**Core Concept**
Dissecting aortic aneurysm is a life-threatening condition where the inner layer of the aorta tears, leading to bleeding into the middle layer and potential rupture. Accurate diagnosis is crucial for timely management.

**Why the Correct Answer is Right**
MRI is the best diagnostic tool for dissecting aortic aneurysm due to its high sensitivity and specificity in visualizing the aortic wall and its layers. MRI can identify the intimal flap, the false lumen, and the true lumen, allowing for precise diagnosis and planning of treatment. The MRI protocol typically involves a thoracic and abdominal aorta sequence with and without contrast.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan, while useful in emergency situations, may not provide the same level of detail as MRI in visualizing the aortic wall layers and intimal flap. CT scan is often used for initial evaluation but may not be as accurate as MRI for diagnosing dissection.

**Option C:** Angiography, particularly in the past, was used to diagnose dissection by injecting contrast into the aorta. However, it is not the best diagnostic tool due to its invasive nature and limited ability to visualize the aortic wall layers. Angiography is more useful for guiding interventions.

**Option D:** X-ray is not a diagnostic tool for dissecting aortic aneurysm. While chest X-ray may show signs of aortic dissection such as mediastinal widening or pleural effusion, it is not specific or sensitive enough for diagnosis.

**Clinical Pearl / High-Yield Fact**
In cases of suspected aortic dissection, MRI should be performed as soon as possible, ideally within 24-48 hours, to accurately diagnose and plan treatment. This can significantly impact patient outcomes.

**✓ Correct Answer: B. MRI**