Hypertrophic Cardiomyopathy ECG

A 28-year-old male athlete presents to the cardiology clinic for pre-participation screening after a family history of sudden cardiac death in a first-degree relative. His 12-lead ECG shows the pattern marked **B** in the diagram: left ventricular hypertrophy by voltage criteria, deep narrow Q waves in the inferior and lateral leads, and giant negative T-wave inversions (>10 mm, dagger-shaped) in the precordial leads V3–V5. Transthoracic echocardiography confirms left ventricular wall thickness of 18 mm at the apex with a left ventricular outflow tract gradient of 35 mmHg at rest. Which of the following genetic mutations is MOST COMMONLY responsible for this clinical presentation?