Irritable Bowel Syndrome (IBS): This condition is characterized by chronic abdominal pain, bloating, and alteration of bowel habits in the absence of any mechanical, inflammatory, or biochemical abnormalities. The long duration of symptoms (8 years) and the presence of defecating urgency are consistent with IBS, which often begins in adolescence.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea, abdominal pain, and urgency. Although IBD can present at any age, the onset in adolescence is not uncommon.
Gastroesophageal Reflux Disease (GERD): While primarily known for causing heartburn, GERD can also lead to symptoms of abdominal pain and changes in bowel habits due to the reflux of stomach contents into the esophagus.
Functional Gastrointestinal Disorders: Other functional disorders like functional diarrhea or functional abdominal pain syndrome could also be considered, given the chronic nature of the symptoms without an identifiable cause.

Infectious Causes (e.g., Giardiasis, Amoebiasis): Chronic infections can cause prolonged gastrointestinal symptoms, including diarrhea and urgency. These conditions are crucial to rule out due to their potential for treatment and the risk of complications if left untreated.
Celiac Disease: An autoimmune reaction to gluten can lead to chronic diarrhea, abdominal pain, and malabsorption. It's essential to consider this diagnosis due to its potential for significant morbidity if not treated with a gluten-free diet.
Colorectal Cancer: Although extremely rare in teenagers, any chronic gastrointestinal symptom warrants consideration of malignancy to avoid missing a potentially curable condition.

Hirschsprung's Disease: Typically diagnosed in infancy, this congenital condition can sometimes present later in childhood or adolescence with chronic constipation or, less commonly, with symptoms of urgency and diarrhea due to enterocolitis.
Neurological Disorders (e.g., Spinal Cord Injury, Multiple Sclerosis): Certain neurological conditions can affect bowel habits, leading to urgency and frequency. These are rare but important to consider, especially if there are other neurological symptoms.
Endocrine Disorders (e.g., Thyrotoxicosis, Diabetes Mellitus): Some endocrine conditions can cause gastrointestinal symptoms, including diarrhea and urgency, due to the metabolic effects of the disorder.