**Core Concept**
Intraabdominal injuries often require prompt and accurate diagnosis to guide urgent surgical intervention. An unstable patient with suspected intraabdominal injury necessitates an investigation that is rapid, non-invasive, and capable of detecting free intraperitoneal fluid.

**Why the Correct Answer is Right**
Ultrasound (USG) is the investigation of choice in an unstable patient with suspected intraabdominal injury due to its speed, bedside accessibility, and ability to detect free intraperitoneal fluid, which is a hallmark of intraabdominal bleeding. The presence of free fluid is a sensitive indicator of intraabdominal injury, and USG can quickly identify this sign, guiding immediate surgical intervention. The focused assessment with sonography for trauma (FAST) exam, a specific ultrasound protocol, is commonly used in this context.

**Why Each Wrong Option is Incorrect**
**Option A:** Diagnostic peritoneal lavage (DPL) is an invasive procedure that requires a catheter to be inserted into the peritoneal cavity, making it unsuitable for an unstable patient. DPL is usually reserved for situations where ultrasound is not available or inconclusive.
**Option C:** CT scan is a more detailed imaging modality that provides excellent visualization of intraabdominal structures but requires the patient to be stable enough to be transported to the scanner and to lie still during the examination. CT scan is not the first-line investigation in an unstable patient.
**Option D:** X-ray abdomen is not sensitive for detecting free intraperitoneal fluid and is therefore not the investigation of choice in suspected intraabdominal injury.

**Clinical Pearl / High-Yield Fact**
In trauma patients, a negative FAST exam does not completely rule out intraabdominal injury, and further investigation may be required. Always consider the clinical context and other diagnostic findings when interpreting ultrasound results.

**✓ Correct Answer: B. USG**