**Core Concept**
Pericardial effusion refers to the accumulation of excess fluid in the pericardial space surrounding the heart, which can lead to cardiac tamponade and potentially life-threatening complications. Accurate diagnosis of pericardial effusion is crucial to initiate timely treatment.

**Why the Correct Answer is Right**
Echocardiography is the preferred diagnostic method for pericardial effusion due to its high sensitivity and specificity in detecting fluid accumulation in the pericardial space. Transthoracic echocardiography (TTE) is a non-invasive and widely available technique that allows for the assessment of pericardial thickness, fluid volume, and cardiac function. It is particularly useful for detecting small amounts of fluid and can guide pericardiocentesis, a procedure to drain the accumulated fluid.

**Why Each Wrong Option is Incorrect**
**Option A:** USG (Ultrasonography) is a broader term that encompasses echocardiography but is less specific in this context. While USG can be used to detect pericardial effusion, it is not as sensitive or specific as echocardiography.
**Option B:** Angiography is primarily used for vascular imaging and is not the primary diagnostic tool for pericardial effusion.
**Option C:** Gallium scan is a nuclear medicine imaging technique that can detect inflammation and infection but is not the preferred method for diagnosing pericardial effusion.

**Clinical Pearl / High-Yield Fact**
Echocardiography is a rapid and bedside-friendly technique that can be used to diagnose pericardial effusion in critically ill patients. It is essential to perform echocardiography in patients with suspected cardiac tamponade to guide urgent pericardiocentesis and improve patient outcomes.

**✓ Correct Answer: D. Echocardiography**