NDUFS1 and Leigh syndrome: Most patients with complex I deficiency have elevated blood and CSF lactate levels due to mitochondrial redox dysfunction[8,21,22] and an increased lactate/pyruvate ratio; the lactate/pyruvate ratio exceeded 20 in this patient, suggesting MRC dysfunction.[23] Patients with LS caused by NDUFS1 mutations also show normal blood lactate and urinary organic acid levels.