A mother brings her 11-year-old son to your office because he easily becomes short of breath while running. She states that he does not seem to be able to play as long a period of time as his friends. The patient’s blood pressure is 140/60 mmHg and he has bounding peripheral pulses. On auscultation of the heart, you detect a harsh, loud continuous murmur heard best below the left clavicle. Which of the following is the most likely diagnosis?