MSFinals-1431

A 29-year-old man was involved in a motorcycle race accident where a rod pierced his abdomen. He underwent emergency surgery and survived. After a year, he complains of abdominal pain and frequent stools, and is diagnosed with anemia. His blood tests show a vitamin B12 level of 41 (160–900 pmol/l) and serum folate of 35 ug/l (4.20 – 18.70 ug/l). He denies any dietary intolerance or rectal bleeding. What is the gold standard test for diagnosing this condition?