31st July 2023

A 29-year-old female presents with tongue pain and swelling

A 29-year-old female presents with tongue pain and swelling. She has also recently started to have regular tingling in her feet, which is not relieved by a change in position. On systems review, there appear to be no gastroenterological symptoms. On examination, there is oedema and loss of papillae on the tongue. There is a slightly reduced sensation in the feet, though the rest of the neurological examination is unremarkable.

She turned vegan four years ago. B12 levels are below the reference range, but folate and haemoglobin levels are normal. Autoantibodies are negative.

What is the most likely cause of the B12 deficiency?

Select the SINGLE most appropriate option from the list below. Select ONE option only.