The Renaissance genius Leonardo da Vinci left us only 10 to 20 paintings; just one resides in the U.S. (National Gallery). Last week, The Washington Post received an anonymous tip that the Museum of Fine Arts in Boston is in the process of authenticating a painting believed to be by the Italian master. "It would change the fortunes of the MFA if they had a real Leonardo," says Alex Nagel, an art history professor at New York University....