A 62-year-old man with a history of hypertension is brought to the emergency room with severe left chest and back pain. His blood pressure is 80/50 mm Hg. Physical examination shows pallor, diaphoresis, and a murmur of aortic regurgitation. An ECG does not show myocardial infarction. An X-ray film of the chest reveals mediastinal widening. Which of the following is the most likely diagnosis?

**Core Concept**
The clinical presentation of severe chest and back pain in a patient with a history of hypertension, along with hypotension, pallor, diaphoresis, and a murmur of aortic regurgitation, suggests aortic dissection. This condition involves a tear in the inner layer of the aorta, leading to blood flow between the layers of the aortic wall.

**Why the Correct Answer is Right**
The presence of mediastinal widening on the chest X-ray is a critical finding that supports the diagnosis of aortic dissection. This is due to the accumulation of blood in the mediastinum as a result of the aortic tear. The murmur of aortic regurgitation is also consistent with aortic dissection, as the tear can compromise the integrity of the aortic valve. The absence of myocardial infarction on the ECG further supports this diagnosis, as the pain and symptoms are not due to coronary artery disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a pulmonary embolism would typically present with sudden onset of dyspnea, cough, and possibly chest pain, but not typically with severe back pain or a murmur of aortic regurgitation.

**Option B:** This option is incorrect because a myocardial infarction would typically present with ECG changes, elevated cardiac enzymes, and possibly chest pain radiating to the arm, but the ECG in this case does not show myocardial infarction.

**Option C:** This option is incorrect because a pneumothorax would typically present with sudden onset of shortness of breath, chest pain, and possibly decreased breath sounds on physical examination, but the mediastinal widening on the chest X-ray is not consistent with a pneumothorax.

**Option D:** This option is incorrect because aortic stenosis would typically present with a systolic ejection murmur, possibly syncope, and possibly angina, but the presence of a murmur of aortic regurgitation and mediastinal widening on the chest X-ray is not consistent with aortic stenosis.

**Clinical Pearl / High-Yield Fact**
Aortic dissection is a medical emergency that requires prompt recognition and treatment. The "3 Ps" of aortic dissection are pain, pulse difference, and pulse pressure, and these should be remembered when evaluating patients with suspected aortic dissection.

**Correct Answer:** Aortic dissection.

✓ Correct Answer: B. Dissecting aneurysm