Understanding Society User Support

"mc" stands for multi-choice item. You can see the full question and the different multi-choice categories for d_hcondn2 here:
https://www.understandingsociety.ac.uk/documentation/mainstage/dataset-documentation/wave/4/datafile/d_indresp/variable/d_hcondn2

The automated labels for that type of items could provide information about both the question and individual response category, but this would typically result in labels that are too long and hence truncated. "mc arthritis" is a compromise that give you information about the item and at the same time alert you to the fact that it is part of a multi-choice set. We initially documented this in the user guide, but it seems it has later been cut.
Jakob