Which structure allows a bladder infection to spread to the kidneys?

The ureters play a critical role in the urinary system as they are the tubes that carry urine from the bladder to the kidneys. In the context of a bladder infection, known medically as cystitis, the bacteria can travel from the bladder through the ureters and ascend toward the kidneys, potentially leading to a more serious infection called pyelonephritis. This pathway allows for the spread of infection, which is why understanding the function of the ureters is essential when considering how infections can escalate within the urinary tract.

The other structures listed do not serve the same role in the context of infection spread. The urethra, while it is the duct that drains urine from the bladder to the outside of the body, does not connect to the kidneys. The bladder itself is the site of the initial infection, and the kidneys are the target organ affected if this infection spreads. Thus, the ureters are the critical pathway for such potential complications in urinary tract infections.