**Core Concept**
Echocardiography is a non-invasive imaging modality that utilizes ultrasound waves to produce images of the heart and its surrounding structures. It is particularly useful for assessing cardiac function and detecting abnormalities such as pericardial effusion, which is the accumulation of fluid in the pericardial space surrounding the heart.

**Why the Correct Answer is Right**
Echocardiography is the investigation of choice for pericardial effusion because it provides real-time, high-resolution images of the heart and its surrounding structures. It allows for the accurate measurement of pericardial fluid thickness and the assessment of cardiac function. Echocardiography is also non-invasive, making it a safe and convenient option for patients. The presence of pericardial effusion can be confirmed by the presence of an echogenic stripe (representing the fluid) between the epicardium and the pericardium.

**Why Each Wrong Option is Incorrect**
**Option A:** MRI (Magnetic Resonance Imaging) is not typically used for the initial assessment of pericardial effusion due to its higher cost and limited availability compared to echocardiography.
**Option B:** CT (Computed Tomography) scans can be used to diagnose pericardial effusion, but they are less sensitive than echocardiography and carry a higher radiation dose.
**Option C:** X-ray is not sensitive for detecting pericardial effusion, as the fluid may not be visible on a standard chest X-ray unless it is large or located in a specific area.

**Clinical Pearl / High-Yield Fact**
In cases of suspected pericardial effusion, it is essential to obtain an echocardiogram as soon as possible to assess cardiac function and guide further management. A large pericardial effusion can lead to cardiac tamponade, a life-threatening condition that requires prompt treatment.

**✓ Correct Answer: D. Echo**