A 51-year-old white woman with a history of pernicious anemia developed increasing lethargy and shortness of breath without neurologic or B symptoms. Investigations revealed the following: white blood cells 2.8 × 10⁹/L, hemoglobin 62 g/L (mean cell volume 126.3 fL), platelets 82 × 10⁹/L, and evidence of hemolysis (unconjugated bilirubin 58 μmol/L, lactate dehydrogenase >2500 U/L, reticulocytes 45 × 10⁹/L, and haptoglobin <0.07 g/L) with negative direct antiglobulin test. Red cell folate was normal, but total vitamin B₁₂ level was decreased at <22 pmol/L; homocysteine and methylmalonic acid were elevated (21.5 μmol/L and 16.6 μmol/L, respectively). Peripheral blood film showed macrocytes and mild fragmented erythrocytes. Bone marrow (BM) aspirate revealed a hypercellular marrow with megaloblastic and dysplastic features in myeloid and erythroid lineages (panels A-B), along with 10% to 15% ring sideroblasts (panels C-E). Marrow cytogenetics revealed a normal female karyotype.

A month after receiving intramuscular vitamin B₁₂ injections daily for 1 week, then weekly, the patient became asymptomatic. Her blood counts and vitamin B₁₂ level normalized, thus arguing against an alternative diagnosis of myelodysplasia (BM aspirate was not repeated). This is a rare presentation of vitamin B₁₂ deficiency with striking megaloblastic features and ring sideroblasts. Rather than presumptively diagnosing myelodysplasia, timely recognition and vitamin B₁₂ supplementation lead to resolution of symptoms and blood work abnormalities.