A 52-year-old male with chronic pancreatitis is admitted to the ICU following an acute exacerbation. He is prescribed morphine 10 mg IV 4-hourly for pain management. On day 3 of admission, he develops severe respiratory depression (RR 8/min, SpO₂ 82% on room air), pinpoint pupils, and altered mental status. His serum creatinine has risen from 1.0 to 2.8 mg/dL over 48 hours. Which metabolite of morphine is most likely responsible for his clinical deterioration?