A 52-year-old woman with a 10-year history of rheumatoid arthritis presents with progressive deformity of her right index finger. On examination, she demonstrates flexion at the proximal interphalangeal joint with compensatory hyperextension at the distal interphalangeal joint, creating a characteristic "buttonhole" appearance. The structure marked **A** in the diagram shows the PIP joint flexion component of this deformity. Which of the following best explains the pathomechanics of the PIP joint flexion in boutonnière deformity?