A nurse is preparing to administer a blood transfusion. Which assessment finding requires stopping the transfusion IMMEDIATELY?

ACUTE HAEMOLYTIC TRANSFUSION REACTION (AHTR) is the most life-threatening transfusion complication and is caused by ABO blood group incompatibility. CLASSIC SIGNS (appear within the first 15-50 mL): Sudden chills and fever; Low back pain (retroperitoneal lysis of red blood cells); Dark/red-brown urine (haemoglobinuria from lysed red blood cell haemoglobin); Chest tightness and dyspnoea; Hypotension and tachycardia; Flank pain; Anxiety and feeling of 'impending doom.' NURSING RESPONSE — SEQUENCE: (1) STOP THE TRANSFUSION IMMEDIATELY — the most critical step; do not flush the line; (2) MAINTAIN IV ACCESS — keep the IV line open with normal saline using a NEW IV tubing set (the blood-containing tubing must not be used to infuse anything further); (3) NOTIFY THE PROVIDER AND BLOOD BANK IMMEDIATELY; (4) MONITOR vital signs every 5 minutes; (5) SEND the blood bag, new tubing, and blood samples to the blood bank for investigation; (6) COLLECT urine sample (haemoglobinuria confirms AHTR); (7) SUPPORT as ordered: IV fluids; vasopressors if hypotensive; maintain urine output above 100 mL/hour to prevent acute tubular necrosis from haemoglobin precipitating in renal tubules; PREVENTION: Two-nurse verification of patient ID and blood product compatibility before every transfusion is the primary prevention measure.