Endocrine Pharmacology USMLE Step 1 Practice Question

A 74-year-old man with type 2 diabetes mellitus, severe peripheral vascular disease, and stage 4 chronic kidney disease (eGFR 18 mL/min/1.73m²) presents for medication adjustment. He has been taking metformin 1000 mg twice daily for three years. Vital signs: BP 142/88 mmHg, HR 76/min, RR 14/min, temp 37°C. Laboratory studies show serum creatinine 3.2 mg/dL, bicarbonate 22 mEq/L, and lactate 1.8 mmol/L. Physical examination reveals diminished bilateral lower extremity pulses. He denies dyspnea or nausea. Which drug should be avoided due to increased lactic acidosis risk in advanced renal failure?