A 23-year-old woman with a history of a childhood heart murmur reports 6 months of progressive exertional dyspnea and new episodic lightheadedness. She has also noticed that her lips sometimes look "bluish" after climbing stairs. Physical exam shows central cyanosis and digital clubbing. Cardiac auscultation reveals a loud, single second heart sound (accentuated P2). Oxygen saturation is 84% on room air. Echocardiography demonstrates severe pulmonary hypertension and right ventricular hypertrophy in the setting of a previously documented congenital left-to-right shunt. Which of the following best explains the pathophysiologic change responsible for her new cyanosis?

(Assume no acute pulmonary embolus or parenchymal lung disease.)