**Core Concept**
The definitive diagnosis of pulmonary embolism (PE) involves identifying the presence of a thrombus within the pulmonary arteries, which can be achieved through imaging modalities that directly visualize the thrombus. This requires a high-resolution imaging technique that can distinguish between the thrombus and surrounding lung tissue.

**Why the Correct Answer is Right**
The most definitive method of diagnosing PE is through **Computed Tomography Pulmonary Angiography (CTPA)**. CTPA uses a contrast agent to visualize the pulmonary arteries and is highly sensitive and specific for detecting clots. The high-resolution images provided by CTPA allow for the direct visualization of the thrombus, making it the gold standard for diagnosing PE. The contrast agent highlights the thrombus, making it distinct from the surrounding lung tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** **Ventilation-Perfusion Scan (V/Q scan)** is a nuclear medicine test that can be used to diagnose PE, but it is not as sensitive or specific as CTPA. V/Q scans can be used in patients who cannot undergo CTPA, but they are not the definitive method.

**Option B:** **D-dimer testing** is a blood test used to rule out PE, but it is not a definitive diagnostic method. A negative D-dimer result can be used to rule out PE, but a positive result does not confirm the diagnosis.

**Option C:** **Echocardiography** can be used to diagnose PE by visualizing the right ventricle and pulmonary artery, but it is not as sensitive or specific as CTPA. Echocardiography can detect signs of right ventricular strain, which can be indicative of PE, but it does not directly visualize the thrombus.

**Option D:** **Chest X-ray** is not a definitive method for diagnosing PE. While a chest X-ray can show signs of pulmonary infarction, it cannot directly visualize the thrombus.

**Clinical Pearl / High-Yield Fact**
Remember that while CTPA is the definitive method for diagnosing PE, it is essential to consider the clinical context and risk factors for PE when interpreting the results. A high clinical suspicion for PE should prompt further diagnostic testing, even if the initial results are negative.

**Correct Answer: CTPA**