A 16-year-old girl has a bald spot on her scalp, and her mother brings her to the PMHNP on the advice of her primary care provider after all medical causes are ruled out. The girl admits to pulling out her hair in class when she is bored, but says she tends to pull it out more often in math class where she is making Cs. This is the first year she is not doing well in math, and her mother says it is "stressing her out." What is the most appropriate diagnosis?