What is the likely diagnosis for a mixed color lesion under a nail bed with irregular borders in an adult?

The presence of a mixed color lesion under a nail bed with irregular borders in an adult strongly suggests melanoma. Melanoma is a type of skin cancer that typically presents as an asymmetrical, multicolored lesion with irregular borders. When located under the nail, this condition is often referred to as subungual melanoma, which can manifest as a dark streak or pigmentation beneath the nail due to abnormal melanocyte proliferation.

The mixed colors observed in the lesion—such as brown, black, or even shades of red or blue—indicate a degree of variation in pigmentation, which is a concerning sign that warrants further investigation. The irregularity of the borders adds to the suspicion of melanoma, as benign conditions such as a nevus typically have smoother and more well-defined edges.

Other potential causes like trauma may present with discoloration under the nail but typically lack the multicolored appearance and irregular borders that characterize melanoma. Lentigo and nevus also generally present with uniform coloration and regular boundaries, making them less likely in this scenario.

Given these features of the lesion, the likelihood that it is melanoma, particularly in an adult, is high, making this the most appropriate diagnosis.