A 16-year-old boy has markedly elevated cholesterol and cutaneous xanthomas which are bumps or plaques caused by lipid cells building up under the skin (shown in picture below). His father had a myocardial infarction before age 40. Which of the following is the most likely underlying abnormality?

Question

BlackTom AI · Accepted Answer

Familial hypercholesterolemia (FH) results from defective hepatic clearance of LDL particles due to mutations in the LDL receptor (LDLR) gene, so...

1) Relevant knowledge points and concepts
- Tendinous and tuberous xanthomas (bumps/plaques over elbows, knees, Achilles, tendons) are classic signs of chronically elevated LDL cholesterol.
- FH is usually autosomal dominant; heterozygotes develop markedly elevated LDL and premature coronary artery disease (MI often <50, sometimes <40), homozygotes present even earlier.
- Pathophysiology: LDLR mutations reduce number or function of LDL receptors on hepatocytes → decreased uptake/clearance of LDL from plasma → persistently high LDL-cholesterol → lipid deposition in skin/tendons and accelerated atherosclerosis.

2) Step-by-step reasoning
- A 16-year-old with marked hypercholesterolemia plus cutaneous xanthomas indicates chronic very high LDL levels.
- Positive family history of MI before age 40 strongly suggests an inherited, autosomal dominant lipid disorder causing premature atherosclerosis.
- The most common and well-known cause that fits these findings is familial hypercholesterolemia due to LDL receptor gene mutation.

3) Calculations
- None required for diagnosis; clinical correlation of signs and family history is diagnostic suspicion. (Lipid panel would show very elevated LDL-C.)

4) Why the correct answer is correct
- Mutation in the LDL receptor gene directly explains impaired LDL clearance, high LDL, xanthomas and early MI — the hallmark triad of FH.

5) Why the other options are incorrect
- a. “An inherited gene defect” is true in a general sense but nonspecific; the precise abnormality is LDLR mutation.
- b. “Deficiency of erythrocytes” (anemia) is irrelevant to xanthomas or hypercholesterolemia.
- d. “Extra copy of chromosome 21” (Down syndrome) is unrelated to severe LDL elevation and tendon xanthomas.

6) Conclusion
The clinical picture and family history point to familial hypercholesterolemia caused by a mutation in the LDL receptor gene; this confirms answer c.

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