You are answering a USMLE-style medical question. Question: An unresponsive 92-year-old man is brought to the emergency department by ambulance and is quickly followed by his grand-daughter. Approx. 1 month ago, he had a radical prostatectomy, pelvic radiation, and several rounds of chemotherapy for aggressive prostate cancer in Nicaragua and was discharged from the hospital with a suprapelvic catheter and a plan for more chemotherapy. He was coming to the United States to spend time with some extended family. According to the grand-daughter his mental status was altered upon arrival and his health began to decline. She is not aware of his past medical status or medications. At the hospital, his blood pressure is 98/60 mm Hg, the pulse id 110/min, the respiratory rate is 21/min, and the temperature is 35.6°C (96.1°F). On physical exam, the patient appears cachectic, drowsy, and only responds to questions by nodding his head. His heart rate is tachycardic with a regular rhythm and his lungs are clear to auscultation bilaterally. The cystotomy incision for his suprapelvic catheter is red and swollen. The patient admitted to the hospital and stabilized and started on broad-spectrum antibiotics. The catheter tube produces no urine over 24 hours and only thick green-white puss can be expressed. The treatment team concludes this patient has urinary tract obstruction. What is the best method to evaluate urinary tract obstruction in this patient? Options: A) Renal biopsy B) Ultrasound C) Abdominal-pelvic CT scan D) Blood analysis Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.