A 62-year-old woman on metformin (2 g/day) for type 2 diabetes is prescribed trimethoprim-sulfamethoxazole (TMP-SMX) for a urinary tract infection. After 3 days, she develops nausea, fatigue, and a serum creatinine rise from 0.9 to 1.4 mg/dL. Which investigation is most appropriate to identify the mechanism of renal dysfunction in this drug interaction?

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