ETFDH and multiple acyl-CoA dehydrogenase deficiency: Patients with late-onset MADD often carry at least 1 missense variation with minor amounts of residual electron transfer flavoprotein/ETFDH activity, which is sufficient to prevent embryonic development of congenital anomalies.1 Unlike other types of MADDs, late-onset MADD diagnosis can be difficult because many patients may not display typical patterns of elevated urinary organic acids and serum acylcarnitine fatty acids during times of well-being.5–7 In plasma, acylcarnitines are characterized by elevations in short-chain, medium-chain, and long-chain acylcarnitines such as C4-C18:2.