A 50-year-old man on ICU develops acute liver failure from an unknown cause. His paracetamol level is undetectable, hepatitis A/B/C serology is negative, autoimmune screen is negative, and ceruloplasmin is normal. He has no history of drug or toxin exposure. Wilson's disease, Budd-Chiari, and ischaemic hepatitis have been excluded. What investigation should be sent to identify a potentially treatable cause?