Fosdenopterin provides an exogenous source of cPMP. cPMP is converted to molybdopterin, which is then converted to molybdenum cofactor (MoCo) and restores molybdenum cofactor biosynthesis. In MOCD Type A, mutations in MOCS1 lead to deficient MOCS1A/B-dependent synthesis of cPMP, which subsequently results in deficient molybdenum-dependent enzyme activity (e.g., sulfite oxidase).