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A 15-year-old male patient presented with dyspnoea on exertion for 6 months. He had no significant medical history. His blood pressure and pulse rate were within normal limits. Cardiac auscultation revealed a grade 4/6 (Levine scale) high-frequency pansystolic murmur over the cardiac apex conducting to left axilla as well as to base of the heart. Haematological and biochemical evaluation including 12-lead ECG were within normal limits. Chest X-ray showed mild cardiomegaly. Transthoracic echocardiography (TTE) with colour Doppler imaging was done (figure 1).
Which of the following is the most likely diagnosis?