A 28-year-old Indian male with a family history of sudden cardiac death presents with chest pain and dyspnea. Clinical examination reveals arachnodactyly, pectus carinatum, and a positive wrist sign. Echocardiography shows the lesion marked **A** in the diagram — annulo-aortic ectasia with effacement of the sinotubular junction. Which of the following molecular mechanisms BEST explains the pathophysiology of this aortic lesion in Marfan syndrome?

Question

Accepted Answer

D. Defective fibrillin-1 destabilizes microfibrils and dysregulates TGF-β signaling, leading to cystic medial necrosis and progressive aortic wall weakening. ## Why option 1 is right

The clinical anchor is that Marfan syndrome is caused by FBN1 mutations encoding fibrillin-1, a key structural component of microfibrils. Defective fibrillin-1 destabilizes microfibrils and dysregulates TGF-β signaling (fibrillin-1 normally sequesters latent TGF-β); excess TGF-β activity drives progressive weakening of the aortic media. The characteristic lesion is cystic medial necrosis — fragmentation and loss of elastic fibers, smooth muscle cell loss, and accumulation of mucoid/proteoglycan material. This pathophysiology directly explains the annulo-aortic ectasia with effacement of the sinotubular junction marked **A** (Revised Ghent Nosology 2010; ACC/AHA Aortic Disease Guidelines 2022).

## Why each distractor is wrong

- **Option 2**: COL3A1 mutations cause Ehlers-Danlos syndrome type IV (vascular EDS), not Marfan syndrome. While both present with aortic aneurysm, the molecular basis is different — type III collagen defect versus fibrillin-1 defect.
- **Option 3**: Lysyl oxidase deficiency is associated with connective tissue laxity in some rare conditions, but it is not the mechanism of Marfan syndrome. Marfan pathophysiology centers on fibrillin-1 and TGF-β dysregulation, not lysyl oxidase.
- **Option 4**: α-1 antitrypsin deficiency causes early-onset emphysema and can predispose to abdominal aortic aneurysm in adults, but it is autosomal recessive and does not explain the systemic skeletal and ocular features (arachnodactyly, ectopia lentis) seen in Marfan syndrome.

**High-Yield:** Marfan syndrome = FBN1 mutation → defective fibrillin-1 → TGF-β dysregulation → cystic medial necrosis → annulo-aortic ectasia and ascending aortic aneurysm (leading cause of death in Marfan).

[cite: Revised Ghent Nosology 2010; ACC/AHA Aortic Disease Guidelines 2022]

Answer

A. Mutation in the COL3A1 gene encoding type III collagen causes defective collagen cross-linking in the aortic media

Answer

B. Autosomal recessive inheritance of α-1 antitrypsin deficiency leads to unopposed neutrophil elastase activity and aortic wall degradation

Answer

C. Deficiency of lysyl oxidase impairs elastin and collagen maturation, resulting in fragmentation of elastic fibers

Explanation

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