Fetal Hypoplastic Left Heart Syndrome

A 33-year-old G3P2 at 22 weeks gestation is referred for fetal echocardiography after an abnormal four-chamber view is noted on routine anomaly scan. Fetal growth is normal, amniotic fluid volume is normal, and extracardiac anatomy is unremarkable. On detailed fetal echocardiography, the structure marked **B** demonstrates a slit-like, almost absent left ventricle that does not form the cardiac apex, with atretic mitral and aortic valves, a hypoplastic ascending aorta, and reversed transverse arch flow on colour Doppler fed by the ductus arteriosus. The right ventricle is dilated and hypercontractile, forming the entire apex. Which of the following best describes the ultrasound findings at the structure marked **B** in hypoplastic left heart syndrome?