An asymptomatic father diagnosed with 3-methylcrotonyl-CoA carboxylase deficiency following his son newborn screening test.

3-methylcrotonyl-CoA carboxylase deficiency (3MCCD) is a hereditary disorder of leucine catabolism caused by pathogenetic variants in the or genes. Typically diagnosed through newborn screening (NBS), 3MCCD is characterized by elevation of 3-hydroxyisovalerylcarnitine (C5OH) in blood as well as increased excretion of 3-methylcrotonylglycine (3-MCG) in urine. While most diagnosed children remain asymptomatic, data on adults are scarce.