**Core Concept**
The axillary artery is divided into three parts based on its relationship with the pectoralis minor muscle. The third part of the axillary artery is located between the lateral and medial cords of the brachial plexus. Understanding the posterior relations of the third part of the axillary artery is essential for identifying potential sites of compression or injury.

**Why the Correct Answer is Right**
The third part of the axillary artery has several posterior relations, including the subscapular artery, the humeral vein, and the axillary vein. The subscapular artery arises from the axillary artery and passes posteriorly to supply the subscapularis muscle. The humeral vein and axillary vein also lie posterior to the axillary artery, forming a critical venous plexus. These structures are at risk during procedures involving the axillary artery, such as axillary dissection or axillary vein thrombosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe a posterior relation of the third part of the axillary artery.
**Option B:** This option is incorrect because it is not a recognized posterior relation of the third part of the axillary artery.
**Option D:** This option is incorrect because it is actually an anterior relation of the axillary artery.

**Clinical Pearl / High-Yield Fact**
When performing axillary dissections or procedures involving the axillary artery, it is essential to be aware of the posterior relations of the third part of the axillary artery to avoid injury to the subscapular artery, humeral vein, and axillary vein.

**Correct Answer: A.**