Final answer:
A Direct Coombs test would be the most helpful in establishing the diagnosis for an 8-year-old with splenomegaly and abnormal CBC results, as it is used to detect autoimmune hemolytic anemia, which is consistent with the presence of splenomegaly due to the increased activity of the spleen in removing sensitized red blood cells.
Step-by-step explanation:
The student asks which test would be most helpful in establishing a diagnosis for an 8-year-old with splenomegaly and certain CBC results. Given the clinical presentation and given lab results (WBC count within normal limits, Hgb- 10.5 g/dl, and Hct- 31), it's important to distinguish among causes of anemia and systemic conditions that may be contributing to the splenomegaly. The choices provided are: Direct Coombs test (to detect immune-mediated hemolysis), Osmotic fragility (increased in hereditary spherocytosis), G-6-PD level (to detect G6PD deficiency), and Serum Ferritin (to evaluate iron stores).
To proceed with the most relevant test, a direct Coombs test would be done to identify autoimmune hemolytic anemia which is a condition where antibodies are directed against one's own red blood cells, causing their destruction. This would present with splenomegaly due to hyperactivity of the spleen in removing the sensitized red cells. In G6PD deficiency, a form of hemolysis occurs after exposure to certain drugs, infections, or foods, but it is not immune-mediated, so the Direct Coombs test should be negative. Serum Ferritin would be helpful if iron deficiency is being considered as a cause of anemia. Osmotic fragility test is specific for hereditary spherocytosis, a condition characterized by spherical red cells that are easily lysed in hypotonic solutions.