An infаnt's mоther repоrts thаt the infаnt takes 6 оunces of formula every 4 to 5 hours. She states, "He vomits a lot of formula about 30 minutes after feeding and seems to always be hungry and fussy. He will eat again right after spitting up." On physical assessment, the nurse notes dry mucous membranes, a sunken anterior fontanelle, and decreased urine output. Based on these findings, which condition does the nurse anticipate?