Glycoprotein Synthesis — N-linked vs O-linked Glycosylation

A 2-year-old boy presents with coarse facial features, gingival hyperplasia, developmental delay, and dysostosis multiplex on skeletal survey. Urine glycosaminoglycans are normal. Fibroblasts cultured from skin biopsy show characteristic inclusion bodies. The structure marked **C** in the diagram (mannose-6-phosphate tag) is absent from lysosomal hydrolases due to deficiency of the enzyme that catalyzes its formation. Which of the following best explains the pathophysiology of this patient's condition?