The San Francisco 49ers' cornerback situation has been interesting. It's evolved since the offseason, and the group seems to be as strong as it has been all season as the team heads into the stretch run. The defense has played at a high level most of the year and has been dominant lately. The cornerbacks are big part of it.Let's look at the current core group:Tramaine Brock: He's been the best cornerback on the roster, answering every challenge as he has gone from a special-teamer to a nickel to a starter all because of injuries.