PLAB-0322

A 20-year-old girl is presented to the OPD with her mother. Her mother reports that she eats very large portions of food most of the time, but takes diet pills and remains depressed because she thinks she is overweight. However, on general physical examination, she appears to be very thin and her blood pressure is lower than normal. Investigations reveal that she has hypokalaemia. What is this girl most likely suffering from?