What is the next diagnostic step if a 35-year-old female taking birth control pills has a positive D-Dimer?

In the scenario presented, where a 35-year-old female taking birth control pills has a positive D-Dimer, the most appropriate next diagnostic step is to perform a CT Angiography (CTA) of the chest.

A positive D-Dimer test indicates the presence of fibrin degradation products in the bloodstream, which can suggest an abnormal clotting process. In the context of this patient's demographic and her use of birth control pills, there is a concern for potential venous thromboembolism, such as a pulmonary embolism (PE). The CTA of the chest is specifically designed to visualize pulmonary arteries and can effectively identify clots in these vessels, making it the gold-standard test for evaluating suspected PE.

Magnetic Resonance Imaging (MRI) is not generally used for the initial evaluation of pulmonary embolism due to its limitations in sensitivity and accessibility compared to CT scans. An ultrasound of the abdomen would not be an appropriate next step, as it does not assess the thoracic vasculature for embolism. Similarly, a chest X-ray might be helpful as an initial evaluation tool but does not have the specificity or sensitivity that CTA provides for diagnosing or ruling out a PE.

Thus, the reason for choosing CT Angiography in this case reflects