A 52-year-old man presents with recurrent myocardial infarction despite optimal statin therapy. His fasting lipid profile shows: total cholesterol 280 mg/dL, LDL-C 180 mg/dL, HDL-C 35 mg/dL, triglycerides 450 mg/dL. Physical examination reveals tendon xanthomas and corneal arcus. Which investigation is most appropriate to confirm the suspected diagnosis?

Explanation

## Clinical Presentation Analysis The patient presents with: - Early recurrent MI (age 52) despite statin therapy - Tendon xanthomas and corneal arcus (pathognomonic signs) - Markedly elevated LDL-C (180 mg/dL) unresponsive to statins - Low HDL-C and elevated triglycerides These findings are classic for **familial hypercholesterolemia (FH)**, specifically the heterozygous form given the presentation in the 5th decade. ## Why LDL Receptor Gene Mutation Analysis? **Key Point:** LDL receptor gene mutations are the primary cause of familial hypercholesterolemia in ~85% of cases, causing defective or absent LDL receptors and impaired clearance of LDL particles. **High-Yield:** Genetic testing for LDL receptor mutations is the gold standard confirmatory test for FH and enables: - Definitive diagnosis - Risk stratification (homozygous vs. heterozygous) - Family screening and cascade testing - Identification of candidates for PCSK9 inhibitors or apheresis ## Differential Investigations | Investigation | Role | Why Not Here | |---|---|---| | Lipoprotein electrophoresis | Identifies lipoprotein patterns (Type IIa, IIb, III, IV, V) | Non-specific; does not confirm FH or identify genetic defect | | ApoB and ApoA-I levels | Assess particle number and quality; useful in metabolic syndrome | Supportive but not confirmatory for FH; does not identify genetic cause | | Lp(a) measurement | Independent CV risk factor; may be elevated in FH | Useful for risk assessment but not diagnostic for FH | **Clinical Pearl:** Tendon xanthomas are virtually pathognomonic for FH and warrant immediate genetic testing. They are absent in other dyslipidemias and indicate severely elevated LDL-C exposure over years. ## Diagnostic Criteria for FH Dutch Lipid Clinic Network criteria (widely used): - Family history of premature CAD or xanthomas - LDL-C >190 mg/dL (untreated) in adults - Physical signs (xanthomas, arcus cornalis) - **LDL receptor gene, APOB, or PCSK9 mutations** (definitive) **Mnemonic: FH-GENE** — Familial Hypercholesterolemia requires GENetic confirmation for Exact diagnosis and management.