**Core Concept**
The posterior cord of the brachial plexus is a critical anatomical structure involved in the innervation of the shoulder region. It is formed by the combination of nerve fibers from the radial nerve, axillary nerve, and upper subscapular nerve.

**Why the Correct Answer is Right**
The posterior cord of the brachial plexus is formed by the merging of the posterior branches of the upper trunk (C5 and C6) and the middle trunk (C7). The radial nerve, which supplies the muscles of the posterior arm and forearm, arises from the posterior cord. The axillary nerve, which innervates the deltoid and teres minor muscles, also originates from the posterior cord. The upper subscapular nerve, which supplies the subscapularis muscle, is the third component of the posterior cord.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the correct nerves that form the posterior cord.
**Option B:** This option is incorrect as it mentions the anterior cord, which is a separate entity formed by the merging of the lateral and medial cords of the brachial plexus.
**Option C:** This option is incorrect as it mentions the medial cord, which is another component of the brachial plexus that forms the ulnar nerve and the medial antebrachial cutaneous nerve.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the posterior cord of the brachial plexus is formed by the combination of nerve fibers from the upper trunk, middle trunk, and the upper subscapular nerve. This knowledge is crucial for understanding the innervation of the shoulder region and the clinical correlation with various neurological deficits.

**Correct Answer: C. C5, C6, and C7 and the upper subscapular nerve.**