Last year a routine health check-up murmur was found in an 11-year old boy without chief complaint of discomfort. Physical examination: BP,110/70mmHg, there is no chest deformity , no thrills are present , A grade II-III/VI ejection-type systolic murmur is heard best at the left sterna border in the second internal costal space S2 is accentuated. Chest X-ray showed : Mild heart enlargement, right atrium and right ventricle to the main, pulmonary artery segment was conspicuous, the lung are ‘’Dancing’’, lung field congestion, reduced aortic shadow. EEG: Right axis deviation, incomplete right bundle branch block What is the most probable diagnosis? Base of diagnosis?