Combined methylmalonic aciduria and homocystinuria, cblC type (Cobalamin C deficiency) is an inherited disorder where the body cannot use vitamin B12 correctly. This is due to the deficiency of multiple enzymes. Onset of symptoms can begin before birth all the way to adulthood. Babies often have restricted growth during development before birth. Heart and brain defects, as well as small head size are present at birth. After birth, infants with this disorder can have poor feeding, poor weight gain, seizures, developmental delay and intellectual disability, a blood problem that results in very large red blood cells, and weak muscles. They may also develop vision problems and skin rashes. Individuals with cobalamin C deficiency can be treated with hydroxycobalamin to help with some of these problems, especially if treatment begins early in life. However, treated individuals may still have motor and language delays and intellectual disability. Cobalamin C deficiency is caused by pathogenic variants in the MMACHC gene.