is often affected in humeral neck fractures, resulting in a " **waiter's tip**" deformity.

## **Correct Answer:** . Axillary nerve