Which diagnostic test should be performed for a 15-year-old female with burning urination after trichomonas vaginitis treatment?

The most appropriate diagnostic test to perform in this scenario is a microscopic examination of urine. This test can help identify the presence of white blood cells, bacteria, and various types of crystals or casts in the urine, which can indicate an infection or other conditions affecting the urinary tract. Given that the patient is experiencing burning urination following treatment for trichomonas vaginitis, a microscopic exam can aid in determining if a urinary tract infection (UTI) or other related pathology is contributing to her symptoms.

A culture of urine could also be beneficial; however, a microscopic examination provides rapid initial insight into the urinary status, which is critical in clinical decision-making, especially if an infection is suspected. This test can be a first step to better understand the cause of the symptoms after treatment.

The urine pH test might give some information about the acid-base status of the urine but does not directly assist in diagnosing infections or other abnormalities. A complete blood count can indicate systemic infection or other issues but does not provide specific information about the urinary tract, making it less relevant in this specific situation. Thus, the microscopic exam is the most targeted choice to investigate the patient's burning urination symptomatology.