Urea cycle disorders are commonly seen in the newborn and paediatric population, however presenting cases in adults are uncommon and are usually misdiagnosed before a correct diagnosis is elicited. Several enzymes are involved in the urea cycle, and a defect in any of them can lead to specific laboratory abnormalities and variable clinical presentations. Deficiency of the enzyme ornithine transcarbamylase (OTC) is the most common inherited urea cycle disorder; however, other less common enzyme deficits have been described. In adults, manifestations range from severe encephalopathy to subtle psychiatric manifestations. Prompt diagnosis and recognition are essential to a successful outcome. In this article, we present the case of a 28-year-old female admitted to an A&E department with encephalopathy of unknown aetiology resulting in a diagnosis of carbomoyl synthetase 1 deficiency as well as an unfortunate outcome.