Name that CBC
Check out the CBC time series and make a diagnosis (without peeking at next graphic).
Pretty straightforward, whether one approaches the data from the perspective of pancytopenia or macrocytic anemia. The quick normalization of the CBC over a period of weeks is a strong clue.
Dx = vitamin B12 deficiency, treated with B12
Most of us are taught that a high RDW is characteristic of nutritional deficiency (B12, folate, iron), as it was in this case. However, anisocytosis is not 100% sensitive for B12 deficiency.
We are also taught that the RDW will further increase upon treatment of the nutritional deficiency… not so with B12 deficiency!