A 68-year-old woman with atrophic gastritis and pernicious anemia for over 10 years has been receiving monthly vitamin B<sub>12</sub> injections for the same duration. Over the past month, she has experienced fatigue upon exertion. Laboratory results show hemoglobin 90g/L, mean corpuscular hemoglobin concentration (MCHC) 26%, mean corpuscular volume (MCV) 67fl, reticulocyte count 0.6%, and serum ferritin 13μg/L. The most likely diagnosis is: