--stands for methymolanic acid, which tends to be elevated in B12 deficiency states. It is a more sensitive (though less specific) test than a standard B12 serum assay, as it roughly correlates to how much usable B12 is available--elimination of MMA metabolically requires B12 as a cofactor, so high MMA indicates there's not a lot of B12 chemically available.

Treatment with methylcobalamin B12 will tend to drive high MMA levels down fairly quickly, but one may still need large doses of B12 for long periods of time to help accomplish whatever tissue repair is possible.