Congenital Heart Disease USMLE Step 1 Practice Question

A 7-year-old boy with a history of cyanotic heart disease presents to the emergency department with acute worsening of dyspnea and cyanosis. Physical examination reveals severe cyanosis, a single loud S2, and diminished pulmonary vascular markings on chest X-ray. Over the past year, his parents report he has had progressively severe exercise intolerance. When examined, he demonstrates a 'boot-shaped' heart on imaging. An echocardiogram reveals absence of the tricuspid valve, a hypoplastic right ventricle, and right-to-left shunting across a patent foramen ovale. The child's arterial oxygen saturation is 65% on room air. Which of the following is the most likely diagnosis?