A 4-month-old male infant presents with progressive cyanosis since 6 weeks of age and a harsh systolic ejection murmur best heard at the left upper sternal border. Echocardiography confirms tetralogy of Fallot. The structure marked **A** in the diagram is a large subaortic ventricular septal defect. Which of the following best explains why this defect, combined with right ventricular outflow tract obstruction, results in the cyanosis observed in this patient?
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