**Core Concept**
Choanal atresia is a congenital anomaly characterized by the abnormal closure of the posterior nasal choanae, which are the posterior openings of the nasal cavity into the nasopharynx. This condition can be unilateral or bilateral and is often associated with other congenital abnormalities.

**Why the Correct Answer is Right**
The investigation of choice for choanal atresia is a CT (Computed Tomography) scan of the sinuses and nasal cavity. This imaging modality allows for the evaluation of the bony and soft tissue structures involved in the atresia. Specifically, CT scans can identify the presence and extent of the bony or membranous obstruction, as well as any associated congenital anomalies.

**Why Each Wrong Option is Incorrect**
**Option A:** X-ray is not the investigation of choice for choanal atresia because it may not provide adequate detail of the bony structures involved.
**Option B:** MRI (Magnetic Resonance Imaging) may be useful in evaluating soft tissue structures, but it is not the primary imaging modality for choanal atresia due to its limited ability to visualize bony structures.
**Option C:** Ultrasound is not typically used for the evaluation of choanal atresia due to the limited accessibility and imaging resolution of the posterior nasal choanae.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of choanal atresia is the presence of nasal regurgitation of formula or breast milk in neonates, which occurs due to the inability to breathe through the nose while feeding.

**Correct Answer: C. MRI (Magnetic Resonance Imaging) may be useful in evaluating soft tissue structures, but it is not the primary imaging modality for choanal atresia due to its limited ability to visualize bony structures.