**Core Concept**
Aortic dissection is a serious medical condition where there is a tear in the inner layer of the aorta, causing blood to flow between the layers of the aorta, potentially leading to aortic rupture. The investigation of choice should be able to rapidly and accurately diagnose the extent and severity of the dissection.

**Why the Correct Answer is Right**
The investigation of choice for aortic dissection is contrast-enhanced computed tomography (CT) angiography. This imaging modality provides detailed information about the aorta, including the location and extent of the dissection, and can also help identify any associated complications such as aortic rupture or cardiac tamponade. CT angiography is the preferred imaging modality because it is rapid, widely available, and can be performed in patients who are unstable.

**Why Each Wrong Option is Incorrect**
**Option A:** While plain X-rays may show signs of aortic dissection, such as mediastinal widening or a "double aortic contour," they are not sensitive or specific enough to be the investigation of choice.
**Option B:** Echocardiography may show signs of aortic dissection, such as an intimal flap or aortic regurgitation, but it is not as sensitive or specific as CT angiography and is also operator-dependent.
**Option C:** MRI may provide detailed information about the aorta, including the extent of the dissection, but it is not as rapidly available as CT angiography and may not be suitable for unstable patients.

**Clinical Pearl / High-Yield Fact**
In patients with suspected aortic dissection, a rapid and accurate diagnosis is crucial to guide management and prevent aortic rupture. CT angiography is the investigation of choice because it is rapid, widely available, and can be performed in patients who are unstable.

**Correct Answer:** C. Contrast-enhanced computed tomography angiography.