**Core Concept**
The Brachial plexus is a complex network of nerves that arise from the spinal cord, providing motor and sensory innervation to the upper limb. The posterior cord is one of the three main divisions of the brachial plexus, along with the lateral and medial cords.

**Why the Correct Answer is Right**
The long thoracic nerve of Bell arises from the roots of the brachial plexus (C5-C7) and supplies the serratus anterior muscle, which is involved in scapular rotation and upward movement of the arm. In contrast, the posterior cord of the brachial plexus primarily contains the axillary nerve, radial nerve, and upper subscapular nerve, which provide motor and sensory innervation to the shoulder and arm.

**Why Each Wrong Option is Incorrect**
**Option A:** The long thoracic nerve of Bell is actually a branch of the roots of the brachial plexus, not the posterior cord.

**Option B:** The axillary nerve is indeed a branch of the posterior cord, which is responsible for innervating the deltoid and teres minor muscles.

**Option C:** The radial nerve is also a branch of the posterior cord, providing motor innervation to the triceps brachii and sensory innervation to the back of the arm.

**Option D:** The upper subscapular nerve is another branch of the posterior cord, which supplies the subscapularis muscle.

**Clinical Pearl / High-Yield Fact**
The posterior cord of the brachial plexus is a critical structure to recognize in surgical and clinical settings, as injuries to this area can result in significant morbidity, including weakness and sensory deficits in the arm.

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