Question category: Gastroenterology
A 28 year old female presents with a history of diarrhoea for numerous years. She has been off work on several occasions due to this. On examination this is unremarkable and her bloods are normal except for a low potassium. She is brought into hospital for assessment and is asked to keep a stool chart which shows no evidence of diarrhoea. A colonoscopy is performed and this reveals brown discolouration of the mucosa and a biopsy reveals pigment laden macrophages. What is the most likely diagnosis?