**Core Concept**
Squamous cell carcinoma (SCC) is a type of **malignant skin tumor** that originates from the **squamous cells** of the epidermis. It is known for its potential to metastasize, particularly to the regional lymph nodes. The underlying principle being tested involves the clinical characteristics and behavior of SCC.

**Why the Correct Answer is Right**
The correct answer, **(D) metastasis is usually to the regional lymph nodes**, is right because SCC tends to spread first to the nearest lymph nodes before potentially metastasizing to other parts of the body. This pattern of spread is a key aspect of its clinical behavior and is crucial for staging and treatment planning. The **metastatic pathway** typically involves the invasion of cancer cells into the lymphatic vessels, which then carry them to the regional lymph nodes.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because basal cell carcinoma is actually the most common malignant skin tumor, not squamous cell carcinoma.
**Option B:** is incorrect because, although SCC is most common in the skin, it can also occur in other squamous epithelium-lined organs, such as the lips, mouth, esophagus, and lungs.
**Option C:** is incorrect because while SCC is more common in transplant patients than in the general population, it is not specified as the most common skin tumor in this group without considering other factors.
**Option E:** is incorrect because the tumor typically has **everted or rolled edges**, but this option is not among the choices provided for evaluation.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that SCC has a higher potential for **metastasis** compared to basal cell carcinoma, especially in certain high-risk locations or in immunocompromised patients. Recognizing the risk factors and early signs of SCC is crucial for timely intervention.

**Correct Answer:** D. metastasis is usually to the regional lymph nodes