What is the most likely cause of a CBC result showing low RBC, high MCV, and abnormal HGB levels?

The most likely cause of a complete blood count (CBC) result showing low red blood cell (RBC) count, high mean corpuscular volume (MCV), and abnormal hemoglobin (HGB) levels is related to vitamin B12 deficiency.

In vitamin B12 deficiency, the body's inability to properly utilize this essential nutrient leads to ineffective erythropoiesis, which results in the production of larger-than-normal red blood cells, hence the increased MCV. This condition often manifests as macrocytic anemia, characterized by the low RBC count and abnormal HGB levels due to the presence of immature or poorly formed red blood cells.

While cold agglutinin could potentially cause variations in CBC results, it typically does not lead to elevated MCV or macrocytic indices; rather, it can cause clumping of RBCs, which may artificially alter counts but not in the same manner as seen with vitamin B12 deficiency. Iron deficiency anemia generally presents with low MCV, reflecting microcytic red blood cells. Hemolytic anemia, on the other hand, usually shows a normocytic picture with normal or elevated reticulocyte counts as the body attempts to compensate for the destruction of red blood cells, rather than high MCV.