Beth, aged 18, has been trying to keep a secret from her family and friends. She is a binge eater. When others are not around, she gorges on cakes, ice cream and other food so fast that she hardly has time to chew. Once she begins there is no stopping until her stomach aches. After her binge, Beth feels disgusted with herself and makes herself vomit, then takes a double dose of laxative. Her life is totally dominated by her obsession with food. Beth is suffering from which disorder?