This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Oncology, Subject name: Medicine
A 45-year-old man is worried about a dark pigmented skin lesion on his arm. The "mole" is 3 mm wide, symmetric with a regular border and even pigmentation. He reports no change in size or other symptoms. Which of the following is the most appropriate next step in management? 
Select and explain. 
A. observation only, B. excisional biopsy, C. punch biopsy, D. chemotherapy, then surgical excision
A: observation only
Answer: A: observation only 
Explanation: (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.