## **Core Concept**
The condition described, winging of the scapula, is a clinical sign where the medial (inner) border of the scapula protrudes outward, resembling a wing. This occurs due to weakness of the muscles that stabilize the scapula against the thoracic wall. The primary muscle responsible for preventing scapular winging is the **serratus anterior**, which is innervated by the **long thoracic nerve**.

## **Why the Correct Answer is Right**
The long thoracic nerve innervates the serratus anterior muscle. This muscle plays a crucial role in stabilizing the scapula against the thoracic wall and rotating it upward. When the serratus anterior muscle is weakened or paralyzed due to injury of the long thoracic nerve, the scapula cannot be held flat against the back, leading to winging of the scapula, especially when pushing against resistance. This clinical presentation is a classic sign of long thoracic nerve injury.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The suprascapular nerve primarily innervates the supraspinatus and infraspinatus muscles, which are involved in shoulder abduction and external rotation, respectively. Injury to this nerve would not result in scapular winging.
- **Option B:** The musculocutaneous nerve innervates the biceps brachii, brachialis, and coracobrachialis muscles, which are involved in elbow flexion and forearm supination. Its injury would not cause scapular winging.
- **Option D:** The dorsal scapular nerve innervates the rhomboids and levator scapulae muscles, which help in scapular rotation and downward movement. While its injury affects scapular movement, it is less commonly associated with the classic presentation of winging of the scapula compared to long thoracic nerve injury.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that scapular winging can be caused by injury to nerves other than the long thoracic nerve, but the scenario described (serratus anterior weakness leading to winging upon pushing against resistance) specifically implicates the long thoracic nerve. Clinicians should also note that scapular winging can be due to other causes, including muscular dystrophy or direct muscle trauma.

## **Correct Answer:** . **Long thoracic nerve**