A 21-year-old woman presents with a 3-month history of a black mole on her right calf. She tells you that the lesion is enlarging and expanding. It began to itch about 3 weeks ago, and it has bled 2 times. She thinks that there may have been a mole near the same spot previously, but she is not certain. Her general health is good; there is no history of chronic illness, hospitalizations, or surgeries. She works as a professional model for a large advertisement agency. She does not take any prescription medication; she does not use tobacco, alcohol, or recreational drugs. Although she has dark hair, she has a fair skin, and she says that she usually burns with even short sun exposure. She does occasionally use a tanning booth prior to modeling events and vacations. There is no family history of skin cancer. VS stable, she looks anxious, but she is otherwise well.
There is a dark brown-black nodule on the right calf 1 cm in diameter.
On the surface of the nodule, there is a tiny area of crusting. There are no hairs. The nodule is asymmetrical, and its border is sharply demarcated; the color is uniform, and the elevation is regular. There is a narrow (1-2 mm) rim of erythema around most of the nodule. She has a sprinkling (about 25-30 in all) of melanocytic nevi on her trunk and legs. There is no significant local or distal lymphadenopathy. The liver is not palpable. The remainder of the physical examination is unremarkable.

What is the most likely diagnosis?