A 34 years old woman who is otherwise healthy has had a fever, nausea, and right-sided back pain for 2 days. The physical exam shows a temperature of 102.2ºF (39.0ºC), blood pressure of 120/60 mmHg, pulse of 110 bpm, respiratory rate of 18 bpm, and right-sided costovertebral angle tenderness to percussion. Dipstick urinalysis is positive for leukocytes, nitrites, and blood. What is the most likely diagnosis?