A 2-year-old boy with Down syndrome presents to the clinic with his mother who reports he becomes short of breath during play. On examination, he has mild cyanosis of the lips and fingertips, a holosystolic murmur best heard at the left lower sternal border, and a single loud S2. Vital signs: BP 95/60 mmHg, HR 118/min, RR 28/min, SpO2 88% on room air. Echocardiography shows a large ventricular septal defect (VSD) with bidirectional shunting and elevated pulmonary artery systolic pressure of 65 mmHg. There is no hepatomegaly. Which of the following best explains why this patient is experiencing mild cyanosis despite a predominantly left-to-right shunt at baseline?

1. A)Pulmonary vascular resistance has progressively increased to approach systemic vascular resistance, creating bidirectional shunting with right-to-left flow during exertionGABARITO
2. B)Right-to-left shunting is the primary hemodynamic pattern due to right ventricular outflow tract obstruction
3. C)The large VSD has caused complete mixing of pulmonary and systemic circulations, resulting in consistent cyanosis independent of shunt direction
4. D)Pulmonary vascular disease has developed with pulmonary vascular resistance now exceeding systemic vascular resistance, causing predominantly right-to-left shunting
5. E)Left-to-right shunting causes chronic pulmonary edema leading to hypoxemia and cyanosis