In a job interview, it's crucial you don't pretend to know things that you don't, but you don't have to just say "no, I don't know about that." Here are three responses that are better than "no", in order of preference.
Show that you're at least familiar with the name. "No, I haven't. Are you doing some sort of enterprise-wide directory integration?" You're showing that you have some understanding of how it's used, and getting more information and it may turn out that you have a different, similar experience, and can turn this into Answer #1 above.
Ask what it is, and how it's used. "I'm sorry, no, I haven't even heard the term. What is LDAP, and how are you using it?" You'll show interest in learning more, and may find out that it's similar to something you've done before, and can upgrade your answer to Answer #2 above.
Please don't use the clichéd answer "No, I don't, but I'm a quick learner!" It's good to try to turn a negative into a positive, but "I'm a quick learner" is meaningless because anyone can say it. Use one of the three above.
Finally, don't think of it as a pass/fail quiz and worry that you're doomed for not knowing. I once asked a candidate, out of the blue, if he knew anything about LDAP, because I had been thinking about it as something my department might use. I thought he was going to have a heart attack as he stammered out his "Uh, uh, no, but, uh, I can learn pretty quick!" I reassured him it wasn't something we were using, but I was just curious. Chances are if you've been called in for an interview, you've already the core basic knowledge that truly is pass/fail.