Lipid Biochemistry USMLE Step 1 Practice Question
A 19-year-old man presents with recurrent tendon xanthomas, corneal opacities, and chest pain. Vital signs: BP 138/88, HR 92, RR 16, Temp 37°C, SpO2 98%. Total cholesterol is 680 mg/dL with LDL 620 mg/dL. Corneal arcus is noted on examination. His father died of myocardial infarction at age 35. Genetic testing reveals homozygous loss-of-function mutations in the LDLR gene. Which of the following best explains his lipid phenotype compared to heterozygous familial hypercholesterolemia?
Answer choices
- AHomozygous state leads to selective accumulation of LDL in macrophages only
- BComplete absence of functional LDL receptors prevents hepatic LDL uptake, resulting in markedly elevated LDL and accelerated atherosclerosisCorrect answer
- CHomozygous mutations cause enhanced compensatory apoB-100 synthesis
- DHomozygous LDLR mutations produce twofold higher LDL levels due to gene dosage effects
- ELoss of LDL receptors triggers increased SREBP-2 feedback inhibition
- FHomozygous LDLR deficiency impairs LDL catabolism exclusively through the alternative scavenger receptor pathway
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