A 68-year-old man with a 10-year history of rheumatoid arthritis presents with progressive dyspnea and orthopnea. Echocardiography shows biventricular wall thickening with preserved ejection fraction and characteristic "apical sparing" on strain imaging. A cardiac biopsy is performed and stained with Congo red. Under polarized light microscopy, the structure marked **B** (apple-green birefringence) is observed in the myocardium. Which of the following best describes the pathophysiologic basis of this patient's cardiac presentation?