user # inxi -r
Repos: Active apt sources in file: /etc/apt/sources.list
deb http://cran.ms.unimelb.edu.au/bin/linux/ubuntu raring/
deb-src http://cran.ms.unimelb.edu.au/bin/linux/ubuntu raring/
Active apt sources in file: /etc/apt/sources.list.d/official-package-repositories.list
deb http://packages.linuxmint.com olivia main upstream import #id:linuxmint_main
deb http://archive.ubuntu.com/ubuntu raring main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu raring-updates main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu/ raring-security main restricted universe multiverse
deb http://archive.canonical.com/ubuntu/ raring partner
deb http://packages.medibuntu.org/ raring free non-free

Synaptic returns an error:
E: Encountered a section with no Package: header
E: Problem with MergeList /var/lib/apt/lists/packages.medibuntu.org_dists_raring_free_i18n_Translation-en
E: The package lists or status file could not be parsed or opened.
E: _cache->open() failed, please report.

Similarly, from the console:

# apt-get install anki
Reading package lists... Error!
E: Encountered a section with no Package: header
E: Problem with MergeList /var/lib/apt/lists/packages.medibuntu.org_dists_raring_free_i18n_Translation-en
E: The package lists or status file could not be parsed or opened.

Edit:

sed -i '/^deb http:\/\/packages.medibuntu.org*/d' /etc/apt/sources.list /etc/apt/sources.list.d/*.list

seems to work for the installation in question. I should have made sure I had tried everything before bothering you all. Sorry.