**Core Concept:** Malignant melanoma is a type of skin cancer that arises from melanocytes, the pigment-producing cells in the skin. The diagnosis of malignant melanoma requires the identification of specific markers or characteristics.

**Why the Correct Answer is Right:** The correct answer, **C. Lymph Node Involvement**, refers to one of the key characteristics of malignant melanoma. Melanoma can invade local tissues and spread to regional lymph nodes through the lymphatic system. This process is known as lymph node involvement, which is a crucial step in the progression of melanoma and is often an early indicator of its presence.

**Why Each Wrong Option is Incorrect:**

A. **Expanding Nodules:** This characteristic is associated with other types of skin lesions, not specifically malignant melanoma.
B. **Changes in Skin Color:** While changes in skin color can occur in malignant melanoma, this option is too broad and does not specifically address the lymph node involvement aspect of the question.
D. **Loss of Skin Elasticity:** This characteristic is unrelated to malignant melanoma and does not address the key markers for this specific type of skin cancer.

**Clinical Pearl:** A high index of suspicion and careful examination are essential in diagnosing malignant melanoma, particularly in its early stages. Malignant melanoma can often mimic benign lesions, making it crucial for clinicians to recognize key markers such as lymph node involvement.

**Correct Answer: C. Lymph Node Involvement**

Malignant melanoma is a potentially lethal form of skin cancer that requires early detection and treatment. Recognizing the key markers of this disease, such as lymph node involvement, is essential for accurate diagnosis and timely management.