Would you consider a nurse practitioner at a dermatologist office to be an expert on melanoma?

Broadly : If the np has the training and experience in doing skin exams to assess for melanoma (abcd), then he or she may be considered a np expert in the local community. But if you're talking about the np being an expert in mohs micrographic surgery, you've found yourself one lucky dermatologist.

Who is an Expert? : In my experience expertise comes with experience and not with a degree. I am a much wiser and smarter physician today than i was ten years ago. If the arnp is working in a dermatology office they are likely to have experience and thus expertise.