A 36-Year-Old Man Is Referred by His Family Physician to the Pulmonary

A 36-year-old man is referred by his family physician to the pulmonary clinic. He complains of shortness of breath following physical activity and decreased capacity for exercise. He says that strenuous exercise including yard work is impossible without sitting down and resting every few minutes. After taking several deep breaths during the physical examination, he begins to wheeze. He is not a smoker and works in an office not exposed to dust, fumes, or other irritants. He appears slightly jaundiced. Serum alpha-1-antitrypsin (AAT) concentration analysis is below normal and is followed up with AAT phenotype and DNA testing that indicates one copy of $S$ and one of $\mathrm{Z}$ mutations with $40 \%$ abnormal AAT production. Urinalysis shows elevated levels of desmosine and isodesmosine. These excreted compounds normally contribute to efficient lung function by which of the following mechanisms?

A) Posttranslational modification of fibrillin
B) Cross-linking elastin
C) Activating elastase
D) Activating AAT
E) Binding type IV collagen to elastin