A 26-year-old woman presents to the emergency department with complaints of fever, chills, and severe left-sided flank pain for the past 48 hours. She reports associated urinary symptoms, including dysuria, urgency, and frequency, that started a week ago. She denies vaginal discharge or abdominal pain. Her medical history is significant for recurrent UTIs, with two similar episodes in the past year. She is sexually active and uses spermicides as contraception. On examination, her temperature is 39.2°C (102.6°F), heart rate is 110 bpm, and blood pressure is 100/70 mmHg. She appears visibly uncomfortable, and palpation of the left costovertebral angle (CVA) elicits significant tenderness. Abdominal examination is unremarkable. Which of the following findings is most specific for acute pyelonephritis in this patient?
