A 56-year-old female with a history of hypertension cannot catch her breath. What should the next test be?

In this scenario, the patient's difficulty in breathing, or dyspnea, combined with her history of hypertension, raises concerns about potential cardiovascular issues, such as heart failure, pulmonary hypertension, or other acute cardiac conditions. A chest CT scan is a highly useful imaging modality that can quickly provide important information regarding both the cardiac structures and the pulmonary circulation.

The chest CT scan can help identify any pulmonary embolism, pneumonia, or other pathologies that might contribute to her symptoms. It can also help assess for any structural abnormalities in the heart or large vessels, which might be exacerbated by her history of hypertension. This makes it a vital test for understanding the underlying cause of her acute respiratory distress.

Other tests, like a bedside echocardiogram, while certainly useful, may not give the most immediate insight into the potential pulmonary pathologies that can often be diagnosed with a chest CT. Exercise treadmill testing is not appropriate in the context of acute dyspnea as it may worsen the patient's condition. A lower extremity Doppler, while useful for assessing venous problems, wouldn't provide information directly related to her respiratory symptoms.

Thus, the chest CT scan is the most appropriate next step to take in this clinical scenario.