**Core Concept:**
Hemiazygos veins are collateral veins that arise from the caudal portion of the superior mesenteric vein and the right renal vein. They connect to the hemiazygos artery and eventually drain into the superior vena cava. In a normal anatomy, these veins are not visible on imaging studies as they are small and numerous.

**Why the Correct Answer is Right:**
The correct answer is right because the hemiazygos veins are not mentioned in the options provided. In the context of the question, we are discussing the exceptions to the presence or function of hemiazygos veins.

A. Hemiazygos vein is always present: This is incorrect because the hemiazygos vein is not always present. In some cases, it may be absent, causing a connection between the superior mesenteric vein and the right renal vein, bypassing the hemiazygos vein.

B. Hemiazygos vein drains into the azygos vein: This is incorrect because the hemiazygos vein actually drains into the superior vena cava, not the azygos vein.

C. Hemiazygos vein is a single large vessel: This is incorrect because the hemiazygos vein is a network of numerous small veins, not a single large one.

D. Hemiazygos vein is seen in all cases: This is incorrect because the hemiazygos vein is not seen in all cases. In some individuals, it may be absent or fused with other veins.

**Why Each Wrong Option is Incorrect:**

A. Absence of hemiazygos vein: This is incorrect because the hemiazygos vein can be absent, which is the case mentioned above.

B. Hemiazygos vein drains into the azygos vein: This is incorrect because the hemiazygos vein actually drains into the superior vena cava.

C. Hemiazygos vein is a single large vessel: This is incorrect because the hemiazygos vein is not a single large vessel but a network of numerous small veins.

D. Hemiazygos vein seen in all cases: This is incorrect because the hemiazygos vein is not seen in all cases. It can be absent or fused with other veins in some individuals.

**Clinical Pearl:**

The hemiazygos vein is a crucial landmark during surgical procedures involving the thoracic cavity, as its presence or absence can significantly affect the surgical approach and outcomes. Knowledge of its anatomy helps surgeons plan and execute procedures safely and effectively.