Which term forms part of the posterior portion of the nose, the orbit, and the floor of the cranium?

• A. Ethmoid bone
• B. Diaphysis
• C. Epiphysis
• D. Endosteum

Answer: (A)

Think of the ethmoid bone as the central bridge between the nose, the eyes, and the skull base. Its perpendicular plate forms the posterior portion of the nasal septum, so it helps define the back part of the nasal cavity. It also has an orbital portion that contributes to the medial wall of each orbit. In addition, the ethmoid contains a cribriform plate that forms part of the floor of the anterior cranial fossa, effectively contributing to the floor of the cranium. This combination of features is why the ethmoid bone fits all three regions.

The other terms aren’t bones themselves and don’t describe a bone that spans these regions—diaphysis and epiphysis refer to parts of a long bone (shaft and ends), and endosteum is the inner lining of bone tissue.