A 58-year-old woman presents with tiredness, a sore tongue and difficulty judging the position of her feet in the dark. She has a macrocytic anaemia, a low serum vitamin B12 concentration and a normal folate concentration; the blood film shows oval red cells and neutrophils with excessive nuclear lobulation. Testing performed late in the assessment identifies antibodies directed against intrinsic factor. Which one of the following is the most likely diagnosis?