In evaluating a patient who complains of acute swelling and tenderness of the calf after a long flight, what is the next best step?

In the context of evaluating a patient who presents with acute swelling and tenderness in the calf following a long flight, the most appropriate and immediate next step is to perform a duplex ultrasound. This imaging modality is essential for assessing the presence of a deep vein thrombosis (DVT), which is a critical condition that can arise in patients who have been immobile for extended periods, such as during long flights. Duplex ultrasound allows for real-time visualization of the venous structures and can help identify any thrombus formation or occlusion.

While the D-dimer test can provide supportive data in the diagnosis of DVT, it is not definitive on its own and can yield false positives due to various other conditions. A physical examination might reveal some clinical findings suggestive of DVT, but it cannot confirm or rule out the diagnosis with certainty. Advising the patient to rest and elevate the leg may help manage symptoms but does not address the urgent need for diagnosing a potential DVT that could lead to serious complications, such as pulmonary embolism. Therefore, conducting a duplex ultrasound is the most effective and necessary action in this clinical scenario.