Final answer:
A nurse should recognize a sunken fontanelle as an indication of moderate to severe dehydration in an infant with rotavirus experiencing acute vomiting and diarrhea.
Step-by-step explanation:
A nurse in an emergency department assessing a 3-month-old infant with rotavirus experiencing acute vomiting and diarrhea must be vigilant for signs of moderate to severe dehydration. One key indicator of this condition is a sunken fontanelle. Increased urine output and moist mucous membranes would generally not be associated with dehydration, and while a decreased heart rate can be associated with dehydration, more commonly in infants, dehydration leads to an increased heart rate due to the body's attempt to maintain blood pressure.
Therefore, the nurse should identify option A) Sunken fontanelle as an indication that the infant has moderate to severe dehydration, which is a critical condition that can lead to more serious symptoms like difficulty waking, seizures, or even death if not treated promptly.