Kidney
From Memory Alpha, the free Star Trek reference.
The kidney is an excretory organ. It functions to maintain the water, the acid-base concentration, and filter the blood; the waste products are excreted as urine. The organ is described by the Malcorians and Gosis' species as the renal organ.
Denobulan lemur kidneys were a delicacy (ENT: "A Night in Sickbay"), as was kidney pie on Earth. (TNG: "The Wounded")
After living with Spock's katra in his head, Leonard McCoy stated that he would have "felt safer giving him one of his kidneys than having what was scrambled in his brain." (Star Trek III: The Search for Spock)
Kidneys were featured in a joke Data told Geordi La Forge in early 2364. (TNG: "Code of Honor")
In 1986, Dr. Leonard McCoy gave a kidney pill to an elderly patient in San Francisco who was suffering from "kidney dialysis." The pill apparently caused the spontaneous growth of a new kidney.
There is a widely circulated tale amongst Starfleet cadets about a man who got invited to an evening of passion with a woman on Risa, only to wake up with a missing kidney. (VOY: "Fury")
