Anorexia nervosa is an eating disorder. It is characterized by perceptual distortions by the teenager which usually leads to restricting their intake of nutrients. Teens suffer with unbearable low body weight which if prolonged could cause permanent physical damage, or in the worst cases death. They have irrational fears of gaining weight and a distorted body image, sometimes with a co-existing body dysmorphic disorder. These children (that is what they really are) control their weight using efforts that tend to squash the joys of being a teenager, and keep them prisoner to their own misperceptions.
Treatment should be in a team approach including the treating therapist, her psychiatrist, her internist, her nutritionist and their parents. Anorexia used to be seen as an act of rebellion, and I think there stil must be some truth to this. However what is important is that this condition be arrested as quickly as it can cause permanent physical damage. There are a few treatment approaches that seem to be useful. Cognitive behavioral treatment, and if the teem has been sexually molested (one out of three are) then a trauma therapy such as EMDR could prove effective.