What is Madonna's last name?

Some people only need one name to get the point across. Among this echelon are some of the greatest within their discipline: Pelé, Slash, Cher, Bono, and Flea, to name a few.

But then there's Madonna, who seems to hover above nearly all other one-namers in pop-culture history. And maybe all of history (sorry, Voltaire).

But she does have a last name, and in fact, she's never changed her name. What's Madonna's surname?