A 55-year-old Latina woman presents to establish care. She recently went to a health fair, where she had some basic serum chemistries drawn. Her serum creatinine was 1.5 mg/dL. On a questionnaire she completed in your waiting room, she noted that she has no known past medical history except for occasional muscular aches, for which she takes indomethacin (2-3 times in the past 8 months). There is no family history of renal disease. Her BP is 142/82 mm Hg, and her body mass index is 31 kg/m2.

What additional study would be most helpful in determining the cause of her elevated creatinine?
a. Urinalysis to assess for proteinuria and hematuria
b. Renal ultrasound to assess for structural abnormalities
c. 24-hour urine collection for creatinine clearance
d. Renal biopsy for histologic assessment