A 28-year-old woman at 28 weeks of pregnancy presents with low back pain and urinary frequency for 1 week, accompanied by low-grade fever (37°C) for 2 days, and occasional urethral pain after urination. Urinalysis shows pH 6.0, SG 1.015, Pro 0.3g/L, WBC 22/μl, RBC 8/μl, and occasional WBC casts/LP. Urine culture reveals Klebsiella species. The most likely diagnosis is: