PDC and lactic acidosis: To the best of our knowledge, this is the fourth case of functional PDC deficiency with a defined mutation in <i>PDP1</i>.<h4>Synopsis</h4>Pyruvate dehydrogenase phosphatase (PDP) regulates pyruvate dehydrogenase complex (PDC) and defective PDP due to <i>PDP1</i> mutations leads to PDC deficiency and congenital lactic acidosis.