Final answer:
A lab result indicating effective treatment for dehydration in a child receiving oral rehydration would be decreased urine specific gravity, which reflects normalization of fluid balance by showing improved kidney function relative to dehydration status.
Step-by-step explanation:
A nurse is caring for a school-aged child who has dehydration and is receiving an oral rehydration solution. An effective treatment regimen for dehydration would result in the normalization of hydration status and electrolyte balance. In such cases, one would expect an improvement in laboratory tests that reflect the patient's hydration status. Therefore, the lab result that indicates that the treatment regimen is effective would be C. Decreased urine specific gravity. This shows that the kidneys are reabsorbing less water, suggesting that body fluids are returning to a more normal state. Conditions such as increased blood urea nitrogen (BUN) or elevated hematocrit levels would typically indicate dehydration due to hemoconcentration, and increased blood sodium levels (hypernatremia) may imply a loss of water greater than a loss of sodium. Hence, decreased urine specific gravity is a sign that rehydration is working, and the body is restoring its fluid balance.