A 22-year-old university student presents to the walk-in centre with a 2-day history of painful urination, increased urinary frequency, and suprapubic discomfort. She is otherwise well and has no vaginal discharge or fever. A urine dipstick shows leukocytes ++ and nitrites +. What is the most appropriate antibiotic choice for uncomplicated lower UTI in this non-pregnant woman, according to NICE guidance?