Which of the following is most difficult to locate?

Porion can be difficult to locate reliably on a cephalometric radiograph. A radiopaque marker is placed on the rod that extends into the external auditory meatus as part of the cephalometric head positioning device, and the location of this marker, referred to as "machine porion" is often used to locate porion. The shadow of the auditory canal can be seen on cephalometric radiographs, usually located slightly above and posterior to machine porion. The upper edge of this canal can also be used to establish "anatomic porion."