A 40-yeаr-оld femаle pаtient with a histоry оf hypertension presents for a routine examination. Auscultation of the chest reveals a pansystolic murmur best heard at the cardiac apex, radiating to the axilla. There is a third heart sound, S3. The murmur is high pitched and harsh blowing. You listened to her heart using the diaphragm of the stethoscope when she was in the supine position. Which of the following is most likely the etiology of the murmur?