A 78-year-old man is admitted with a 2-day history of confusion, lethargy, and reduced urine output. His observations are: BP 90/50 mmHg, HR 110 bpm, RR 22/min, Temp 38.5°C. Blood tests show: Na+ 130 mmol/L, K+ 5.8 mmol/L, Urea 25 mmol/L, Creatinine 350 µmol/L (baseline 110 µmol/L). Urinalysis shows +protein, +leucocytes. What is the most likely diagnosis?