A 52-year-old woman with rheumatoid arthritis (on long-term corticosteroids and methotrexate) presents for elective total knee replacement. She has mild renal impairment (eGFR 55 mL/min/1.73m²) and is on chronic bisoprolol. During induction with thiopental and fentanyl, rocuronium 0.6 mg/kg IV is given for intubation. After 45 minutes of surgery, the surgeon requests continued neuromuscular blockade. A repeat dose of rocuronium 0.15 mg/kg is administered. At the end of surgery (2.5 hours total), train-of-four monitoring shows only 1 out of 4 twitches present. The anesthesiologist is concerned about prolonged blockade. Which factor is MOST likely responsible for the prolonged neuromuscular blockade in this patient?