Final answer:
The combining form that refers to "kidney" is nephr/o, which relates to the kidneys and their functional units, the nephrons.
Step-by-step explanation:
The combining form that means "kidney" is nephr/o. This term is used in various medical contexts when referring to the kidneys. The kidneys are pivotal in the urinary system, which also includes the ureters, urinary bladder, and urethra, responsible for filtering waste products and excess water from the blood and excreting them from the body in urine. Each kidney contains over a million tiny units called nephrons that filter blood and form urine. The other options provided, such as ur/o (which relates to urine), glomerul/o (which relates to the glomeruli within the nephron), and pyel/o (which relates to the renal pelvis of the kidney), while related to kidney function or parts, do not denote the kidney itself.