Okay, let's tackle this question. The question is asking which sinus infection most commonly leads to periorbital cellulitis. Hmm, periorbital cellulitis is an infection of the eyelid and surrounding tissues, right? I remember that there are two types of orbital cellulitis: preseptal (periorbital) and postseptal (which is more serious). The common sources for infections spreading to the orbit are the paranasal sinuses, especially the ethmoid and frontal sinuses.

Wait, the ethmoid sinuses are close to the orbit. The ethmoid air cells are located between the nose and the eye, so an infection there could easily spread through the thin bone into the periorbital area. The frontal sinuses are above the eyes, but the ethmoid is more anterior and closer. Also, in children, the frontal sinuses are underdeveloped, so maybe ethmoid is more common in them. Let me think about the options again. The question is about the most common cause. So ethmoid sinusitis would be the answer here. The other sinuses like maxillary or sphenoid are less likely to cause periorbital cellulitis. So the correct answer should be ethmoid sinus infection.

**Core Concept** Periorbital cellulitis is an infection of the eyelid and surrounding soft tissues, typically caused by bacterial spread from adjacent structures. The **ethmoid sinuses**, located between the nasal cavity and the orbit, are the most common source due to their proximity and thin bony walls.

**Why the Correct Answer is Right** Ethmoid sinusitis causes periorbital cellulitis via direct extension through the lamina papyracea, a thin bony wall separating the ethmoid sinus from the orbit. This is particularly common in children, whose ethmoid sinuses are more developed and anatomically closer to the orbit than frontal sinuses. *Staphylococcus aureus* and *Streptococcus* species are frequent pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** Maxillary sinusitis is less likely to cause periorbital involvement due to its more posterior location.
**Option B:** Frontal sinusitis is rare in children (underdeveloped sinuses) and less common in adults for this complication.
**Option C:** Sphenoid sinusitis typically spreads to the cavernous sinus, not the orbit.

**Clinical Pearl / High-Yield Fact** Remember the "E" for ethmoid: **E**thmoids are **E**xposed and **E**asy to infect, making them the leading cause of orbital complications. Distinguish periorbital (eyelid swelling, no proptosis) from postseptal cellulitis (proptosis, ophthalmoplegia), which requires urgent imaging.

**Correct Answer: C. Ethmoid sinusitis**

✓ Correct Answer: A. Ethmoidal