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In a case of sudden onset dyspnea following surgery in a patient who has been immobile, the best diagnostic study is a V/Q (ventilation-perfusion) scan. This choice is particularly relevant because one of the main concerns in such scenarios is the development of a pulmonary embolism (PE), which is a potential complication due to immobility after surgery.

The V/Q scan helps to assess the ventilation and perfusion of the lungs to identify mismatches that indicate possible embolic events. A significant perfusion defect without corresponding ventilation suggests that a blood clot is occluding a major pulmonary artery, leading to reduced blood flow and subsequent hypoxia, a hallmark of PE.

While other diagnostic studies can provide useful information in this context, such as a chest CT angiography (CTA) which is often employed to confirm a diagnosis of PE, the V/Q scan can be preferable in situations where CTA may not be immediately available, or in patients who may not tolerate contrast material due to renal implications or allergies.

Doppler ultrasound of the leg is useful for assessing deep vein thrombosis but does not directly evaluate the lungs. An echocardiogram can provide insights into cardiac function and can hint at the presence of strains related to PE, but