If you make a request:

http://www.musicbrainz.org/ws/2/discid/arIS30RPWowvwNEqsqdDnZzDGhk-?inc=record
ings+artists+artist-credits+url-rels

The list of media does not include disc ids, so there's no way to know which one applies.

If you add '+discids' to the query:

http://www.musicbrainz.org/ws/2/discid/arIS30RPWowvwNEqsqdDnZzDGhk-?inc=releas
+es+recordings+artists+artist-credits+url-rels+discids

you get an error:

discids is not a valid option for the inc parameter for the discid resource
unless you specify one of the following other inc parameters: releases

If you add 'releases' too:

http://www.musicbrainz.org/ws/2/discid/arIS30RPWowvwNEqsqdDnZzDGhk-?inc=releas
+es+recordings+artists+artist-credits+url-rels+discids

You get another error:

releases is not a valid inc parameter for the discid resource.