A 3-year-old boy from Delhi presents with recurrent respiratory tract infections and failure to thrive. His mother reports he tires easily during play and has a persistent cough. On examination, there is a continuous 'machinery' murmur best heard at the left infraclavicular region. Chest X-ray shows cardiomegaly with pulmonary congestion. Echocardiography confirms a left-to-right shunt with increased pulmonary blood flow. What is the most likely diagnosis?