Final answer:
The combining form for the sac that stores urine is 'cyst/o', which refers to the urinary bladder that stores urine until excretion.
Step-by-step explanation:
The combining form for the sac that stores urine is cyst/o. This prefix is derived from the Greek word "kystis", which means bladder, and is used to refer to the urinary bladder, a hollow organ that stores urine until it is ready to to be excreted. The urinary bladder is part of the urinary system, which includes the kidneys, ureters, and urethra. After urine forms in the kidneys, it travels down the ureters and is stored in the bladder. The bladder has a peritoneal covering only over the dome and features a one-way valve mechanism at the junction of the ureters and the bladder to prevent reflux. The full term "cystitis" for instance, refers to the inflammation of the bladder.