Final answer:
The highest priority nursing intervention for a severely dehydrated child with gastroenteritis is to administer intravenous fluids to correct dehydration, with an ordinary NaCl solution or a mixture of isotonic saline and Na lactate depending on specific electrolyte losses.
Step-by-step explanation:
When admitting a 5-year-old with severe gastroenteritis and dehydration to the hospital, the highest priority nursing intervention is to correct dehydration. This can be achieved with the administration of fluids. For mild dehydration, oral rehydration solutions (ORS) are recommended and can be effective in preventing further dehydration. However, in severe cases of gastroenteritis with significant dehydration, as described in the scenario, the child may require intravenous fluids. According to clinical recommendations, an ordinary NaCl solution may be given parenterally to repair the fluid losses, and in certain conditions where high Na and HCO3 loss is experienced, a mixture of isotonic saline and Na lactate should be administered intravenously. This intervention is crucial because dehydration can adversely affect vital organ functions and lead to serious complications.