Final answer:
Untreated cases of cystitis may progress to pyelonephritis, an infection of the kidneys.
Step-by-step explanation:
Untreated cases of cystitis may progress to pyelonephritis, which is an infection of the kidneys. Bladder infections, if left untreated, can spread to the kidneys and cause more severe symptoms and complications. It is essential to seek medical treatment for cystitis to prevent the progression of the infection.