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A 4-year-old boy presents for a well-child visit. His mother notes that he breathes fast and his lips turn "dusky" when he runs or plays hard. The symptoms resolve once he stops the activity. On examination, he has a II/VI left upper sternal border systolic murmur that radiates to the back; a faint click is heard. Which of the following is the most likely cause of this child’s exercise intolerance?
A. Asthma
B. Atrial septal defect
C. Pulmonary valve stenosis
D. Tricuspid atresia
E. Ventricular septal defect