A 66-year-old male presents the emergency room accompanied by his wife who claims that he has been acting confused. the man is complaining of a sudden onset of severe weakness and malaise and has a dry cough and diarrhea. his temperature is 102.8°f and his blood work indicates his sodium level at 126 meq/l (normal 135-145 meq/l). based on this assessment, the nurse suspects the patient has: