A 6-month-old female infant from rural Maharashtra presents with progressive cyanosis (SpO₂ 65% on room air), failure to thrive, and squatting posture during episodes of dyspnea. Clinical examination reveals a single loud S₂ and a systolic ejection murmur at the left upper sternal border. Chest X-ray shows a 'boot-shaped' heart with right ventricular hypertrophy and decreased pulmonary vascular markings. Echocardiography confirms tetralogy of Fallot (TOF) with severe right ventricular outflow tract (RVOT) obstruction. The infant is currently on no medications. What is the most appropriate immediate management?