Which condition is characterized as an infection of the urinary bladder?

Cystitis refers specifically to the inflammation of the urinary bladder, often due to infection. It is characterized by symptoms such as frequent urination, urgency, pain during urination, and sometimes lower abdominal discomfort. The infection typically occurs when bacteria ascend the urethra into the bladder, leading to inflammation and irritation.

In contrast, pyelonephritis pertains to an infection of the kidneys, glomerulonephritis is primarily associated with inflammation of the glomeruli in the kidneys, and nephrotic syndrome is a collection of symptoms indicating kidney damage, which does not directly relate to a bladder infection. Thus, cystitis is the correct answer as it specifically identifies the infection of the urinary bladder.