Which of the following is NOT one of the five skull bones that comprise the base?

The correct answer is identified as the frontal bone because it does not form part of the base of the skull. The base of the skull consists primarily of the ethmoid, sphenoid, and the petrous and mastoid portions of the temporal bone.

The ethmoid bone is a delicate structure situated at the roof of the nasal cavity and part of the orbits. The sphenoid bone is complex and contributes significantly to the floor of the cranial cavity, as well as to the eye socket. The petrous and mastoid portions of the temporal bone are also critical in providing support and space for the inner ear structures. These components together make up the foundational structure that supports the brain and aligns with the cervical spine.

In contrast, the frontal bone is primarily located at the forehead and does not play a role in the base of the skull. Understanding the anatomical positioning of these bones is essential for recognizing how they collectively support the cranial architecture and protect the brain.