A 42 year old man presents with numbness and a tingling sensation in both lower limbs, associated with generalised weakness and fatigability for 3 months.

He also experienced occasional malodorous, bulky stools and lost 8 kg of weight during the preceding year. There was no history of alcohol abuse, rectal bleeding, or joint pains.

His full blood count shows a Hb of 8.7 g/dL and MCV of 112 fL. A blood film shows a megaloblastic anemia with hypersegmented neutrophils.