A dermatologist treats skin, hair and nails. That covers acne, eczema, psoriasis, hair loss, pigment changes, keloids and skin cancer, and it covers the medical side and the cosmetic side of each one.
Book an appointment when a rash has not settled in two weeks, when a mole or a dark streak under a nail changes, when hair is thinning at the temples or the crown, when acne is leaving marks that outlast the spots, or when a scar keeps growing after the wound has closed. Anything that bleeds, grows or changes gets seen sooner rather than watched.
Bring three things with you. A photo of the problem at its worst, because skin rarely behaves on the day of the appointment. A list of everything that touches the area, including hair grease, edge control, shampoo, and any product a barber or stylist uses on you. And the names of relatives with the same problem, since much of what a dermatologist treats runs in families.