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?

Determine if the toddler is voiding.

Request evaluation of the toddler's serum electrolytes.

Initiate isotonic fluids with 20 mEq/L potassium chloride.

Collect a stool sample from the toddler.