**Core Concept**
The concept of "secondaries" in the pancreas refers to metastatic disease originating from a primary tumor in another organ. Metastatic disease to the pancreas is relatively rare, but it is more common than primary pancreatic cancer in some cases.

**Why the Correct Answer is Right**
The most common primary leading to secondaries in the pancreas is lung cancer. This is due to the fact that the pancreas is a common site for metastasis from lung cancer, particularly from squamous cell carcinoma and small cell carcinoma. The pancreas is a frequent site for metastasis from lung cancer due to its rich blood supply and the presence of specific receptors on pancreatic cells that facilitate the adherence and growth of metastatic tumor cells. The liver and lungs are the most common sites for metastasis from lung cancer, but the pancreas is also a significant site, especially in advanced disease.

**Why Each Wrong Option is Incorrect**
**Option B:** Breast cancer can metastasize to the pancreas, but it is less common compared to lung cancer. Breast cancer metastases to the pancreas are often associated with a poor prognosis.
**Option C:** Colon cancer can also metastasize to the pancreas, but it is relatively rare. The pancreas is not a typical site for metastasis from colon cancer, which more commonly metastasizes to the liver and lungs.
**Option D:** Stomach cancer can metastasize to the pancreas, but it is even less common than breast or colon cancer. Stomach cancer metastases to the pancreas are often associated with advanced disease and a poor prognosis.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with a pancreatic mass, it is essential to consider the possibility of metastatic disease from a primary tumor in another organ, particularly lung cancer. A thorough history and physical examination, along with imaging studies and laboratory tests, can help identify the primary source of the metastasis.

**✓ Correct Answer: A. Lung**