**Core Concept**
Pulmonary embolism (PE) is a life-threatening condition caused by a blood clot obstructing blood flow in the lungs. Accurate diagnosis is crucial for timely management. The best test to detect PE should be highly sensitive and specific.

**Why the Correct Answer is Right**
The D-dimer test is a rapid and non-invasive method to detect the presence of fibrin degradation products, which are indicative of clot formation. However, it lacks specificity, and a positive result can be seen in various conditions such as recent surgery, trauma, or malignancy. The correct answer is the test that combines the D-dimer result with clinical probability to improve specificity and guide further diagnostic testing.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chest X-ray is not a reliable test for PE, as it may show indirect signs such as atelectasis or pleural effusion but cannot confirm the diagnosis.
* **Option B:** Electrocardiogram (ECG) may show nonspecific changes such as tachycardia or T-wave inversion in PE, but it is not a definitive test.
* **Option C:** Computed Tomography Pulmonary Angiography (CTPA) is a sensitive test for PE but is not the first-line test due to its high radiation exposure and cost.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient for PE, remember the Wells score, which is a clinical prediction rule that combines symptoms such as dyspnea, tachycardia, and pleuritic chest pain with clinical probability to guide diagnostic testing.

**Correct Answer: C. Computed Tomography Pulmonary Angiography (CTPA)**