A 35 year old Caucasian woman presents with severe, intermittent abdominal pain for two days, in association with nausea and vomiting. She also complains of progressive weakness of both lower limbs. Her husband claims that she has "not been herself" for the last few days.

Her medical, surgical, and family histories are unremarkable. She only drinks socially and does not smoke. She had started the Atkins diet a month ago.

Serial ECGs are completely normal, as is a random plasma glucose assay, serum electrolyte levels, renal and liver profiles, and a noncontrast CT study of the brain.

A sample of urine is submitted for investigation. It is initially clear, but then turns into a dark red color upon standing.