**Core Concept**
Inferior vena cava (IVC) obstruction leads to increased pressure in the lower limbs and abdominal veins. To compensate for this, collateral veins open up to bypass the obstructed IVC. This phenomenon is a result of the body's attempt to maintain venous return and prevent edema.

**Why the Correct Answer is Right**
The key collateral veins that open up in case of IVC obstruction are the azygos system, hemiazygos system, and the ascending lumbar veins. The azygos system, which includes the azygos vein and hemiazygos vein, plays a crucial role in draining blood from the lower limbs and abdominal veins into the superior vena cava. The ascending lumbar veins also provide an alternative route for blood to return to the heart via the azygos system. This collateral circulation helps to reduce the pressure in the lower limbs and abdominal veins, thereby preventing edema.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the collateral circulation in case of IVC obstruction.
**Option B:** This option is incorrect as it does not describe a collateral vein that opens up in case of IVC obstruction.
**Option C:** This option is incorrect as it does not accurately describe the collateral veins that open up in case of IVC obstruction.

**Clinical Pearl / High-Yield Fact**
In cases of IVC obstruction, the azygos system is often enlarged, which can be visualized on imaging studies such as CT scans or ultrasound. This is a key clinical finding that can help diagnose IVC obstruction.

**Correct Answer: D.**