Which description matches heterozygous FH?

• A. FF; defective LDL receptors; LDL levels above 600 mg/dL
• B. Ff; 50% defective LDL receptors; LDL levels 300-350 mg/dL
• C. ff; normal LDL receptors; LDL levels below 100 mg/dL
• D. Ff; 1 normal gene, 1 defective gene; LDL 300-350 mg/dL

Answer: (D)

Heterozygous FH occurs when one normal LDL receptor gene and one defective LDL receptor gene are present. This means liver cells still have some functional receptors, but their activity is reduced, so the clearance of LDL from the blood is impaired enough to raise LDL cholesterol substantially, though not to the extreme levels seen when both copies are defective. That’s why the description that specifies one normal gene and one defective gene with LDL around 300–350 mg/dL fits best. In contrast, having both genes defective leads to very high LDL levels (often above 600 mg/dL) characteristic of the homozygous form, while having both normal genes gives normal LDL levels.