A 7-year-old boy with uncorrected tetralogy of Fallot (TOF) is evaluated for progressive dyspnea and exercise intolerance. Echocardiography shows a ventricular septal defect (VSD) with right ventricular outflow tract (RVOT) obstruction. His parents ask about the natural history and prognosis if surgery is not performed. Which of the following pathophysiologic changes occurs FIRST and is the PRIMARY driver of the right-to-left shunt in TOF?