A nurse is assisting with the admission of a 2-year-old toddler who has acute gastroenteritis. Which of the following actions should the nurse take first?

Initiate isotonic fluids with 20 mEq/L potassium chloride.

Request evaluation of the toddler's serum electrolytes.

Determine if the toddler is voiding.

Collect a stool sample from the toddler