**Core Concept**
The lung is a unique organ that is susceptible to systemic diseases due to its extensive vascular network and high metabolic rate. This makes it a common site for metastasis from various primary cancers.

**Why the Correct Answer is Right**
The correct answer involves the understanding of the pathophysiology of metastasis. Cancer cells from primary tumors can break away and enter the bloodstream or lymphatic system, eventually reaching distant organs, including the lungs. The lungs are particularly prone to metastasis due to their high blood flow and the fact that cancer cells can easily lodge in the capillary bed. This process is facilitated by the expression of certain adhesion molecules on the surface of cancer cells, which allows them to interact with and adhere to the endothelial cells lining the pulmonary capillaries.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while the liver is a common site for metastasis, it is not characteristically involved in the same way as the lungs.
**Option B:** This option is incorrect because the spleen is primarily involved in the filtering of the blood and the storage of lymphocytes, rather than being a common site for metastasis.
**Option C:** This option is incorrect because while the brain is a site for metastasis, it is not characteristically involved in the same way as the lungs due to the blood-brain barrier.

**Clinical Pearl / High-Yield Fact**
The lungs are a common site for metastasis from primary tumors in the breast, colon, stomach, and pancreas. This is due to the high blood flow to the lungs and the fact that cancer cells can easily lodge in the capillary bed.

**Correct Answer:** D.