"Methylmalonic" is an adjective used to describe a compound called methylmalonic acid. It is important for our body's metabolism and energy production. Full definition
Without it, the buildup of methylmalonic acid in the blood can cause weakness and slowed development, kidney and liver damage, and even seizures and stroke. (sciencemag.org)
A kit used to collect exhaled breath for metabolic analysis in a study of methylmalonic acidemia (MMA). (sciencedaily.com)
There is a clear association between B12 deficiency and proton pump inhibitors, so I checked his B12 level and also checked for methylmalonic acid, which is the best test for early B12 deficiency. (ultrawellnesscenter.com)