This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Lipoproteins, Subject name: Biochemistry
A patient was found to have high LDL, increased total cholesterol. But normal levels of LDL-Receptors. What is the most probable cause: 
Select and explain. 
A. Apo B100 mutation, B. Apo E defect, C. LCAT Deficiency, D. Lipoprotein lipase Deficiency
A: Apo B100 mutation
Answer: A: Apo B100 mutation 
Explanation: 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.