3-methylcrotonyl-coa carboxylase deficiency is a rare inherited disorder affecting leucine metabolism. It presents with a variable clinical picture, from metabolic crises in infancy to asymptomatic adults. The condition is caused by mutations in the MCCC1 or MCCC2 gene, inherited in an autosomal recessive manner. Symptoms can include poor appetite, lack of energy, irritability, weakness, nausea, vomiting, feeding difficulties, delayed development, lethargy, and hypotonia. If untreated, it can lead to seizures, breathing difficulties, and comas.