This is L-Isoleucine.
The question is from Berkeley Review Ochem ch3 passage 1 question 2:
"If the side chain chiral center were changed, the new structure would be switched to which of the following?
a. an enantiomer of isoleucine.
b. a diastereomer of isoleucine.
c. an epimer of isoleucine.
d. identical to isoleucine."
My answer was c, but BR's answer is b. I thought the definition of an epimer is a diastereomer that only differs by 1 stereocenter and has a hydrogen substituent. See orgohack's post. What gives? Bad question? Do I have an outdated version of the book? BerkeleyReviewTeach care to comment?