Which of the following is the most likely risk factor for the development of urinary tract infections in individuals assigned female at birth?

Spermicide use is a well-established risk factor for urinary tract infections (UTIs) in individuals assigned female at birth. Spermicides, particularly nonoxynol-9, alter the normal vaginal flora by suppressing Lactobacillus species that maintain an acidic vaginal pH. This disruption allows uropathogens, most notably Escherichia coli, to colonize the periurethral area and ascend into the bladder. Spermicide use is especially risky when combined with diaphragm use, further elevating UTI incidence. Hot tub use can introduce bacteria near the perineum but is not considered a primary independent risk factor with the same evidence base as spermicides. Intrauterine devices (IUDs) are not a recognized risk factor for lower UTIs; they are associated with pelvic inflammatory disease if insertion occurs in the setting of undiagnosed infection, but not with bladder infections. Nulliparity is not associated with increased UTI risk; in fact, some studies suggest parity-related anatomical changes may influence pelvic floor disorders rather than UTI frequency. Sexual intercourse, a short urethra, hormonal changes, and spermicide use remain the most clinically important modifiable risk factors for recurrent UTIs in this population.