Your DNS administrator has changed the DNS entry for RouterB. You clear the DNS cache on RouterA and ping routerb.sybex.com from RouterA, but you still ping the original address prior to the change. All other DNS addresses work fine and the entry resolves correctly on your laptop. What is the problem?

If the cache is cleared after the change has been made to DNS and you still get the same IP address, the reason is most likely that there is a host entry configured. The command show running-config will show you if there is an entry. The router is not likely to be configured to the wrong DNS server because other domains resolve correctly. The DNS administrator could have made an error, but it resolves correctly on your laptop. The domain name is not the problem because it resolves correctly on your laptop.