Which test is most helpful to diagnose kidney failure?

Following an infection with rhinovirus, the patient reports initial improvement followed by significant congestion and upper teeth pain. The nurse anticipates which diagnosis?

The 58-year-old patient presents to the emergency department complaining of worsening back pain that started a few days ago. He states, “I thought maybe I had slept wrong and gotten a spasm, but I’m not sure. I didn’t fall or get hit or anything. The ibuprofen I took definitely didn’t help.” His blood pressure is 174/96 mmHg and heart rate is 96 bpm. He reports being out of his antihypertensive medication for the past week while he was out of town. Other history is notable for arthritis in the hands and a 44-pack-year smoking history. He drinks socially but denies illicit drug use. Which assessment is most important?