It's a tree in a man's lung.
This Thursday on "Grey's Anatomy," April confronts Christina about a breach of protocol. Check out Christina's explanation in the sneak peek video clip below.
Did you miss an episode? Check out our Grey's Anatomy weekly recaps. Join us on Twitter @soapoperafan and Facebook. Come tweet and Facebook with us about "Grey's Anatomy," all the other soap operas, "Glee" and "Vampire Diaries!"