**Core Concept**
The imaging study of choice for para-nasal sinuses should provide high-resolution images of the bony structures and mucosal details. This requires a modality that can effectively visualize the complex anatomy of the sinuses, including the frontal, maxillary, ethmoid, and sphenoid sinuses.

**Why the Correct Answer is Right**
The correct answer is CT (Computed Tomography) scan with thin slices and 1-2 mm collimation, as it offers the best balance between spatial resolution and radiation dose. CT scans can accurately depict the bony structures, mucosal thickening, and air-fluid levels within the sinuses. The thin slice thickness and high-resolution reconstruction algorithms enable detailed visualization of the sinus ostia, septations, and mucosal disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** MRI (Magnetic Resonance Imaging) is not the preferred modality for evaluating the bony structures of the sinuses, although it can provide excellent soft-tissue contrast and is useful in evaluating the orbits and intracranial complications.
* **Option B:** X-ray is not sufficient for evaluating the para-nasal sinuses due to its limited spatial resolution and inability to accurately depict the bony structures and mucosal details.
* **Option C:** Ultrasound is not typically used for evaluating the para-nasal sinuses due to its limited depth penetration and inability to provide high-resolution images of the bony structures.

**Clinical Pearl / High-Yield Fact**
When ordering a CT scan of the sinuses, it is essential to request thin slices (1-2 mm) and high-resolution reconstruction algorithms to ensure accurate visualization of the sinus anatomy and mucosal disease.

**Correct Answer:** C. CT scan.