Marfan Syndrome Wrist and Thumb Signs

An 18-year-old male college volleyball player presents to cardiology after pre-participation screening revealed a systolic murmur, tall stature, and abnormal body habitus. He has a family history of sudden cardiac death and aortic dissection in male relatives. On examination, the structure marked **A** (positive Walker-Murdoch wrist sign with thumb-fifth finger overlap) is demonstrated along with ectopia lentis, aortic root dilatation (48 mm at sinuses of Valsalva), and mitral valve prolapse. Which of the following best explains the pathophysiologic basis for the skeletal findings demonstrated by structure **A** in this patient?