"... where the client relies on a trusted source (their ISP's DNS server..."
Well, for several major ISP's, maybe even most ISP's worldwide, you can't trust them not to use their DNS servers to show you ads or a sponsored "search engine" if you type a domain name that is not registered.
If a user really wanted a "trusted source" for DNS RR's she would be best to contact the appropriate authoritative nameserver directly, not an ISP's resolver.