**Core Concept**
Acute aortic dissection is a life-threatening condition involving a tear in the intima of the aortic wall, leading to hematoma and separation of the aortic layers. This can result in direct involvement of valves or coronary arteries, causing complications such as aortic regurgitation (AR), mitral regurgitation (MR), or myocardial ischemia due to coronary artery involvement.

**Why the Correct Answer is Right**
Aortic dissection can lead to **aortic regurgitation (AR)** due to dissection involving the aortic valve or root. **Mitral regurgitation (MR)** may occur if the dissection extends to the left ventricle or affects the mitral valve apparatus, especially in proximal dissections. **Pericardial effusion** can result from rupture of the aortic wall or inflammatory response. However, **acute myocardial infarction (AMI)** is not a direct manifestation of dissection; it occurs only if a coronary artery is involved (e.g., via a dissecting plaque), which is rare. Thus, AMI is not a typical or expected feature of aortic dissection and is more commonly due to coronary artery disease.

**Why Each Wrong Option is Incorrect**
Option A: Pericardial effusion can occur due to rupture or inflammation in dissection, especially in descending aortic involvement.
Option B: Aortic regurgitation (AR) is a common manifestation due to dissection involving the aortic root or valve.
Option D: Although AMI is not common, it can occur if a coronary artery is dissected, making it a rare but possible feature.

**Clinical Pearl / High-Yield Fact**
Mitral regurgitation (MR) is a **rare** feature of aortic dissection, and its presence should prompt consideration of aortic root involvement, but it is not a hallmark. Most dissections present with chest pain, hypertension, and signs of aortic regurgitation. AMI is not a typical feature and should not be assumed in dissection.

✓ Correct Answer: C. MR