A12-month-old african american girl with sickle cell disease is brought to the emergency department because of the sudden onset of high fever, irritability, and tachypnea 6 hours ago. her diet consists of iron-fortified formula and pureed fruits and vegetables. her mother states that she only misses an occasional dose of her penicillin prophylaxis. she appears ill. her temperature is 39°c (102.2°f), pulse is 180/min, and respirations are 48/min. examination shows conjunctival pallor. a grade 2/6 systolic murmur is heard best at the left sternal border. laboratory studies show a hematocrit of 23%, leukocyte count of 23,000/mm³, and platelet count of 250,000/mm³. the most appropriate initial step in management is administration of which of the following?