A young girl presents with abdominal pain and a recent change in bowel habit, with passage of mucus in stool. There is no associated blood in stool and symptoms are increased with stress. The most likely diagnosis is:

IBS is a disorder for which no pathognomonic abnormalities are identified. Females are more commonly affected. People of all age groups are affected, but most have the onset of symptoms before 45 years. Patients usually presents with recurrent lower abdominal pain, abdominal bloating and altered bowel habits. Stool is accompanied by large amount of mucus, Bleeding is not a feature. Symptoms occur at times of stress or emotion. Ref: Harrison's principles of internal medicine 18th edition, Chapter 296; Current medical diagnosis and treatment 2012, Chapter 35; Current medical diagnosis and treatment 2012, Chapter 15.