Which of the following bony landmarks is not palpable externally?

The inferior occipital protuberance is located on the posterior aspect of the skull, specifically the occipital bone. While it serves as an important attachment point for ligaments and muscles, it is not easily palpable externally because it is situated beneath a layer of muscle and other tissues. The structure's position makes it less accessible to examination, requiring more invasive or thorough techniques, such as imaging, to identify.

In contrast, the glabella, mandible fossa, and inferior nuchal line can be palpated with relative ease. The glabella is located between the eyebrows on the frontal bone, the mandibular fossa can be found just anterior to the ear where the mandible articulates with the skull, and the inferior nuchal line is located at the base of the skull where muscles attach. These structures are closer to the surface and have less tissue coverage, making them readily accessible during a physical examination.