reno

One root, and it is short enough to look like a seam. Reno is a single indivisible root. There is no re- prefix in it and no -no ending: the word is ren- plus the noun -o, and that is the whole of it. Two renoj filter the sango and sit against the dorso, which is why kidney trouble is felt as backache rather than as anything you would call stomach ache.

The river is not this word. Rejno is the Rhine, and it is a different word in three separate ways: it carries a j, it takes a capital letter as a proper name does, and it comes from Celtic by way of German rather than from Latin. The two only look alike in English, where the Rhine has no j to warn you. Esperanto capitalises rivers and countries and leaves ordinary nouns lower case, so la Rejno and la reno never collide on the page.

The body words around it. Organo is the general word for an organ, stomako the stomach, koro the heart. Rena is the adjective and renŝtono a kidney stone, built the transparent way with ŝtono rather than borrowed whole from the Latin the way English borrowed renal calculus.