Final answer:
Dialysis patients often become anemic primarily due to a decreased production of erythropoietin (EPO) by the kidneys, leading to reduced red blood cell production and an inadequate supply of oxygen to the body's tissues.
Step-by-step explanation:
The primary reason dialysis patients are anemic is due to a decreased production of erythropoietin (EPO), a hormone that is critical for the production of red blood cells (RBCs). Dialysis is often required for individuals with chronic kidney disease or renal failure; these conditions can significantly reduce the kidneys' ability to produce EPO. Consequently, this leads to ineffective hematopoiesis, resulting in insufficient numbers of RBCs and subsequent anemia.