This case report describes a 20-year-old woman with Turner's syndrome who presented with reduced effort tolerance limited by dyspnea. She had previously been on pediatric cardiology follow-up for congenital subvalvular aortic stenosis first diagnosed at age 7. Unfortunately she defaulted after two visits before any intervention could be done. Transthoracic echocardiography demonstrated severe aortic incompetence (AI) with a membrane-like structure in the left ventricular outflow tract (LVOT). The mean pressure gradient across the LVOT on continuous wave Doppler was 41 mmHg. The membranous interventricular septum appeared aneurysmal and it was observed that the "subaortic membrane" had a connection to the anterolateral papillary muscle via a strand of chordal tissue. Further images were captured using two-dimensional and three-dimensional transthoracic and transesophageal echocardiography (iE33, Philips Medical Systems, Andover, MA, USA). After a review of the literature it was concluded that this appeared to be an accessory mitral valve (AMV) leaflet causing LVOT obstruction associated with AI. AMV tissue is a rare congenital malformation causing LVOT obstruction. Because it is so unusual, it may not be immediately recognizable even in a high volume echocardiography laboratory. The clue which helped with the diagnosis was the strand of chordal tissue which connected the mass to the papillary muscle. This anomaly is often associated with LVOT obstruction.