Infective Endocarditis

What is the optimal timing for administering antibiotic prophylaxis before surgery?

A 28-year-old female presents with progressive dyspnea on exertion and palpitations for the past 6 months. She has a history of recurrent sore throat episodes during childhood. On examination, she has an irregularly irregular pulse, and auscultation reveals a mid-diastolic rumbling murmur at the apex with presystolic accentuation. ECG shows atrial fibrillation with controlled ventricular rate and left atrial enlargement. Echocardiogram demonstrates mitral valve stenosis with commissural fusion, leaflet thickening, and a mitral valve area of 1.2 cm². What is the most likely diagnosis?