A patient was found to have high LDL, increased total cholesterol. But normal levels of LDL-Receptors. What is the most probable cause:

This is Familial defective Apo B100, also known as Autosomal Dominant Hypercholesterolemia IIa. There is a mutation in the gene encoding Apo B100, specifically in LDL-receptor binding domain of Apo B100.