You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.

Category: HOSPITALS 
Clue: The Cleveland Clinic tops U.S. News' 1996 ranking of hospitals in this, the study of the heart
cardiology

Category: QUEEN ELIZABETH II 
Clue: It's Queen Elizabeth's official London residence
buckingham palace

Category: PREHISTORIC CRITTERS 
Clue: If Horton climbed his family tree, he'd go from elephant to stegodon to stegolophodon to this

mastodon