A 21-year-old male has been progressively short of breath and fatigued. He has not seen a physician since childhood. Physical examination reveals a loud IV/VI systolic ejection murmur at the left lower sternal border much louder with Valsalva as well as a mid-systolic murmur at the cardiac apex. An echocardiogram shows an interventricular septal thickness of 35 mm. Which of the following is the likely cause of the murmur at the cardiac apex?
A. Left ventricular outflow tract obstruction
B. Tricuspid regurgitation
C. Austin-Flint murmur
D. Venturi effect
E. A ventricular septal defect