A 3-month-old girl is brought to the emergency department with a 3-week history of poor feeding. She is at the 10th percentile for height and below the 5th percentile for weight. On examination, temperature is 37 ˚C (98.6 ˚F), pulse is 120/min, respirations are 50/min, and blood pressure is 90/30 mm Hg. She appears to be dyspneic. Physical examination shows a continuous murmur heard at the upper left sternal border. Peripheral pulses are bounding. A chest x-ray shows cardiomegaly with prominence of the left ventricle. Which of the following is the most likely diagnosis?