**Core Concept**
Pericardial effusion refers to the accumulation of excess fluid in the pericardial space, which can lead to cardiac tamponade and compromise cardiac function. The diagnosis of pericardial effusion requires a combination of clinical evaluation, imaging studies, and laboratory tests.

**Why the Correct Answer is Right**
The correct answer is **Echocardiography**. Echocardiography is a non-invasive imaging modality that allows for the direct visualization of the pericardial space and the assessment of pericardial fluid accumulation. It is particularly useful in the emergency setting, as it can quickly diagnose cardiac tamponade and guide pericardiocentesis. Echocardiography can also provide information on cardiac function, including the presence of right ventricular diastolic dysfunction, which is a hallmark of cardiac tamponade.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chest X-ray is not sensitive for the diagnosis of pericardial effusion, as it requires a significant amount of fluid accumulation to be visible.
* **Option B:** Electrocardiogram (ECG) may show nonspecific changes in the setting of pericardial effusion, but it is not diagnostic.
* **Option C:** Cardiac enzymes are not specific for the diagnosis of pericardial effusion, as they can be elevated in a variety of cardiac conditions.

**Clinical Pearl / High-Yield Fact**
In the setting of cardiac tamponade, the pericardial pressure is elevated, leading to a decrease in cardiac output and potentially life-threatening hypotension. Echocardiography is essential in this scenario to quickly diagnose cardiac tamponade and guide pericardiocentesis.

**Correct Answer: E. Echocardiography**