Marfan Syndrome

A 7-year-old boy is brought to the pediatric clinic for evaluation of tall stature and unusual hand appearance. On examination, the structure marked **A** in the diagram is noted—the child's thumbs protrude beyond the ulnar border of the palm when opposed across it. His arm span exceeds his height by 1.08 times. Echocardiography reveals an aortic root Z-score of 2.1 at the sinuses of Valsalva. Genetic testing confirms a heterozygous mutation in the FBN1 gene on chromosome 15q21. Which of the following best explains the underlying pathophysiology of this patient's connective tissue disorder?