A 52-year-old woman with type 2 diabetes on metformin (1000 mg twice daily) and hypertension on lisinopril presents with a 3-day history of acute diarrhea. She self-started ciprofloxacin 500 mg twice daily for suspected traveler's diarrhea. On day 2 of antibiotic use, she develops nausea, fatigue, and malaise. Serum creatinine has risen from 0.9 to 1.8 mg/dL. What is the most likely cause of her clinical deterioration?