AKT-2326

A 65-year-old woman presents for follow up after being prescribed trimethoprim for a suspected urinary tract infection by an out-of-hours doctor two weeks ago. She reports experiencing lower abdominal pains and bloating for the past six months, which have become increasingly severe and often wake her from sleep. She denies vomiting or diarrhea but notes increased frequency of urination. She has a poor appetite and has lost no weight. On examination, there is no clinical evidence of anemia or jaundice, and PR examination is normal. Which investigation is most likely to reveal the cause of this patient’s symptoms?