A patient asks about contraception and using an IUD. The nurse explains the IUD best prevents conception by:

A. Blocking the opening to the cervix: Intrauterine devices are placed inside the uterine cavity and do not physically block the cervical opening. Cervical barriers such as diaphragms or cervical caps function at the level of the cervix rather than within the uterus. The contraceptive effect of an IUD occurs after sperm enter the reproductive tract. B. Increasing the mobility of the uterus: Uterine mobility is not altered by the presence of an intrauterine device. IUDs do not affect uterine movement, peristalsis, or positioning. Their action is related to local inflammatory and biochemical effects within the uterine environment. C. Preventing sperm from reaching the fallopian tubes: IUDs do not create a mechanical barrier that blocks sperm transport through the cervix or uterus. Sperm may still enter the uterine cavity, but their function and survival are impaired. Fertilization is prevented through changes in the uterine and tubal environment. D. Not keeps sperm from reaching the egg by creating a hostile environment within the uterus: IUDs create a local inflammatory response in the uterus that is toxic to sperm and ova. Copper IUDs release ions that reduce sperm motility and viability, while hormonal IUDs thicken cervical mucus and alter the endometrium. These changes interfere with fertilization and implantation.