**Core Concept:**
The brachial plexus is a network of nerves that control the muscles of the upper limb. It is formed by the union of the ventral rami of C5 to T1 spinal nerves. The posterior cord contains motor neurons that innervate the muscles of the arm and posterior shoulder girdle.

**Why the Correct Answer is Right:**
Injuries to the posterior cord of the brachial plexus, caused by a knife wound to the neck, would disrupt the supply of motor neurons to the muscles of the arm and posterior shoulder girdle. This results in paralysis of the muscles innervated by these neurons.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because the ulnar nerve, which is separate from the brachial plexus and supplies the muscles of the forearm, would not be affected by a neck injury involving the brachial plexus.
B. This option is incorrect because the radial nerve, another separate nerve that supplies the muscles of the forearm, would not be affected by a neck injury involving the brachial plexus.
C. This option is incorrect because the suprascapular nerve, a separate branch of the brachial plexus that supplies the rotator cuff muscles, would not be affected by a neck injury involving the brachial plexus.
D. This option is correct, as the injury to the posterior cord of the brachial plexus would result in paralysis of the muscles innervated by this cord, which includes the muscles of the arm and posterior shoulder girdle.

**Clinical Pearl:**
In cases of brachial plexus injuries, careful examination of the affected limb should be done to identify other associated symptoms and signs, such as loss of sensation, weakness, or deformity, which can provide further information about the extent of the injury. In some cases, nerve conduction studies or electromyography may be necessary to confirm the diagnosis and guide management.

**Correct Answer:** D. The posterior cord of the brachial plexus supplies the muscles of the arm and posterior shoulder girdle, and its injury would result in paralysis of these muscles. The correct answer is D, as it accurately describes the affected muscles due to a neck injury involving the brachial plexus.