A 40-year-old woman presents with 5 days of high fever, low back pain accompanied by frequency of urination, dysuria, and urgency. She took ciprofloxacin for 2 days without improvement. No prior similar episodes. A renal pelvis stone was detected 1 month ago but left untreated. Exam: temperature 39°C, right costovertebral angle tenderness. Urinalysis: urinary protein (+), WBC 20-30/HP, occasional WBC casts, urine specific gravity (USG) 1.025. The most likely diagnosis is: