A 34 year-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 102.2°F (39.0°C), blood pressure of 120/60, pulse of 110, respiratory rate of 18, and right-sided costovertebral angle tenderness to percussion. Dipstick urinalysis is positive for leukocytes, nitrites, and blood. What is the most likely diagnosis?