A 2-month-old infant is brought to the clinic because of poor feeding, sweating, and difficulty breathing. The parents state that she was doing very well, and has actually been a “very easy going baby”, until about a week ago. They assumed that she was developing a “cold”, but it has not passed, and the symptoms have been worsening. Cardiac examination reveals a loud, harsh systolic murmur with a thrill that is heard best at the left sternal border. The most likely underlying abnormality is