Review Question - QID 213569

A 12-year-old boy is brought to the emergency department with a 2-month history of increasing difficulty with physical activity. Specifically, he says that he gets short of breath quickly with exertion even though he used to participate in all activities without a problem. When asked, he endorses sometimes coughing up flecks of blood. His past medical history is unclear because he was recently adopted from abroad and has not yet established care. Physical exam reveals blue discoloration of his lips and fingernails as well as a holosystolic murmur best heard at the lower left sternal border. Which of the following physiologic mechanisms is primarily responsible for this patient's current presentation?