Chronic Rheumatic Mitral Stenosis - Fish-Mouth Deformity

A 45-year-old woman with a history of acute rheumatic fever at age 10 presents with 3 years of progressive exertional dyspnoea, orthopnoea, and recent haemoptysis. On examination, she has an irregularly irregular pulse, a loud first heart sound, an opening snap, and a mid-diastolic rumble at the apex. Echocardiography reveals a mitral valve area of 0.9 cm² with a Wilkins score of 10. The gross autopsy specimen shown demonstrates the structure marked **C** — the fish-mouth deformed mitral valve orifice with commissural fusion, leaflet thickening, and chordal shortening. Which of the following pathophysiological mechanisms is responsible for the development of this characteristic deformity?