Membranous septal aneurysm causing severe right ventricular outflow obstruction in an adult with trisomy 18.

Membranous ventricular septal aneurysm is a known entity but rarely causes severe right ventricular outflow obstruction. We report a 40-year-old female with trisomy 18 who developed severe right ventricular outflow obstruction caused by an enormous membranous septal aneurysm associated with unrepaired inlet ventricular septal defect with perimembranous extension.