Which of the following diagnostic tests provides a definitive diagnosis of a pulmonary embolism?

A. CT Pulmonary Angiography: This imaging modality uses intravenous contrast to provide high-resolution visualization of the pulmonary vasculature. It is the gold standard for definitively identifying an intraluminal filling defect caused by a thrombus. It offers superior sensitivity and specificity compared to other non-invasive diagnostic tests. B. Chest X-ray: Radiographic imaging of the chest is often normal or shows non-specific findings like atelectasis in cases of pulmonary embolism. It is primarily used to rule out other causes of chest pain or dyspnea, such as pneumonia or pneumothorax. It cannot visualize the blood vessels clearly enough to confirm a clot. C. Spirometry: This test measures lung volumes and airflow to diagnose obstructive or restrictive lung diseases like asthma or COPD. It provides no information regarding the pulmonary circulation or the presence of a vascular obstruction. It is not an appropriate tool for the diagnosis of an acute embolic event. D. Electrocardiogram (ECG): An ECG may show signs of right heart strain, such as the S1Q3T3 pattern, but these are inconsistent and non-specific. Its primary role in the workup is to exclude myocardial infarction as the cause of the patient's symptoms. It provides indirect clues but is not a definitive diagnostic for PE.