Choice A rationale:

Elevated serum EPO levels.

Rationale: Primary polycythemia, also known as polycythemia vera, is characterized by the overproduction of red blood cells independent of normal regulatory mechanisms.

In response to the increased red blood cell count, the body typically tries to compensate by decreasing the production of erythropoietin (EPO), a hormone that stimulates red blood cell production.

Therefore, elevated serum EPO levels are often seen in primary polycythemia as the body tries to regulate the overproduction of red blood cells.

Choice B rationale:

Decreased platelet count.

Rationale: Platelet count is typically not decreased in primary polycythemia.

In fact, it may be increased due to the overall increase in blood cell production.

Choice C rationale:

Normal hemoglobin levels.

Rationale: Hemoglobin levels are typically elevated in primary polycythemia due to the increase in red blood cell mass.

Choice D rationale:

Low red blood cell count.

Rationale: In primary polycythemia, the hallmark is an elevated red blood cell count, not a low count.

The condition is characterized by the overproduction of red blood cells.