A 61-year-old woman was evaluated because of 2 days of anuria, fever, anorexia, and progressive decline in mental status. On admission, she appeared confused with a GCS score of E2V2M4, and her blood ammonia level was elevated (176 μg/dL). Abdominal computed tomography showed bilateral hydronephrosis and bladder fullness with wall thickness in spite of indwelling bladder catheter. Her catheter was obstructed by blood clot. Blood cultures, blood clot culture, and one urine culture all yielded Proteus mirabilis. Obstructive urinary tract infection complicated with septic shock was diagnosed. After treatment, her ammonia level normalized.