**Core Concept**
The clinical measurement of circulating plasma cholesterol concentrations involves the quantification of various lipoproteins, including low-density lipoprotein (LDL), high-density lipoprotein (HDL), and very-low-density lipoprotein (VLDL). Among these, a specific lipoprotein is responsible for the majority of the circulating cholesterol.

**Why the Correct Answer is Right**
The major contributor to routine clinical measurements of circulating plasma cholesterol concentrations is LDL. This is because LDL is the primary carrier of cholesterol in the bloodstream, accounting for approximately 60-70% of total cholesterol. LDL particles are produced in the liver and are responsible for delivering cholesterol to peripheral tissues. Elevated levels of LDL are associated with an increased risk of atherosclerotic cardiovascular disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** HDL is often referred to as "good" cholesterol, but it is not the major contributor to circulating plasma cholesterol concentrations.
* **Option B:** VLDL is a lipoprotein that transports endogenous triglycerides from the liver to peripheral tissues, but it is not the primary carrier of cholesterol in the bloodstream.
* **Option D:** Chylomicrons are lipoproteins that transport exogenous triglycerides from the intestine to the liver, but they are not the major contributor to circulating plasma cholesterol concentrations.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the ratio of total cholesterol to HDL (TC/HDL) is a useful clinical marker for assessing cardiovascular risk. A higher TC/HDL ratio is associated with an increased risk of cardiovascular disease.

**Correct Answer: C. LDL. Low-density lipoprotein.**