**Core Concept**
The development of paranasal sinuses is a complex process that involves the formation and growth of air-filled cavities within the bones of the face. The timing and sequence of sinus development are critical in understanding various clinical conditions, including sinusitis and congenital anomalies.

**Why the Correct Answer is Right**
The first sinus to develop is the maxillary sinus, which forms from an outpouching of the oral ectoderm called the nasolacrimal duct. This occurs during the 2nd month of fetal development, when the maxillary process begins to grow and eventually forms the maxillary bone. The maxillary sinus then expands and grows, eventually becoming the largest of the paranasal sinuses.

**Why Each Wrong Option is Incorrect**
**Option A:** The frontal sinus is actually the last of the paranasal sinuses to develop, typically forming around 6-8 years of age.
**Option B:** The ethmoid sinus is composed of multiple small air cells, but it is not the first sinus to develop. It grows and expands from the nasal cavity during fetal development.
**Option C:** The sphenoid sinus is the smallest of the paranasal sinuses and develops last, typically around 10-15 years of age.

**Clinical Pearl / High-Yield Fact**
The paranasal sinuses are lined with mucous membranes that produce mucus, which helps to humidify and warm the air we breathe. The sinuses also contain tiny hair-like structures called cilia, which help to move mucus and debris out of the sinuses.

**Correct Answer:** A. Maxillary sinus.