You are answering a USMLE-style medical question. Question: Two hours following an elective cholecystectomy, a 43-year-old woman has fever and chills. The patient received cephalexin for antibiotic prophylaxis and one unit of packed red blood cells during the procedure. She underwent a hysterectomy 9 years ago for leiomyomata uteri. She has a 5-year history of hypertension treated with lisinopril. Her temperature is 39.5°C (102.3°F), pulse is 90/min, respirations are 18/min, and blood pressure is 125/90 mm Hg. Examination shows a mildly tender abdominal wound without erythema. The lungs are clear to auscultation. Cardiac examination shows no abnormalities. She had a Foley catheter and peripheral line access placed at the time of the procedure. Which of the following is the most likely cause of this patient's symptoms? Options: A) Adverse effect of medication B) Malignant hyperthermia C) Urinary tract infection D) Transfusion reaction Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.