An amino acid metabolic disorder that is classified by inadequate levels of the enzyme 3-methylcrotonyl-CoA carboxylase that helps break down proteins containing the amino acid leucine. This disease has_symptom muscular hypotonia (weak muscle tone), has_symptom muscular atrophy, has_symptom feeding difficulties, has_symptom recurrent episodes of vomiting and diarrhea, and has_symptom lethargy.