Tetralogy of Fallot, right ventriculogram, right anterior oblique projection. The infundibular septum (IS) is arrowed. This is one of those few cases when the IS does not develop any hypertrophy, and remains as it began, a thin septum separating the aortic from the pulmonary valves, extending below them a little to contibute in the normal to closure of the upper ventricular septum. In ordinary tetralogy it is tempting to look at the hypertrophied IS (below, arrowed) as simply the crista displaced.
In fact, the normal crista (below,arrowed) is a thin structure. It is not a thick band of muscle: on the other side of it is the aortic root.
