A 32-year-old woman presents with a 3-month history of progressive dyspnea, chest pain, and recurrent syncope. On examination, she has a loud systolic murmur best heard at the left sternal border. Chest X-ray shows a boot-shaped heart with reduced pulmonary vascularity. ECG reveals right axis deviation and right ventricular hypertrophy. What is the imaging modality of choice to confirm the diagnosis and assess the severity of the defect?