An 8-year-old boy is admitted with chest tightness and palpitations for 1 week. Past medical history is unremarkable. Physical examination: BP 120/70 mmHg, heart rate 75 bpm, thin build, mild cyanosis of the lips, coarse breath sounds in both lungs without rales, heart rate 75 bpm with regular rhythm, and a grade III/6 systolic murmur at the left 3rd intercostal space. Investigations: CXR shows prominence of the pulmonary artery segment, CBC reveals WBC 10.5×10<sup>9</sup>/L, ECHO demonstrates a 5 mm ventricular septal defect (VSD) with left-to-right shunt, and ECG shows sinus rhythm. Based on these findings, the diagnosis is: