**Core Concept**
The azygos system is a group of veins that drain blood from the posterior wall of the thorax and abdominal wall into the superior vena cava. The azygos vein is a significant vessel that plays a crucial role in the venous drainage of the thoracic cavity.

**Why the Correct Answer is Right**
The last tributary of the azygos vein is the hemiazygos vein, which is a significant vessel that drains blood from the left side of the thoracic wall. The hemiazygos vein merges with the azygos vein at the level of T8 and T9 vertebrae, forming a common vessel that continues as the azygos vein. This anastomosis is an essential part of the venous drainage system of the thoracic cavity.

**Why Each Wrong Option is Incorrect**
**Option A:** The accessory hemiazygos vein is a tributary of the azygos vein, but it is not the last tributary. It drains blood from the left side of the thoracic wall and merges with the hemiazygos vein before joining the azygos vein.

**Option B:** The ascending lumbar vein is a tributary of the azygos vein, but it is not the last tributary. It drains blood from the abdominal wall and merges with the azygos vein in the thoracic cavity.

**Option C:** The right superior intercostal vein is a tributary of the azygos vein, but it is not the last tributary. It drains blood from the upper right intercostal spaces and merges with the azygos vein in the thoracic cavity.

**Clinical Pearl / High-Yield Fact**
Remember that the azygos system is a vital part of the venous drainage of the thoracic cavity, and its tributaries provide a significant pathway for blood to return to the heart.

**Correct Answer:** C. Right superior intercostal vein.