An 82-year-old previously healthy male presents with exertional dyspnea and lower extremity edema. Physical examination reveals a IV/VI systolic murmur at the right upper sternal border peaking late in systole with no audible S2 heart sound. The murmur radiates to the cardiac apex where it sounds holosystolic but does not increase with handgrip maneuvers. The carotid upstroke is weak and delayed. Echocardiography confirms the presence of aortic valve stenosis with a mean pressure gradient of 45 mm Hg and an aortic valve area of 0.8 cm2.
Which of the following is the degree of aortic stenosis present?