What causes morquio syndrome?

AR, MPS IV: Both parents are carriers of the defective mutation, an autosomal recessive gene. Each child of carriers will have 1 chance in 4 of inheriting the disorder (that is where 25% comes from). All humans are carriers of at least 6 pathogenic variants (have the capacity to cause disease if present in a double dose--no normal gene present). see http://www.chop.edu/conditions-diseases/morquio-syndrome