Preseptal cellulitis is infection located anterior to which structure?

• A. Conjunctival fornix
• B. Lacrimal sac
• C. Sclera
• D. Orbital septum

Answer: (D)

The key idea is that the orbital septum acts as the boundary between the eyelid/soft-tissue spaces and the orbit itself. Preseptal cellulitis is an infection located anterior to this membrane, so it involves the eyelid skin and superficial periorbital tissues but does not involve the orbital contents. In contrast, orbital cellulitis lies posterior to the orbital septum and includes the eye socket’s contents, which can produce more serious signs like painful eye movements, restricted extraocular movements, proptosis, and potential vision changes.

Understanding the boundary helps distinguish these conditions. The conjunctival fornix is part of the conjunctiva and not the dividing line for preseptal versus orbital involvement. The lacrimal sac is a drainage structure located medially and behind the septum, not the boundary that defines preseptal infection. The sclera is part of the globe itself and lies behind the conjunctiva and eyelids; its involvement would indicate a different pattern of disease. Therefore, being anterior to the orbital septum correctly identifies preseptal cellulitis.