**Core Concept**
The first investigation of choice in blunt trauma abdomen aims to quickly identify potential life-threatening injuries, such as bleeding or hollow viscus rupture. A rapid and accurate assessment is crucial in this scenario to guide further management and prevent delayed diagnosis.

**Why the Correct Answer is Right**
The correct answer is focused on quickly assessing the patient's hemodynamic stability and identifying potential sources of bleeding. The FAST (Focused Assessment with Sonography for Trauma) examination is a bedside ultrasound technique that evaluates for free intraperitoneal fluid, which can indicate bleeding or hollow viscus rupture. This examination is rapid, non-invasive, and can be performed simultaneously with other trauma assessments.

*Option A:* X-ray of the abdomen is not the first line of investigation in blunt trauma abdomen as it may not detect free intraperitoneal fluid and can be delayed by the need to move the patient to the radiology department.

*Option B:* CT scan of the abdomen is not the first line of investigation due to its longer duration, the need for contrast, and the potential for delay in diagnosis.

*Option C:* Ultrasound of the liver is not the first line of investigation as it may not detect injuries to other abdominal organs and is more time-consuming than FAST.

*Option D:* MRI of the abdomen is not the first line of investigation due to its longer duration, the need for contrast, and the potential for delay in diagnosis.

**Clinical Pearl / High-Yield Fact**
In cases of blunt trauma abdomen, the ABCDE approach should be followed, with FAST being an integral part of the assessment. This approach includes assessing Airway, Breathing, Circulation, Disability, and Exposure.

**Correct Answer: C. Ultrasound of the abdomen (FAST examination)**