What defect leads to elevated plasma β lipoprotein levels in familial hypercholesterolemia?

Familial hypercholesterolemia is primarily caused by a defect in the low-density lipoprotein (LDL) receptor, which is responsible for clearing LDL cholesterol from the bloodstream. The specific defect leading to elevated plasma β lipoprotein levels in this condition is associated with a malfunction or absence of the receptor that binds ApoB-100. This apoprotein is essential for the uptake of LDL particles by hepatic cells. When the receptors are defective, the LDL particles remain in circulation instead of being absorbed, causing an accumulation of cholesterol and β lipoproteins in the plasma.

This accumulation is due to the inability of the liver to effectively remove LDL from circulation, leading to higher levels of plasma cholesterol and associated lipoproteins. Thus, the correct answer highlights the crucial role of the ApoB-100 receptor in regulating cholesterol levels, making the defect in this receptor a key factor in familial hypercholesterolemia.