**Core Concept**
The azygos system, including the azygos vein and its tributaries, is a vital component of the venous drainage of the thoracic wall. The azygos vein plays a crucial role in returning deoxygenated blood from the posterior walls of the thorax to the superior vena cava, thereby facilitating the return of blood to the heart.

**Why the Correct Answer is Right**
The last tributary of the azygos vein is the hemiazygos vein. The hemiazygos vein, along with the accessory hemiazygos vein, drains into the azygos vein, providing venous drainage from the left side of the thoracic wall. This is in contrast to the right side, which is primarily drained by the azygos vein itself. The hemiazygos vein is a crucial tributary due to its role in draining the left hemithorax, which includes the left lung, left mediastinal structures, and the left intercostal spaces.

**Why Each Wrong Option is Incorrect**
**Option A:** The azygos vein's primary tributaries are the right ascending lumbar vein, the right subcostal vein, and the right bronchial veins. The hemiazygos vein is not among these primary tributaries.
**Option B:** The accessory hemiazygos vein is a tributary of the hemiazygos vein, not the azygos vein itself. It drains into the hemiazygos vein, which then drains into the azygos vein.
**Option C:** The superior intercostal vein is a tributary of the brachiocephalic vein, not the azygos vein. It drains the upper two intercostal spaces on the left side.

**Clinical Pearl / High-Yield Fact**
The azygos system is often affected in conditions such as superior vena cava syndrome, where obstruction of the superior vena cava can lead to collateral venous circulation through the azygos system. Understanding the anatomy of the azygos system is crucial in diagnosing and managing such conditions.

**Correct Answer:** C. The hemiazygos vein.