A 42-year-old Indian male presents with recurrent myocardial infarction and prominent xanthomas on his Achilles tendons and knuckles. Lipid profile shows LDL cholesterol of 320 mg/dL despite atorvastatin 80 mg daily. The structure marked **C** in the diagram (LDL) is not being cleared effectively from the circulation. Genetic testing confirms a heterozygous mutation in the LDL receptor gene on chromosome 19. Which of the following therapeutic agents would be most effective in this patient by increasing the recycling and availability of LDL receptors on peripheral tissues?

Explanation

## Why PCSK9 inhibitor (evolocumab) is right The structure marked **C** (LDL) is cleared from circulation via the LDL receptor (apoB-100 binding) on peripheral tissues. In familial hypercholesterolemia (FH), heterozygous mutations in the LDL receptor gene (chromosome 19) cause impaired LDL clearance, leading to markedly elevated LDL cholesterol (200–400 mg/dL in heterozygotes) and premature coronary artery disease by age 40–50 years, with characteristic tendon xanthomas and corneal arcus. PCSK9 is a serine protease that normally degrades LDL receptors intracellularly, reducing their recycling to the cell surface. PCSK9 inhibitors (monoclonal antibodies such as evolocumab and alirocumab) block PCSK9-mediated degradation, allowing more LDL receptors to be recycled and remain on the cell surface. This dramatically increases LDL clearance capacity, reducing circulating LDL by 50–70%, and is the gold-standard add-on therapy for statin-resistant or high-risk FH patients. [Harper 32e Ch 25; KD Tripathi 9e Ch 45] ## Why each distractor is wrong - **Bempedoic acid**: A uricosuric agent that inhibits xanthine oxidase and reduces uric acid; it has modest LDL-lowering effects (~10–15%) via AMPK activation but does not directly increase LDL receptor recycling and is not indicated for FH management. - **Inclisiran (PCSK9 siRNA)**: While inclisiran is a newer PCSK9-silencing agent that also increases LDL receptor recycling, it is not yet widely available in India and is not the standard first-line PCSK9 inhibitor; evolocumab and alirocumab are the established monoclonal antibody options in Indian clinical practice. - **Proprotein convertase inhibitor (separate from PCSK9)**: Other proprotein convertases (e.g., PCSK7) do not have the same established role in LDL receptor degradation; only PCSK9 inhibition is proven to increase LDL receptor recycling and is guideline-recommended for FH. **High-Yield:** PCSK9 inhibitors block degradation of LDL receptors → more receptors recycled to cell surface → increased LDL clearance; essential add-on in statin-resistant FH. [cite: Harper 32e Ch 25; KD Tripathi 9e Ch 45]