What does the term nucleated red blood cell (NRBC) refer to?

The term nucleated red blood cell (NRBC) specifically refers to a red blood cell that has not yet lost its nucleus. In the development of red blood cells, they undergo several stages in the bone marrow, starting from the hematopoietic stem cell to the mature erythrocyte. NRBCs are essentially immature red blood cells that are in an earlier stage of development, where they still contain a nucleus.

Once red blood cells mature, they undergo a process called enucleation, during which they expel their nucleus to maximize space for hemoglobin, the protein that carries oxygen. Therefore, NRBCs are typically found in the erythropoietic (red blood cell-producing) stages of development before they reach maturity. Their presence in the peripheral blood can indicate certain medical conditions, including anemia or bone marrow stress, as they usually remain in the bone marrow.

The other options do not accurately describe NRBCs; a mature red blood cell is enucleated, while white blood cells developing into their final forms and red blood cells that have already lost their nuclei are distinct from NRBCs.