A 33-year-old teacher presents who complains of being short of breath with mild exercise (i.e., walking across campus to class). He reports no chest pain, has a BMI=21 kg/m2, is a nonsmoker, and his blood pressure is 116/64 mm Hg. Physical examination reveals a Grade 3/6 crescendo-decrescendo systolic murmur that radiates to the neck. These findings are most consistent with: