A client receiving a blood transfusion reports sudden onset of lower back pain, chills, and feeling flushed 15 minutes into the transfusion. What is the FIRST nursing action?

ACUTE HAEMOLYTIC TRANSFUSION REACTION (AHTR): MECHANISM: ABO incompatibility — transfused red cells are destroyed by recipient's antibodies; severe inflammatory cascade; SYMPTOMS: Lower back pain (classic — renal pain from haemoglobinuria); fever/chills; flushing; hypotension; haematuria; LIFE-THREATENING: Can cause acute renal failure, DIC, shock, and death; IMMEDIATE RESPONSE: STOP the transfusion FIRST — every additional millilitre of incompatible blood worsens the reaction; keep IV line patent (NS); notify RN and blood bank immediately; vital signs; obtain urine specimen; keep blood bag and tubing for blood bank analysis; return blood unit to blood bank; anticipate: IV fluids (maintain renal perfusion); diuretics; vasopressors if needed; CONTRAST WITH FEBRILE NON-HAEMOLYTIC REACTION: Just fever and chills without back pain/haematuria — less serious; COMPARE WITH ALLERGIC: Urticaria/hives — slow or stop transfusion, give diphenhydramine, may restart; ANAPHYLAXIS: Airway symptoms — stop permanently, epinephrine; KEY: ANY symptomatic blood reaction = STOP transfusion FIRST.