**Core Concept**
Focused Assessment with Sonography in Trauma (FAST) is an ultrasound-based bedside examination used to quickly assess for free intraperitoneal fluid in trauma patients. It relies on the ability of ultrasound waves to penetrate soft tissues and detect fluid collections.

**Why the Correct Answer is Right**
The FAST examination utilizes ultrasound to evaluate four specific regions: the right upper quadrant (hepatorenal space), the left upper quadrant (splenorenal space), the pericardial area, and the suprapubic region. Ultrasound waves are transmitted through the skin and soft tissues, allowing the examiner to visualize fluid collections, such as blood, in these areas. The suprapubic region is particularly useful for detecting free fluid in the pelvic cavity. The use of ultrasound in FAST provides a rapid, bedside assessment that can be performed by trained emergency medicine or surgical residents.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scans are not typically used for bedside assessment in trauma patients due to the need for patient transport and radiation exposure.
**Option C:** MRI is not suitable for FAST due to its limited availability, longer scanning time, and contraindications in trauma patients with metallic objects.
**Option D:** X-rays are not sensitive for detecting free fluid in the peritoneal cavity and are not used for FAST.

**Clinical Pearl / High-Yield Fact**
In addition to detecting free intraperitoneal fluid, FAST can also be used to evaluate for cardiac tamponade by visualizing pericardial effusion. A pericardial effusion greater than 4 mm is considered significant and warrants further evaluation and intervention.

✓ Correct Answer: B. USG Ultrasound.