**Core Concept**
Metastatic disease to the neck without a visible primary malignancy can be caused by various factors, including the primary tumor's location, size, and histology. In the neck, metastases can arise from primary sites in the head and neck, lungs, breast, and gastrointestinal tract.

**Why the Correct Answer is Right**
The most common cause of secondaries in the neck without an obvious primary malignancy is metastatic spread from a primary tumor in the lung. This is because the lung is a common source of metastases due to its rich blood supply and the fact that many primary tumors can metastasize to the lung via the bloodstream. The lymph nodes in the neck, particularly the jugulo-digastric and jugulo-omohyoid nodes, are often involved in this process.

**Why Each Wrong Option is Incorrect**
* **Option A:** While the breast is a common source of metastatic disease, it is less likely to present with neck secondaries without an obvious primary.
* **Option B:** The gastrointestinal tract is a common source of metastatic disease, but it is less likely to present with isolated neck secondaries without an obvious primary.
* **Option D:** The head and neck are a common source of metastatic disease, but they are more likely to present with local symptoms and signs rather than isolated neck secondaries.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with neck secondaries without an obvious primary malignancy, a thorough history and physical examination should be performed to identify potential primary tumor sites, and imaging studies such as chest X-rays or CT scans should be considered to evaluate for pulmonary primary tumors.

**Correct Answer:** C.