RESUMEN
Mucopolysaccharidosis type IIIB (Sanfilippo's B; OMIM no.: 252920) is a lysosomal storage disorder caused by defective degradation of heparan sulfate. The enzyme that has decreased function in this disease is α-N acetylglucosaminidase. This enzyme is encoded by the NAGLU gene. A 9-year-old male patient was referred to us with speech disability, developmental delay, hepatomegaly, mild learning disability, and otitis media with effusion complaints. Whole exome sequencing (WES) was performed because of consanguinity between the parents of the patient and the lack of specific prediagnosis. As a result of the patient's WES analysis, a homozygous mutation was detected in the NAGLU gene. The leukocyte enzyme activity was then evaluated to confirm the diagnosis. Alpha-N acetylglucosaminidase deficiency was found. Alpha-N acetylglucosaminidase activity was 0.2 nmol/mLh. WES is a successful diagnostic method in the diagnosis of the mild clinical diseases with recessive inheritance. In addition, our case is a good example of genotype to phenotype diagnosis. Because in storage diseases, the diagnosis is made by leukocyte enzyme analysis first, and then the result is confirmed by gene analysis. The opposite situation occurred in our case.