A 45-year-old man presents with progressive weakness of grip and a characteristic hand deformity. Clinical examination reveals hyperextension at the metacarpophalangeal joints with flexion at the interphalangeal joints of the 4th and 5th fingers bilaterally. Sensory loss is noted over the medial 1.5 fingers. The structures marked **C** in the diagram are responsible for the loss of motor function. Which of the following best explains the mechanism of the observed deformity?