A 6-year-old girl from Kerala presents with jaundice, dark urine, and bone pain. She has a history of recurrent transfusions since age 2 years. Physical examination reveals hepatosplenomegaly, frontal bossing, and maxillary prominence. Serum bilirubin is 4.2 mg/dL (indirect), serum ferritin is 2800 ng/mL, and serum creatinine is 1.4 mg/dL (elevated for age). Abdominal ultrasound shows hepatic cirrhosis and pancreatic atrophy. Which complication is most directly responsible for the elevated creatinine?