What diagnostic test should be performed immediately for a 15-year-old female with burning urination following treatment for trichomonas vaginitis?

The immediate diagnostic test appropriate for a 15-year-old female experiencing burning urination after treatment for trichomonas vaginitis would be the microscopic exam of urine. This test is critical because it can help identify the presence of any underlying conditions that might be contributing to the symptoms, such as a urinary tract infection (UTI) or other abnormalities in the urine sediment, such as white blood cells, red blood cells, or organisms.

Performing a microscopic examination allows the healthcare provider to evaluate the urine sample for signs of infection or inflammation, which could explain the ongoing symptoms of burning urination. It provides quicker results than cultures and can guide immediate management decisions.

Urine culture, while important for confirmed diagnoses, takes longer to yield results and may not be necessary as an initial step if a microscopic exam shows clear evidence of an infection. A CT scan of the abdomen and a pelvic ultrasound are more advanced imaging techniques, typically reserved for cases with specific indications or complications and not for initial assessment of urinary symptoms after vaginal infection treatment.