Okay, let's tackle this question. The user provided a question about the first sinus to develop but didn't list the options. Hmm. First, I need to recall the embryological development of the paranasal sinuses.

The maxillary sinus is the first to develop. It starts forming around the 12th week of gestation. The ethmoid sinuses follow, beginning around the 3rd month. The frontal sinuses start developing later, around 2 years of age. The sphenoid sinuses are the last, developing after puberty.

So the correct answer should be the maxillary sinus. Now, for the explanation. The core concept here is embryology of the paranasal sinuses. The user might have options like maxillary, ethmoid, frontal, sphenoid. The correct answer is maxillary. Each wrong option needs a brief explanation. For example, ethmoid develops next, frontal later, and sphenoid last. The clinical pearl is that maxillary is first, ethmoid second, frontal third, sphenoid last. Mnemonics could help here. Need to structure the answer with the required sections and ensure it's concise within the character limit.

**Core Concept**
The paranasal sinuses develop in a specific embryological sequence. The **maxillary sinus** is the first to form, followed by the ethmoid, frontal, and sphenoid sinuses. This sequence reflects the growth of the facial skeleton and pneumatization of the skull.

**Why the Correct Answer is Right**
The **maxillary sinus** begins developing during the **12th week of gestation** as an outpocketing of the nasal epithelium. It is the earliest and most consistently developed sinus in utero. Its early formation is critical for supporting the growth of the maxilla and providing structural integrity to the midface. Unlike other sinuses, its development is not delayed by postnatal factors.

**Why Each Wrong Option is Incorrect**
**Option A:** *Ethmoid sinus* develops after the maxillary sinus, starting in the **3rd trimester** and continuing postnatally.
**Option B:** *Frontal sinus* begins forming **postnatally**, around **2 years of age**, making it the third to develop.
**Option C:** *Sphenoid sinus* develops **after puberty**, as the sphenoid bone pneumatizes later than other facial bones.

**Clinical Pearl / High-Yield Fact**
Remember the sequence with the mnemonic **"Maxillary, Ethmoid, Frontal, Sphenoid" (MEFS)**. The maxillary sinus is fully developed at birth, while others mature gradually. This sequence is vital for interpreting pediatric sinus imaging and understanding developmental anomalies.

**Correct Answer: A. Maxillary sinus**

✓ Correct Answer: A. Maxillary sinus