Which one of the following word roots means "kidney"?

The word root that means "kidney" is "nephr-." This term is derived from the Greek word "nephros," which specifically refers to the kidney. It is commonly used in medical terminology to form various terms related to kidney function, conditions, and anatomy. For example, "nephrology" is the branch of medicine that deals with kidney health and diseases.

The other roots do not refer to the kidney. "Uret-" pertains to the ureter, which is the tube that carries urine from the kidneys to the bladder. "Cyst-" relates to the bladder or any cystic structure, often used to describe fluid-filled sacs. "Urethr-" refers to the urethra, the duct through which urine is excreted from the bladder. Understanding these distinctions helps in accurately interpreting medical terminology and its applications in clinical settings.