According to the International 10-20 system, which landmark is the most challenging to locate?

In the context of the International 10-20 system, the inion is often regarded as the most challenging landmark to accurately locate. The inion refers to the most prominent point of the external occipital protuberance at the back of the skull. Its variability in prominence among individuals can make it difficult to identify consistently compared to other landmarks.

The other landmarks mentioned have more definitive anatomical locations. The nasion is the bridge of the nose, which is a readily visible surface landmark, making it easier to locate. The apex of the ear, which is typically identified at the uppermost point of the pinna, is also more consistently identifiable due to its positioning relative to the ear's structure. The vertex, being the highest point on the skull, can usually be found using the mid-sagittal line and can be palpated with relative ease.

Given these considerations, the inion's anatomical variability and less discernible position contribute to its classification as the most challenging landmark within the 10-20 system.