US Presidential Trivia: Who is the only U.S. President to have a doctorate?

U.S. Presidents, as you can likely imagine, have a history of being very well educated.  You'd hope that this would be the case - it is, after all, the smartest office in the land.  However, of the 43 men that have been President, only one has held a Doctorate.  So, from Pocket Trivia: US Presidents - Who is the only American President to hold a Doctorate?  Is it John Kennedy, Bill Clinton, Richard Nixon or Woodrow Wilson?