A 28-year-old male developed nausea, fatigue, low-grade fever, and arthralgia 3 days after taking ciprofloxacin for a cold 15 days ago. Urinalysis showed glycosuria, urinary protein, 15-20 RBCs/HPF, Scr 360 μmol/L, low serum potassium (K), low serum phosphorus (P), and low urine osmolality. Bilateral enlarged kidneys were noted. Which diagnostic test is most appropriate?