**Core Concept**
Type III hyperlipoproteinemia is a rare genetic disorder characterized by the accumulation of intermediate-density lipoprotein (IDL) and chylomicron remnants in the plasma. This condition results from a deficiency or dysfunction of the lipoprotein lipase enzyme, which is essential for the breakdown and clearance of triglyceride-rich lipoproteins.

**Why the Correct Answer is Right**
The most characteristic finding in Type III hyperlipoproteinemia is the presence of elevated levels of IDL and chylomicron remnants in the plasma. This is due to the impaired activity of lipoprotein lipase, which normally hydrolyzes triglycerides in these lipoproteins, leading to their clearance from the circulation. As a result, these triglyceride-rich lipoproteins accumulate in the plasma, causing hypertriglyceridemia and other characteristic features of the disorder.

**Why Each Wrong Option is Incorrect**
* **Option A:** Elevated levels of very-low-density lipoprotein (VLDL) are more characteristic of Type IV hyperlipoproteinemia, which is associated with a deficiency of apolipoprotein C-II, a cofactor for lipoprotein lipase.
* **Option B:** High levels of low-density lipoprotein (LDL) cholesterol are more typical of familial hypercholesterolemia, which is caused by mutations in the LDL receptor gene.
* **Option C:** Markedly elevated levels of chylomicrons are more characteristic of Type I hyperlipoproteinemia, which is associated with a deficiency of lipoprotein lipase.

**Clinical Pearl / High-Yield Fact**
Type III hyperlipoproteinemia is often associated with atherosclerotic cardiovascular disease, which can be attributed to the accumulation of IDL and chylomicron remnants in the arterial wall.

**Correct Answer:** D. Elevated levels of intermediate-density lipoprotein (IDL) and chylomicron remnants.