uk-drcog MCQ (SBA): Suspected Ovarian Cancer

A 62-year-old postmenopausal woman presents to the GP with a 2-month history of bloating, abdominal distension, and early satiety. She has lost 3 kg. On examination, there is a palpable pelvic mass and shifting dullness suggestive of ascites. Which is the single most appropriate initial investigation?