Rare inherited disorder marked by the inability to process certain proteins and lipids correctly due to impaired metabolism of methylmalonic acid. An autosomal recessive disorder caused by a mutation on the MUT, MMAA, MMAB, MMADHC, or MCEE genes. Typically presents early in infancy as the patient consumes increased amounts of protein. Common initial findings include vomiting, dehydration, lethargy, developmental delay, hypotonia, failure to thrive, and hepatomegaly. Progressive complications include difficulty feeding, pancreatitis (acute, chronic), chronic kidney disease, and intellectual disabilities. Cases can be mild or life-threatening. Treatment includes a specialized diet and, in severe cases, liver and/or kidney transplantation may be required.