**Core Concept**
In cases of pelvic fracture with pelvic hematoma, the primary concern is the potential compromise of the urinary tract, particularly the urethra and bladder. The presence of a pelvic hematoma and inability to pass urine since trauma suggests a possible urethral injury or bladder rupture.

**Why the Correct Answer is Right**
The correct approach in such cases is to perform a retrograde urethrogram (RUG) to assess the integrity of the urethra. This involves the insertion of a catheter into the bladder through the urethra and the injection of contrast material to visualize the urethral lumen. If the urethra is intact, the contrast material will flow freely through the urethra and into the bladder. However, if there is a urethral injury, the contrast material will leak out of the urethra, indicating a false passage.

**Why Each Wrong Option is Incorrect**
* **Option A:** Performing a voiding cystourethrogram (VCUG) may not be the best initial step in this scenario, as it requires the patient to void, which may exacerbate a potential urethral injury.
* **Option B:** Administering a diuretic may not address the underlying cause of the urinary retention and may even worsen the situation if there is a significant extravasation of urine.
* **Option D:** Inserting a suprapubic catheter may be necessary if the urethra is compromised, but it is not the first step in evaluating the patient.

**Clinical Pearl / High-Yield Fact**
In cases of pelvic fracture with pelvic hematoma and urinary retention, a retrograde urethrogram should be performed before attempting to pass a catheter or insert a suprapubic catheter to assess the integrity of the urethra.

**Correct Answer: C. Administering a diuretic.