Postrenal acute kidney injury Quiz: Ace Your Exams

A 56-year-old woman presents to the emergency department with four hours of suprapubic pressure and difficulty initiating urination. Medical history is significant only for seasonal allergies, which are currently managed with over-the-counter diphenhydramine. The patient does not drink alcohol or use tobacco products. Temperature is 37°C (98.6°F), blood pressure is 160/87 mmHg, heart rate is 101 beats per minute, respiratory rate is 18 breaths per minute, and oxygen saturation is 99% on room air. Body mass index (BMI) is 35 kg/m². On physical examination, there is mild left costovertebral angle tenderness. The suprapubic region is soft without a palpable mass and shows only minimal tenderness on deep palpation. Cardiopulmonary and the remainder of the abdominal exam are unremarkable. Initial laboratory studies show a creatinine of 2.0 mg/dL (baseline 0.7 mg/dL). Which of the following additional tests is most likely to quickly delineate the anatomic region of pathology?