AKT-3593

A 72-year-old man comes to his General Practitioner complaining of increasing fatigue and shortness of breath over the past few months. He reports no current medication use but mentions experiencing back pain in recent weeks. Upon examination, initial tests show a serum creatinine level of 654 µmol/l (normal range: 60–120 µmol/l). What diagnostic test would be most beneficial in determining a diagnosis?