7 year old female, came with insidious-onset, progressive dyspnea on exertion for 1.5 years. Physical examination revealed mildly elevated jugular venous pressure with prominent A-wave and palpable hepatomegaly. Chest X-ray showed generalized cardiomegaly.
Electrocardiography showed normal sinus rhythm with incomplete RBBB and left ventricular hypertrophy strain pattern. Echocardiography revealed features of biventricular RCMP showing bilaterally enlarged atrial chambers, biventricular hypertrophy and diastolic dysfunction with a normal left ventricular ejection fraction (LVEF). What diagnosis and treatment should be the suggested?