A 63-year-old woman is seen in the emergency room with acute shortness of breath. There is no history of heart or lung problems in the past. She was recently diagnosed with breast cancer and is undergoing active treatment. On examination, her blood pressure is 120/80 mm Hg, pulse 100/min, and heart and lungs are normal. There are no clinical signs of deep venous thrombosis (DVT). Which of the following investigations is most likely to rule out a pulmonary embolism (PE)?

**Core Concept**
The question is testing the ability to diagnose pulmonary embolism (PE) in a patient with a high risk of thromboembolic events, such as cancer patients undergoing treatment. The underlying principle is that a specific investigation can be used to rule out PE in this context.

**Why the Correct Answer is Right**
The D-dimer test is a useful investigation in ruling out PE in patients with a low pre-test probability of disease, such as those with a normal chest X-ray and no clinical signs of DVT. A negative D-dimer test has a high negative predictive value (NPV), meaning that it can safely rule out PE in these patients. The D-dimer test measures the level of fibrin degradation products in the blood, which are elevated in the presence of a thrombus.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not a specific investigation for ruling out PE, as it may be normal even in the presence of a significant embolus. While a chest X-ray can show indirect signs of PE, such as the Westermark sign or Hampton's hump, it is not a reliable test for ruling out PE.

**Option B:** Echocardiogram may show signs of right ventricular strain or dysfunction in the presence of a large PE, but it is not a sensitive test for ruling out PE, especially in patients with smaller emboli.

**Option C:** CT pulmonary angiogram (CTPA) is a sensitive and specific test for diagnosing PE, but it is not the best choice for ruling out PE in a patient with a low pre-test probability of disease, as it involves radiation exposure and is more expensive than the D-dimer test.

**Clinical Pearl / High-Yield Fact**
In cancer patients undergoing treatment, the risk of thromboembolic events is increased due to the release of pro-coagulant factors from the tumor. A one-time D-dimer test can be used to rule out PE in these patients, but repeat testing is not recommended due to the high risk of false positives.

**Correct Answer:**
D-dimer test.

✓ Correct Answer: C. normal ventilation-perfusion lung scan