A teenage girl complains of burning on urination after treatment for trichomonas vaginitis. What diagnostic test should be done immediately?

In this scenario, where a teenage girl is experiencing burning on urination after being treated for trichomonas vaginitis, the most immediate diagnostic test that should be performed is a microscopic examination of the urine. This test is essential because it can help identify the presence of urinary tract infections (UTIs), which can manifest with similar symptoms, such as dysuria or burning during urination.

Given her previous diagnosis of trichomonas vaginitis, which is sexually transmitted, there is a potential risk for concurrent infections or complications, including a UTI, which may develop despite treatment for trichomonas. A microscopic examination will allow for the detection of leukocytes, bacteria, and other abnormalities in the urine that can indicate an infection.

While other tests may be relevant in the broader context—such as a urine pregnancy test to rule out pregnancy complications, a herpes simplex virus culture to assess for genital herpes, or a repeat chlamydia test to check for other STIs—these tests are not as immediately pertinent to her acute symptom of burning urination. The focus should be on understanding whether a UTI is present, making the urine microscopic exam the most appropriate immediate choice.