Combined malonic and methylmalonic acidemia is a rare inherited metabolic disorder caused by a deficiency of the enzyme methylmalonyl-CoA mutase. This enzyme is responsible for breaking down certain proteins and fats in the body. Without it, the body is unable to properly process these substances, leading to a buildup of malonic and methylmalonic acids in the blood and urine. Symptoms of this disorder can include poor feeding, vomiting, lethargy, seizures, and developmental delays. Treatment typically involves a combination of dietary modifications, vitamin supplementation, and medications.