A two-year old boy presented with episodes of becoming dusky. On examination, there was central cyanosis and clubbing. There was no pallor, oedema or respiratory distress. The hea was normal sized with a parasternal heave. A systolic thrill was palpable over the left middle sternal border. First hea sound was normal and only the aoic component was audible in the second hea sound. Liver was not enlarged - What would be the likely diagnosis ?