**Core Concept**
The adrenal glands are located near the kidneys and have a unique blood supply and drainage system. The **right adrenal vein** is a key structure in this system, and its drainage is crucial for understanding adrenal function and pathology.

**Why the Correct Answer is Right**
The **right adrenal vein** drains directly into the **inferior vena cava**, which is a major vein that carries deoxygenated blood from the lower body to the heart. This direct drainage allows for the quick transport of adrenal hormones into the systemic circulation.

**Why Each Wrong Option is Incorrect**
**Option A:** The **renal vein** is not the correct answer because the right adrenal vein does not drain into it.
**Option B:** The **hepatic vein** is also incorrect as it is not directly related to adrenal vein drainage.
**Option C:** The **superior mesenteric vein** is not relevant to adrenal vein drainage.

**Clinical Pearl / High-Yield Fact**
Remembering the direct drainage of the **right adrenal vein** into the **inferior vena cava** is crucial for understanding conditions like **Cushing's syndrome** and **pheochromocytoma**.

**Correct Answer:** D. Inferior vena cava