## **Core Concept**
The question pertains to the common sites of systemic metastasis for various types of cancers. Different cancers have predilections for metastasizing to specific organs due to factors like blood flow, organ-specific growth factors, and cell adhesion molecules.

## **Why the Correct Answer is Right**
Lung cancer, particularly non-small cell lung cancer (NSCLC) and small cell lung cancer (SCLC), has a high propensity for systemic metastasis. The lungs are a common site for metastasis from many cancers due to their rich blood supply and the fact that venous drainage from many parts of the body passes through the lungs. However, when considering primary sites that commonly metastasize systemically, lung cancer itself is notorious for early and widespread metastasis, often to the brain, bones, liver, and adrenal glands.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain cancers like breast, prostate, and lung can metastasize to bones, bones are not the most common site for systemic metastasis overall when considering all cancer types.
- **Option B:** The liver is a very common site for metastasis, especially from gastrointestinal tract cancers (e.g., colorectal cancer), but stating it as the commonest site for systemic metastasis might not be accurate when comparing across all cancer types.
- **Option C:** This option seems to be missing, but typically, the brain is a site for metastasis, particularly from lung, breast, and melanoma, but it's not usually the most common site overall.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the most common sites for cancer metastasis vary by the primary cancer site. However, the "mnemonic" for common sites of metastasis can include the "4-4-1" rule for some cancers (e.g., lung, liver, and bone being common sites), but specifically, lung cancer often metastasizes to the brain, making early detection and management critical.

## **Correct Answer:** . Lung