**Core Concept**
Orbital metastasis is a rare condition where cancer cells spread to the orbit, the bony cavity containing the eyeball and its associated structures. This can occur through the hematogenous route, where cancer cells are carried through the bloodstream, or through direct extension from adjacent tissues.

**Why the Correct Answer is Right**
The most common primary sites for orbital metastasis are the breast, prostate, and lung cancers. Breast cancer, in particular, is known to frequently metastasize to the orbit, often through the hematogenous route. The cancer cells from the breast can spread to the orbital bones and soft tissues, leading to symptoms such as proptosis (bulging of the eye), eye pain, and vision disturbances.

**Why Each Wrong Option is Incorrect**
* **Option A:** Although melanoma can metastasize to the orbit, it is not the most common primary site for orbital metastasis.
* **Option B:** Lymphoma can also metastasize to the orbit, but it is not the most common primary site.
* **Option D:** Renal cell carcinoma can metastasize to the orbit, but it is not the most common primary site.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to consider orbital metastasis in the differential diagnosis of patients with proptosis, especially in patients with a history of breast or prostate cancer.

**Correct Answer:** C.