**Core Concept**
The azygos system is a network of veins that drain deoxygenated blood from the thoracic wall and the abdominal wall into the superior vena cava. The azygos vein itself is formed by the union of the right ascending lumbar and right subcostal veins, and it receives blood from several tributaries.

**Why the Correct Answer is Right**
The left superior intercostal vein is actually a tributary of the brachiocephalic vein (also known as the innominate vein), which is a major vein of the thorax that drains into the superior vena cava. In contrast, the right superior intercostal vein is a tributary of the azygos vein, as are the right posterior intercostal veins and the accessory hemiazygos vein. The azygos system is primarily responsible for draining the right side of the thoracic wall, whereas the left side is drained by the left superior intercostal vein via the brachiocephalic vein.

**Why Each Wrong Option is Incorrect**
**Option A:** The right posterior intercostal veins are indeed tributaries of the azygos vein, which receives blood from these veins to drain the right side of the thoracic wall.
**Option B:** The right superior intercostal vein is a tributary of the azygos vein, which is essential for draining the right side of the thoracic wall.
**Option D:** The accessory hemiazygos vein is also a tributary of the azygos vein, which helps to drain the left side of the thoracic wall.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the azygos system is primarily responsible for draining the right side of the thoracic wall, whereas the left side is drained by the brachiocephalic vein. This knowledge can be useful in identifying potential sites of thrombosis or obstruction in the venous system.

**✓ Correct Answer: C. Left superior intercostal**