The correct answer is:

Computed tomography angiography (CTA) scan

Educational Objective:

Know the appropriate use of imaging and other diagnostic tests in a patient with suspected venous thromboembolic disease.

Key Point:

Wells criteria guide the diagnostic workup of patients with suspected venous thromboembolism.

Explanation:

A common scoring system to assess the likelihood of pulmonary embolism (PE) is the Wells Clinical Prediction Rule. Three points are awarded for the presence of calf swelling or tenderness, and 3 points for absence of an alternative diagnosis. In addition, 1.5 points are awarded for tachycardia, recent history of immobilization, or history of DVT/PE. One point is awarded for hemoptysis or evidence of malignancy.

This patient has a Wells score of 4.5 (evidence of DVT =3) and a predisposing factor (recent hospitalization =1.5). In addition, she has modest tachycardia (95/min) and below-normal oxygen saturation, not otherwise explained.

Based on these findings, and the history of exertional dyspnea, it is likely she has suffered a pulmonary embolism.

The key question, here, is... which test is likely to be more informative, computed tomography angiography (CTA) or doppler ultrasound of the left leg?

The selection of ultrasound would give no information about a pulmonary embolus or its hemodynamic burden. The CTA would confirm both an embolism and, by inference, a clot in the left leg. Where CTA is not available, one would confirm a thrombus by doppler and infer an embolic event from the clot.

Given the clinical findings, a D-dimer test would be superfluous. One would not conclude that she has not had a pulmonary embolus if the test was negative. 

Due to its low rates of sensitivity and specificity, a ventilation-perfusion scan is not a cost-effective test for the evaluation of possible PE.