A 23 year-old female with asthma has increasing shortness of breath. What is the most appropriate initial diagnostic evaluation?

The most appropriate initial diagnostic evaluation in this scenario is peak flow measurement. This test is particularly useful in assessing asthma control and the severity of airflow obstruction. It allows for the quantification of the peak expiratory flow rate (PEFR), which can help determine whether the patient's condition is stable or worsening.

In patients with asthma, changes in peak flow can indicate exacerbations and help guide treatment decisions. Assessing peak flow is quick, simple, and can often be done in an office or clinic setting or even at home, making it an ideal first step when a patient presents with increasing shortness of breath.

While a chest x-ray can be used to rule out other conditions related to respiratory distress, such as infections or pneumothorax, it is not typically the immediate priority in evaluating an asthma exacerbation. A sputum gram stain is more useful in diagnosing bacterial infections or pneumonia, and a ventilation-perfusion scan would be indicated in cases where there is suspicion of pulmonary embolism, which is less likely in the context of worsening asthma symptoms. Therefore, peak flow measurement is the most direct way to evaluate the severity of the asthma exacerbation and guide further management.