This is a medical research study to test a medication in adult patients with a disease called medium-chain acyl-CoA dehydrogenase (MCAD) deficiency caused by at least one copy of the 985A>G mutation. The medication is glycerol phenylbutyrate, called Ravicti, which is currently FDA approved for the treatment of urea cycle disorders. Previous research suggests that Ravicti may also be effective in the treatment MCAD deficiency. This study will investigate the safety and efficacy (how well it works) of Ravicti in patients with MCAD deficiency caused by having at least one copy of the 985A>G mutation.