A 28-year-old woman from rural India presents with progressive dyspnea on exertion for 6 months and orthopnea. She has a history of recurrent sore throat in childhood. On examination, she has an irregular pulse (rate 110/min), a loud S1, an opening snap at the apex, and a diastolic murmur best heard at the apex in the left lateral decubitus position. Chest X-ray shows pulmonary congestion and a straightened left heart border. Echocardiography reveals a mitral valve area of 1.2 cm² with commissural fusion and restricted leaflet motion. What is the most likely pathological finding in the mitral valve?