(a) Source: (Kasper, pp. 500-502) Observation alone is adequate for this lesion. The "ABCD" rules are helpful in distinguishing benign skin lesions from malignant melanoma. (A) asymmetry, benign lesions are symmetric; (B) border irregular, most nevi have clear-cut borders; (C) color variation, benign lesions have uniform color; (D) diameter, >6 mm is more likely to be malignant. In addition, recent rapid change in size is also helpful in distinguish benign from malignant lesions. Thickness of the tumor is the most important prognostic factor in the majority of cases, and ulceration indicates a more aggressive cancer with a poorer prognosis. Although cumulative sun exposure is a major factor in melanoma (e.g., more frequent near the equator), it cannot explain such things as the more common occurrence of some types in relatively young people. It is possible that brief, intense exposure to sunlight may contribute to, or initiate, carcinogenic events.