A 72-year-old man presents with progressive exertional dyspnea, episodic chest pain with minimal exertion, and occasional lightheadedness. Physical examination reveals a harsh, grade 4/6 systolic crescendo-decrescendo murmur best heard at the right second intercostal space that radiates to the carotids, as well as a delayed carotid upstroke. Transthoracic echocardiography demonstrates a markedly calcified aortic valve with an estimated valve area of 0.8 cm2. Which of the following is the most likely underlying etiology of his aortic stenosis?