Multiple OXPHOS deficiency in the liver of a patient with CblA methylmalonic aciduria sensitive to vitamin B12

Summary An adult patient with methylmalonic aciduria due to defective cobalamin synthesis (CblA) responsive to vitamin B 12 presented suddenly with severe visual impairment ascribed to optic atrophy followed by a fatal multiorgan failure and lactic acidosis but low methylmalonic acid in plasma and urine. Multiple deficiency of oxidative phosphorylation was found in the patient's liver. We suggest that patients with B 12 ‐sensitive methylmalonic aciduria who have a milder clinical course should be carefully monitored for long‐term complications.