Lipid Biochemistry USMLE Step 1 Practice Question
A 41-year-old woman presents for routine lipid screening. Vital signs are normal (BP 118/76, HR 72, RR 14). Fasting labs show HDL 65 mg/dL, triglycerides 95 mg/dL, and LDL calculated at 85 mg/dL using the Friedewald equation. Repeat measurement by direct immunoassay yields LDL 110 mg/dL. She denies statin use and has no history of recent illness or dietary changes. Which factor best explains this discrepancy between the two calculation methods?
Answer choices
- AThe patient's triglycerides are too low to use the Friedewald equation accurately
- BThe patient has a VLDL remnant abnormality causing equation overestimation
- CThe patient's HDL includes lipoprotein X, inflating the estimate
- DThe patient has elevated Lp(a) particles that are counted as LDL in the direct methodCorrect answer
- ELaboratory calibration error occurred between the two measurements
- FThe patient has Type III hyperlipoproteinemia with abnormal apoE isoforms affecting particle composition
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