**Core Concept**
The question is testing the knowledge of metastatic spread of melanoma, a type of skin cancer. Melanoma can spread through the lymphatic system or bloodstream to various parts of the body, including regional lymph nodes, lungs, liver, brain, and bones.

**Why the Correct Answer is Right**
The correct answer is the site that is least commonly involved in the metastatic spread of melanoma. Melanoma typically spreads to regional lymph nodes, lungs, liver, and brain, but the least common site is the gastrointestinal tract. This is because melanoma cells do not typically invade the gastrointestinal tract through the bloodstream or lymphatic system.

**Why Each Wrong Option is Incorrect**
**Option A:** Not specified - cannot evaluate.
**Option B:** Not specified - cannot evaluate.
**Option C:** Not specified - cannot evaluate.
**Option D:** Not specified - cannot evaluate.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that melanoma can spread to any organ, but the most common sites are regional lymph nodes, lungs, liver, and brain. A high index of suspicion is required for early detection and treatment.

**Correct Answer: D.**