**Core Concept**
The best initial investigation for a patient with a history of blunt abdominal trauma is an ultrasound (USG) of the abdomen. This is because ultrasound is a non-invasive, quick, and widely available imaging modality that can effectively evaluate for free intraperitoneal fluid, which is a common finding in cases of abdominal trauma.

**Why the Correct Answer is Right**
Ultrasound is particularly useful in the initial evaluation of blunt abdominal trauma because it can quickly identify free intraperitoneal fluid, which may indicate internal bleeding or other serious injuries. This is especially important in the acute setting, where timely intervention can be lifesaving. Ultrasound can also be used to evaluate for solid organ injuries, such as those to the liver or spleen.

**Why Each Wrong Option is Incorrect**
**Option B:** While a CT scan is a valuable imaging modality in the evaluation of blunt abdominal trauma, it is not the initial investigation of choice. CT scans are more sensitive than ultrasound for detecting injuries to the abdominal organs, but they are also more expensive and expose patients to radiation.

**Option C:** A complete hemogram (CBC) may be useful in assessing for hemodynamic instability or indicating the need for blood transfusions, but it is not a suitable initial investigation for evaluating the extent of abdominal trauma.

**Option D:** An abdominal X-ray may be useful in evaluating for free air under the diaphragm, indicating a perforated viscus, but it is not a reliable method for detecting other types of abdominal injuries.

**Clinical Pearl / High-Yield Fact**
In cases of blunt abdominal trauma, the FAST (Focused Assessment with Sonography for Trauma) exam can be used to quickly evaluate for free intraperitoneal fluid. The FAST exam involves a series of ultrasound views that evaluate for fluid in the pericardial space, subphrenic space, and peritoneal cavity.

**✓ Correct Answer: A. USG**