Which presentation is most concerning for skin cancer?

The most concerning presentation for skin cancer is the dark pigmentation of a solitary nail that has developed quickly and without trauma. This scenario raises significant red flags for potential melanoma, particularly subungual melanoma, which can manifest as a dark streak or pigmentation under the nail. Rapid changes in nail pigmentation, especially in the absence of any trauma, are worrisome because they may indicate malignancy. Melanomas can arise from various parts of the skin, and the nails are not exempt from this possibility.

In contrast, the other options represent various benign or stable conditions. The blue, round, nonpalpable discoloration that has been present since birth is likely a nevus (a benign mole), especially since it has not changed over time. A stable, 5 mm black mole with round, regular borders typically indicates a benign melanocytic nevus (common mole) rather than a malignant one, since the characteristics described are not suggestive of the irregularity associated with melanoma. Lastly, a stable 3 mm red spot that has been unchanged for several years also does not raise immediate concern for skin cancer, as the longevity and stability of the lesion suggest it is likely harmless.