A 5-yeаr-оld mаle is brоught tо the pediаtric clinic because his parents are concerned about his apparent developmental delay, frequent infections, and abnormal physical features. The child is short in stature with a large head and broad facial features. He has a protruding abdomen and short neck. His joints appear stiff. Genetic studies reveal a mutation consistent with Hunter syndrome. This is a type of _____.