What imaging study is typically used to diagnose a posterior urethral injury?

The retrograde urethrogram (RUG) is the imaging study of choice to diagnose a posterior urethral injury. This procedure involves the insertion of a contrast medium into the urethra, followed by imaging to visualize the anatomy of the urethra and any potential disruptions or injuries. It is particularly useful in cases of trauma or suspected injuries where the urethra may be compromised, providing clear delineation of the injury's extent and location.

The retrograde urethrogram is preferred because it specifically focuses on the urethra, allowing for detailed assessment of both anterior and posterior urethral injuries. Additionally, it can help rule out injuries before any attempt at catheterization, which is crucial to avoid exacerbating a potential injury.

Imaging studies such as a CT scan of the abdomen or ultrasound of the kidneys can provide information about other abdominal and pelvic injuries, but they do not specifically target urethral injuries. An abdominal X-ray may reveal pelvic fractures but is not reliable for visualizing the urethra itself, thus lacking the specificity required for diagnosing posterior urethral injuries. Therefore, the retrograde urethrogram stands out as the most appropriate choice for this scenario.