BACKGROUND Anorexia nervosa has been referred to as the "great pretender" of medicine and is often misdiagnosed. We present a rare genetic disorder that was misdiagnosed as anorexia nervosa. This case highlights the diagnosis of mitochondrial neurogastrointestinal encephalomyopathy in a patient with previously diagnosed anorexia nervosa, which was discovered due to the metabolic abnormalities and intolerance of nutrition encountered during refeeding. CASE REPORT A 24-year-old woman with a previous diagnosis of type 2 diabetes mellitus and psychogenic anorexia and recent diagnosis of avoidant restrictive food intake disorder presented to a medical stabilization unit for weight restoration and treatment of medical complications associated with extreme malnutrition. She reported being underweight her entire life, and had a body mass index of 12 kg/m2 for a year prior to admission. She reported a long-standing intolerance to enteral feeding and had not gained weight when prescribed parenteral nutrition. She reported a desire to gain weight and denied any body dysmorphia. Upon nutritional rehabilitation, the patient was found to have extreme insulin resistance, with serum glucose values exceeding 400 mg/dL, and multiple neurologic findings that were incongruent with the medical complications of anorexia nervosa. These metabolic abnormalities, with her physical examination findings, allowed for a diagnosis of mitochondrial neurogastrointestinal encephalomyopathy. CONCLUSIONS This case report highlights the clinical presentation of mitochondrial neurogastrointestinal encephalomyopathy, including the newly described extreme insulin resistance, and showcases the importance of avoiding confirmation bias and working through differential diagnoses that are inconsistent with a clinical presentation to arrive at the correct diagnosis.