BCKDHB and maple syrup urine disease: Maple syrup urine disease (MSUD) is a rare autosomal recessive inherited disorder of branched‐chain amino acid metabolism caused by mutations in BCKDHA, BCKDHB, and DBT that encode the E1α, E1β, and E2 subunits of the branched‐chain α‐ketoacid dehydrogenase (BCKD) complex.