You are a healthcare provider working in a primary care clinic. A 65-year-old male patient comes in for a routine check-up. During the physical examination, you auscultate his heart and detect a harsh, crescendo-decrescendo murmur at the upper right sternal border. The patient denies any chest pain but reports occasional shortness of breath with exertion. His blood pressure is 140/82 mmHg, and his pulse is regular at 75 beats per minute.
What is the most likely cause of the murmur detected during the physical examination of the 65-year-old male patient?