2 year ldo boy is evaluated for easy bruisning, open wounds following minor trauma. The skin is difficult to suture due its extreme fragility. Physical exam: hyperextensible skin, mutiple ecchymoses over forearms and pretibial regions, and an umbilical hernia. A biopsy reveals a defect in *extracellular processing of collagen*. Which of the following steps of collagen synthesis is most likely impaired in this patient?