EW's Michael Ausiello has the official description of what's wrong with Izzie (although you'll have to figure it out still), along with a fun tidbit pertaining to Derek's proposal ...

Q: Got any clues as to what's ailing Izzie on Grey's Anatomy?

A: Sources confirm to me exclusively that Izzie will learn in this week's episode that she has ***** * ********** ******** that has spread to her ***** and *****.

Q: I need some Mer-Der proposal scoop, please!

A: Derek's second attempt to pop the question goes haywire, but the third time's the charm - Patrick Dempsey (sort of) said so himself on The View yesterday!

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.