A 35-year-old woman with a history of recurrent urinary tract infections presents with fever (38.5°C), flank pain, and costovertebral angle tenderness. Urine culture grows a gram-negative bacillus that is oxidase-negative, ferments lactose, and produces pink colonies on MacConkey agar. The organism is resistant to ampicillin but sensitive to third-generation cephalosporins. Which of the following is the most likely causative organism?