A boy fell down from a tree and has fracture of neck of humerus. He cannot raise his arm because of the involvement of:
**Core Concept**
The question is testing the understanding of the anatomy and function of the muscles and nerves involved in the movement of the shoulder joint. The neck of the humerus is a common site for fractures, and the inability to raise the arm suggests involvement of the muscles or nerves responsible for abduction of the arm.
**Why the Correct Answer is Right**
The muscles responsible for abduction of the arm are the deltoid and supraspinatus muscles. The suprascapular nerve innervates these muscles and is responsible for their function. When the neck of the humerus is fractured, it can cause damage to the suprascapular nerve, leading to weakness or paralysis of the muscles involved in arm abduction. This is because the suprascapular nerve passes close to the neck of the humerus and can be injured during the fracture.
**Why Each Wrong Option is Incorrect**
* **Option A:** The axillary nerve is responsible for innervation of the deltoid muscle, but it is not primarily involved in abduction of the arm. The axillary nerve is more involved in sensation and motor function of the shoulder joint.
* **Option B:** The radial nerve is responsible for innervation of the triceps muscle and is not primarily involved in abduction of the arm.
* **Option C:** The musculocutaneous nerve is responsible for innervation of the biceps brachii and brachialis muscles, which are involved in flexion of the elbow, not abduction of the arm.
**Clinical Pearl / High-Yield Fact**
The suprascapular nerve is a common site for injury during shoulder trauma, and damage to this nerve can lead to weakness or paralysis of the muscles involved in arm abduction.
**Correct Answer:** C.