**Core Concept**
The long thoracic nerve of Bell is responsible for innervating the serratus anterior muscle, which plays a crucial role in stabilizing the scapula against the thoracic wall and facilitating movements such as forward elevation and upward rotation of the arm.

**Why the Correct Answer is Right**
The serratus anterior muscle helps to rotate the scapula upward and around the chest wall, allowing the arm to be lifted overhead. When the long thoracic nerve of Bell is injured, the serratus anterior muscle becomes paralyzed, leading to winging of the scapula. This is because the muscle is unable to maintain its normal attachment to the thoracic wall, causing the scapula to protrude outward and appear "winged." The serratus anterior muscle is supplied by the long thoracic nerve of Bell, which arises from the C5-C7 spinal nerve roots.

**Why Each Wrong Option is Incorrect**
**Option B:** The ulnar nerve primarily innervates the intrinsic muscles of the hand and forearm, and its injury is associated with symptoms such as claw hand and weakness of the hand muscles.

**Option C:** The lower subscapular nerve innervates the subscapularis muscle, which helps to stabilize the shoulder joint and rotate the arm medially. Injury to this nerve would not result in winging of the scapula.

**Option D:** The thoracodorsal nerve innervates the latissimus dorsi muscle, which is involved in adduction and extension of the shoulder joint. Its injury would not cause winging of the scapula.

**Clinical Pearl / High-Yield Fact**
Winging of the scapula can also be caused by other conditions such as muscular dystrophy, polio, or a congenital defect. However, the most common cause is an injury to the long thoracic nerve of Bell.

**✓ Correct Answer: A. Long thoracic nerve of Bell**