A 6-year-old girl from Mumbai is brought to the clinic for routine school examination. She is asymptomatic and thriving well. On auscultation, a fixed, widely split second heart sound (S2) is noted. There is a soft ejection systolic murmur at the left upper sternal border. Chest X-ray shows mild cardiomegaly with increased pulmonary vascular markings. Echocardiography confirms a left-to-right shunt at the atrial level with a Qp:Qs ratio of 2:1. What is the most likely diagnosis?