**Core Concept**
Echocardiography is a non-invasive imaging modality used to visualize cardiac structures and detect various cardiac conditions, including pericardial effusion. It relies on the reflection of ultrasound waves to create detailed images of the heart and surrounding tissues.

**Why the Correct Answer is Right**
Pericardial effusion refers to the accumulation of fluid in the pericardial space, which can lead to cardiac tamponade. Echocardiography can detect even small amounts of fluid in the pericardial space due to its high sensitivity. The correct answer is 20 mL, as this is the minimum amount of fluid that can be detected by echocardiography. This is made possible by the ability of ultrasound waves to detect changes in tissue density and reflectivity.

**Why Each Wrong Option is Incorrect**
**Option A:** 5 mL - This is an incorrect answer as echocardiography is not sensitive enough to detect such small amounts of fluid.
**Option B:** 50 mL - While echocardiography can detect larger amounts of fluid, 50 mL is not the minimum amount that can be detected.
**Option C:** 100 mL - This is also an incorrect answer as it is a larger amount of fluid than the minimum detectable by echocardiography.

**Clinical Pearl / High-Yield Fact**
It is essential to note that echocardiography is a valuable tool in the diagnosis of pericardial effusion, especially in cases where the patient presents with symptoms such as shortness of breath or chest pain. A high index of suspicion and prompt echocardiography can help diagnose and manage cardiac tamponade.

**Correct Answer:** C. 30 mL.