A form of methylmalonic acidemia with homocystinuria, an inborn error of vitamin B12 (cobalamin) metabolism characterized by megaloblastic anemia, lethargy, failure to thrive, developmental delay, intellectual deficit and seizures. cblC type methylmalonic acidemia with homocystinuria is caused by mutations in the MMACHC gene (1p36.3) and is transmitted in an autosomal recessive manner.