**Core Concept**
Sister Mary Joseph nodules are small, palpable nodules found at the umbilicus, often associated with abdominal malignancies. They represent metastatic deposits from a primary tumor elsewhere in the body, most commonly from the gastrointestinal tract.

**Why the Correct Answer is Right**
The formation of Sister Mary Joseph nodules is thought to occur through the hematogenous spread of cancer cells from a primary tumor to the navel. This is a result of the rich vascular supply to the umbilicus, which allows cancer cells to seed and form metastatic deposits. The most common primary sites for Sister Mary Joseph nodules are the stomach, colon, and pancreas.

**Why Each Wrong Option is Incorrect**
**Option A:** While other metastatic sites like the liver and lungs are common in cancer patients, they are not typically associated with Sister Mary Joseph nodules.
**Option B:** Sister Mary Joseph nodules are not a manifestation of primary umbilical tumors, but rather a sign of metastatic disease from another site.
**Option C:** The presence of Sister Mary Joseph nodules does not necessarily indicate a specific type of cancer, but rather suggests the presence of a visceral malignancy.

**Clinical Pearl / High-Yield Fact**
Sister Mary Joseph nodules are a classic sign of abdominal malignancy and should prompt a thorough investigation for a primary tumor elsewhere in the body.

**Correct Answer: D.**