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Marfan variable expressivity — MRCPCH TAS MCQ

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HardGeneticsMarfan variable expressivityMRCPCH TAS

A 15-year-old boy is tall with lens subluxation and aortic root dilatation. His father had an aortic dissection at 35 years but has no ocular history. What best explains the different manifestations in father and son?

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Correct answer: B — Variable expressivity of an autosomal dominant disorder

Marfan syndrome is usually autosomal dominant due to FBN1 variants, and affected relatives may show variable expressivity. Anticipation would imply earlier or more severe disease from repeat expansion, which is not the mechanism in Marfan syndrome. The pearl is that the same pathogenic variant can produce different organ involvement within a family.

Reference: Nelson Textbook of Pediatrics; RCPCH Theory examination syllabi; Lissauer's Illustrated Textbook of Paediatrics