**Core Concept:** Blunt abdominal trauma is a type of injury caused by blunt force, typically involving the abdomen and pelvis. Hematuria refers to the presence of blood in the urine, which can be a symptom of internal organ damage in trauma patients. Investigations in this context aim to identify the cause and severity of abdominal trauma.

**Why the Correct Answer is Right:**

In a patient with blunt abdominal trauma presenting with hematuria, the most suitable investigation is **C:** Ultrasonography (USG). USG is a non-invasive, radiation-free, and cost-effective imaging technique that can visualize the internal organs, blood vessels, and surrounding tissues. It is particularly useful for assessing the liver, spleen, kidneys, and urinary system. Ultrasound can detect free fluid, injuries, and hemoperitoneum (collection of blood in the peritoneal cavity).

**Why Each Wrong Option is Incorrect:**

A. **X-ray:** X-rays are useful for detecting fractures and gas in the gastrointestinal tract but are not ideal for assessing soft tissues and organs. The sensitivity of X-rays is low for detecting blunt abdominal trauma and hematuria.

B. **Computed Tomography (CT):** While CT is a more sensitive method for detecting abdominal injuries, it is not the first-line investigation for blunt abdominal trauma patients with hematuria. Ultrasonography is more accessible, cost-effective, and radiation-free, making it the preferred initial investigation in this scenario.

D. **Blood tests:** Blood tests, including complete blood count (CBC) and coagulation profile, are useful for assessing general health status, but they do not directly diagnose or stage blunt abdominal trauma or hematuria.

**Clinical Pearl:** In blunt abdominal trauma cases, a triad of tenderness, guarding, and rebound tenderness can suggest significant injury, but these signs alone are not definitive. Ultrasound remains the initial imaging modality to assess for injuries in hematuria patients. If the ultrasound is inconclusive or not available, further imaging modalities like CT and MRI can be considered.