AUH and 3-methylcrotonyl-CoA carboxylase deficiency: However, the C5OH index is a biomarker for several IMDs, including MCD (BTD and HLCS deficiency), 3-hydroxy-3-methylglutaryl-CoA lyase (HMG-CoA) deficiency, 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency, and 3-methylglutaconyl-CoA hydratase deficiency [22].