MRCP-0853

An 12 year old girl is taken to the clinic for a review. Despite being normally fit and well and not visiting a doctor for the past four years, her father is concerned about her behaviour in the past week. The girl has expressed plans to run for president, is active with planning all through the night and seems overtly excited. When asked, she admits to smoking cannabis once a few months ago and has drank alcohol a few times in the past year. The last time she had alcohol was a week ago. Prior to her deterioration a few weeks ago her father describes her as a happy, well-adjusted, sociable girl. Which one of the following is the most likely diagnosis?