In order to determine whether the levels of methylmalonic acid in different body fluids might be biomarkers for the diagnosis of MMA-induced cognitive impairment, the levels of methylmalonic acid in urine, serum, serum total exosomes and serum neuronal-derived exosomes were detected in healthy children, non-inherited HHcy patients and cblC-MMA patients. Here, CBLC is linked to Cognitive impairment.