Fatal infantile lactic acidosis with methylmalonic aciduria is a rare neurometabolic disease characterized by infantile onset of severe encephalomyopathy, lactic acidosis and elevated methylmalonic acid urinary excretion. Clinically it manifests with severe psychomotor delay, hypotonia, failure to thrive, feeding difficulties and dystonia. Epilepsy and multiple congenital anomalies may be associated. The disease is Monarch Disease Ontology term MONDO_0009504 (mitochondrial DNA depletion syndrome 9). Also known as: SUCLG1 mitochondrial DNA depletion syndrome, mitochondrial DNA depletion syndrome caused by mutation in SUCLG1, mitochondrial DNA depletion syndrome type 9.