Mrs. Salter is an 81-year-old lady who presents for evaluation of near-syncopal episodes. Her 12-lead ECG, complete blood count, and comprehensive metabolic panel are all essentially normal. A urinalysis shows some trace protein but is otherwise normal. A 24-hour rhythm monitor demonstrates rare premature ventricular contractions. Mrs. Salter has been in good health and had her first episode at her aerobics class. She only gets lightheaded when she exerts herself. Her heart auscultates for a grade III/VI systolic crescendo-decrescendo murmur at the second intercostal space, right sternal border. The AGACP considers that the likely cause of the syncope is: