Among the options provided, the patient who is most likely to develop a urinary tract infection (UTI) is a woman who has completed menopause.
Postmenopausal women tend to have lower levels of estrogen, which can lead to changes in the vaginal flora and urinary tract, making them more susceptible to infections. Although UTIs can occur in both males and females of various ages, females (particularly postmenopausal women) are at a higher risk compared to younger males and pre-menstrual females.
The other options listed (a man in his early 20s, a girl who has not yet started menstruating, and a boy with a history of irritable bowel syndrome) typically have lower associations with UTIs compared to postmenopausal women.