## **Core Concept**
Pulmonary embolism (PE) is a condition where one or more arteries in the lungs become blocked by a blood clot, typically originating from the deep veins of the legs. The diagnosis of PE often involves imaging studies, with computed tomography pulmonary angiography (CTPA) being a common modality. Various radiological signs can suggest the presence of PE.

## **Why the Correct Answer is Right**
The correct answer, **Westermark sign**, is a radiological sign that refers to a sharply defined, avascular area seen on a chest radiograph, suggestive of pulmonary infarction or oligemia due to pulmonary embolism. This sign is considered when there is a sudden cutoff of a pulmonary artery branch with a clear area beyond it, indicating reduced blood flow.

## **Why Each Wrong Option is Incorrect**
- **Option A:** The **Hampton hump** is indeed a sign associated with pulmonary embolism, representing a wedge-shaped opacity seen on chest radiographs, thought to represent pulmonary infarction. However, it is not the best answer here given the specificity of the question towards another sign.
- **Option B:** The **Knock-Knock sign** or **Pullen sign** might relate to other conditions or could be a distractor; it's not commonly associated with PE in radiological literature.
- **Option C:** The **Sailor sign** does not directly relate to a commonly recognized radiological sign for PE.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that while chest radiographs are often used as an initial screening tool, they are rarely diagnostic for PE. However, they can help in ruling out other causes of symptoms. The **Westermark sign** and **Hampton hump** are classic signs, but their sensitivity for PE is low. CT pulmonary angiography remains the gold standard for diagnosis.

## **Correct Answer:** . Westermark sign