A Study to Evaluate the Effect of Aficamten in Pediatric Patients With Symptomatic Obstructive Hypertrophic Cardiomyopathy (oHCM).

Inclusion Criteria: * Period 1: Treatment Period * Males and females between 12 and \< 18 years of age at screening and at Day 1. * Body weight ≥ 45 kg for the initial cohort and then body weight ≥ 35 kg after at least 10 participants in the initial cohort have undergone dose titration up to Week 4 without observed events of LVEF \< 50% at the starting dose of 5 mg qd. * Core laboratory confirmation of the following oHCM echocardiographic criteria at screening: * Left ventricular (LV) hypertrophy with nondilated LV chamber in the absence of other cardiac disease. * LV end-diastolic wall thickness that meets a threshold of: * Z-score \> 2.5 in the absence of family history OR * Z-score \> 2 in the presence of positive family history or positive genetic test. * LVEF ≥ 60% AND Valsalva LVOT-G ≥ 50 mmHg. * oHCM of sarcomeric origin confirmed by genetic testing or, if unable to confirm by genetic testing, oHCM of sarcomeric origin may be presumed in the absence of history of metabolic disorders, mitochondrial cardiomyopathies, neuromuscular disease, malformation syndromes, infiltrative diseases/inflammation, and endocrine disorders (such as Fabry's disease, Noonan syndrome with left ventricular hypertrophy, and amyloid-cardiomyopathy). * New York Heart Association (NYHA) Class ≥ II at screening. * Adequate acoustic windows for echocardiography. * Participants on beta blockers, verapamil, diltiazem, or disopyramide should have been on stable doses for more tha…