Arrhythmia responding to Adenosine

A 17 day old baby presented with complaints of vomiting every feed and excessive crying. Child had BCG vaccination just 3 days ago and parents were attributing excessive cry due to pain following vaccination. But on examination there was a heart rate of 300, min, though no murmur could be heard with respiratory distress and hepatomegaly. Chest x-ray showed cardiomegaly. Electrocardiogram {ECG} showed narrow QRS complex with absent P wave. Child was given oxygen and IV adenosine at a dose of 0.1mg, kg was given by rapid flush technique. Cardiac rhythm reverted to normal sinus rhythm. Further ECG was done which showed presence of delta wave which is a slurred upstroke in the QRS complex with short PR interval. Echocardiography and Holter monitoring were normal. Child was put on propranolol and is doing fine since then.



What is the diagnosis?