Final answer:
The synthesis of erythropoietin is primarily stimulated by low oxygen levels in the blood, not by dehydration, high oxygen levels, low altitude, or an increased red blood cell count.
Step-by-step explanation:
Erythropoietin (EPO) is a hormone produced by the kidneys that stimulates the production of red blood cells in response to hypoxic (low oxygen) conditions in the body. Dehydration can indeed lead to an increased hematocrit, but the primary driver for EPO synthesis is the oxygen level in the blood. High blood oxygen levels would generally not lead to an increase in EPO production. Living at a low altitude means that there is more oxygen available in the atmosphere, which typically would decrease the need for additional red blood cells. Conversely, an increased red blood cell count would signal that there is already an ample amount of red blood cells supplying the body's oxygen needs, and there would be no requirement to produce more EPO. Therefore, none of these conditions would typically prompt an increase in EPO production. In fact, a decrease in oxygen levels, such as that experienced in high altitude or after blood loss, would be more likely to prompt an increase in EPO synthesis.