Why echo and not angiography?

From UTD:

Aortic disease

As recommended in the 2010 American College of Cardiology/American Heart Association/American Association for Thoracic Surgery thoracic aorta guidelines, echocardiography is recommended at initial diagnosis and at six months to assess the aortic root and ascending aorta in patients with MFS

Approach to diagnosis of MFS

MFS is most commonly diagnosed using the 2010 revised Ghent Criteria. These are based on the presence or absence of family history, physical examination, imaging of the aorta, and genetic testing in some cases.

The revised Ghent nosology puts greater weight on aortic root dilatation/dissection and ectopia lentis as the cardinal clinical features of MFS and on testing for mutations in FBN1. For the aortic criteria, aortic root Z score calculators are available for children and adults.

In the absence of family history of MFS

For individuals without a family history of MFS, the presence of one of any of the following criteria is diagnostic for MFS:

• Aortic criterion (aortic diameter Z ≥2 or aortic root dissection) and ectopia lentis.

• Aortic criterion (aortic diameter Z ≥2 or aortic root dissection) and a causal FBN1 mutation.

• Aortic criterion (aortic diameter Z ≥2 or aortic root dissection) and a systemic score ≥7.

• Ectopia lentis and a causal FBN1 mutation that has been identified in an individual with aortic aneurysm.

https://www.uptodate.com/contents/genetics-clinical-features-and-diagnosis-of-marfan-syndrome-and-related-disorders

https://pubmed.ncbi.nlm.nih.gov/20591885/

TL;DR Echo for aortic diameter

Marfan syndrome (you know, hinted at by the familial tall stature and weak hypermobile joints) is associated with a dilated/aneurysmal aortic root, which can worsen, dissect and/or rupture if not monitored.