## **Core Concept**
The question tests the knowledge of imaging modalities used for screening lung metastases (secondaries). Lung metastases are a common complication of various cancers, and early detection is crucial for management.

## **Why the Correct Answer is Right**
The correct answer, **CT (Computed Tomography) of the chest**, is the ideal method for screening lung metastases. This is because CT scans have higher sensitivity and specificity compared to chest X-rays, especially for small lesions. CT scans can detect lesions as small as 1-2 mm and provide detailed information about the number, size, and location of metastases. This information is critical for staging and planning treatment.

## **Why Each Wrong Option is Incorrect**
- **Option A: Chest X-ray** - While chest X-rays are often used as a preliminary screening tool due to their low cost and wide availability, they have lower sensitivity for detecting small lung metastases compared to CT scans.
- **Option B: MRI (Magnetic Resonance Imaging)** - MRI is not typically used for screening lung metastases because it is less sensitive than CT for detecting small lung lesions. MRI may be used in certain situations, such as when CT is contraindicated or when evaluating mediastinal or vascular invasion.
- **Option D: PET (Positron Emission Tomography) scan** - While PET scans are useful for detecting metabolically active tumors and can help identify metastatic disease, they are not typically used as a screening tool for lung metastases due to their high cost, limited availability, and the need for combined PET/CT imaging for accurate anatomical localization.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **CT scans of the chest are the preferred initial imaging modality for detecting lung metastases** in patients with a history of cancer. This is because early detection of lung metastases can significantly impact treatment planning and patient outcomes.

## **Correct Answer:** . **CT Chest**