A 33-year-old patient has developed dyspnea during physical exertion, palpitations, disruptions of heart rate, swollen legs. In the childhood the patient had a case of acute rheumatic fever that required in-patient treatment. There were no further requests for medical care. Objectively: heart rate is 92/min., rhythmic; BP is 110/70 mm Hg. At the apex the I heart sound is increased, triple rhythm, diastolic murmur. What heart disease is most likely?