**Core Concept**
Choanal atresia is a congenital condition characterized by the partial or complete obstruction of the posterior nasal choanae, which are the openings between the nasal cavity and the nasopharynx. The preferred imaging modality for diagnosing choanal atresia is crucial for accurate assessment and planning of treatment.

**Why the Correct Answer is Right**
The preferred imaging modality for choanal atresia is high-resolution computed tomography (CT) of the paranasal sinuses and the posterior nasal cavity. CT scans provide detailed images of the bony structures, including the nasal cavity, sinuses, and the posterior choanae. This allows for accurate assessment of the extent and location of the atresia, as well as any associated anomalies, such as bony or soft tissue obstructions.

**Why Each Wrong Option is Incorrect**
**Option A:** Magnetic Resonance Imaging (MRI) is not the preferred imaging modality for choanal atresia, as it may not provide sufficient detail of the bony structures involved.
**Option B:** Plain radiographs, such as a lateral neck X-ray, may show some signs of choanal atresia, but they are not sufficient for accurate diagnosis and assessment.
**Option C:** Ultrasound is not a suitable imaging modality for choanal atresia, as it is not effective in visualizing the bony structures of the nasal cavity and sinuses.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that high-resolution CT scans of the paranasal sinuses and posterior nasal cavity are the gold standard for diagnosing choanal atresia, and should be performed in any patient suspected of having this condition.

**Correct Answer:** C. Computed Tomography (CT) scan