The physician gives orders to infuse 1 unit (250 mL) of packed red blood cells over 4 hours. Calculate the flow rate in milliliters per hour.

Choice A reason: Flow rate is volume divided by time; 250 mL over 4 hours equals 62.5 mL/hr, ensuring safe transfusion pacing to avoid fluid overload or hemolysis. Choice B reason: 125 mL/hr assumes 250 mL over 2 hours; this doubles the ordered rate, risking circulatory overload and transfusion reactions in a standard protocol. Choice C reason: 250 mL/hr infuses the unit in 1 hour; this rapid rate exceeds safe limits, potentially causing hypertension or pulmonary edema in vulnerable patients. Choice D reason: 500 mL/hr is far too fast, implying 250 mL in 30 minutes; this dangerous speed could trigger severe hemolytic reactions or cardiovascular collapse.