Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)


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Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)


Population genetics

Mendelian genetics and punnett squares

Hardy-Weinberg equilibrium

Inheritance patterns

Independent assortment of genes and linkage

Evolution and natural selection

Genetic disorders

Down syndrome (Trisomy 21)

Edwards syndrome (Trisomy 18)

Patau syndrome (Trisomy 13)

Fragile X syndrome

Huntington disease

Myotonic dystrophy

Friedreich ataxia

Turner syndrome

Klinefelter syndrome

Prader-Willi syndrome

Angelman syndrome

Beckwith-Wiedemann syndrome

Cri du chat syndrome

Williams syndrome

Alagille syndrome (NORD)


Polycystic kidney disease

Familial adenomatous polyposis

Familial hypercholesterolemia

Hereditary spherocytosis

Huntington disease

Li-Fraumeni syndrome

Marfan syndrome

Multiple endocrine neoplasia

Myotonic dystrophy


Treacher Collins syndrome

Tuberous sclerosis

von Hippel-Lindau disease


Polycystic kidney disease

Cystic fibrosis

Friedreich ataxia

Gaucher disease (NORD)

Glycogen storage disease type I

Glycogen storage disease type II (NORD)

Glycogen storage disease type III

Glycogen storage disease type IV

Glycogen storage disease type V


Mucopolysaccharide storage disease type 1 (Hurler syndrome) (NORD)

Krabbe disease


Niemann-Pick disease types A and B (NORD)

Niemann-Pick disease type C

Primary ciliary dyskinesia

Phenylketonuria (NORD)

Sickle cell disease (NORD)

Tay-Sachs disease (NORD)



Wilson disease

Fragile X syndrome

Alport syndrome

X-linked agammaglobulinemia

Fabry disease (NORD)

Glucose-6-phosphate dehydrogenase (G6PD) deficiency


Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)

Lesch-Nyhan syndrome

Muscular dystrophy

Ornithine transcarbamylase deficiency

Wiskott-Aldrich syndrome

Mitochondrial myopathy

Autosomal trisomies: Pathology review

Muscular dystrophies and mitochondrial myopathies: Pathology review

Miscellaneous genetic disorders: Pathology review


Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)


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USMLE® Step 1 questions

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High Yield Notes

5 pages


Mucopolysaccharide storage disease type 2 (Hunter syndrome) (NORD)

of complete


USMLE® Step 1 style questions USMLE

of complete

A 4-year-old boy is brought to the clinic for evaluation of increasing head size and difficulties at school. The patient has had trouble interacting with his peers and is unable to communicate with full sentences. His teacher has reported several episodes of aggressive behavior towards other students as well. His birth was unremarkable, and he started walking at the age of 15 months. His parents have noticed that he tends to walk with an unsteady gait over the last month. His past medical history is significant for recurrent upper respiratory infections since the age of 8 months. Vitals are within normal limits. Weight and height are tracking at the 30th percentile, but head circumference is above the 90th percentile. His facial features are notable for a prominent forehead, flat nose bridge and enlarged lips, gums and tongue. Echocardiogram reveals left ventricular hypertrophy. Abdominal examination shows hepatosplenomegaly and an umbilical hernia. Ophthalmic examination is normal.  Accumulation of which of the following metabolites is responsible for this patient’s symptoms?

External References

First Aid








Hunter syndrome p. 58, 86

External Links


Content Reviewers

Rishi Desai, MD, MPH


Salma Ladhani, MD

Stefan Stoisavljevic, MD

Hunter syndrome is a rare genetic metabolic disorder that occurs when lysosomal enzymes are absent.

Hunter syndrome is also known as mucopolysaccharidosis type 2.

Mucopolysaccharides or glycosaminoglycans are types of complex sugars, which are produced by the cells and exported to the extracellular space.

Examples include heparan sulfate and dermatan sulfate.

Both heparan sulfate and dermatan sulfate can be found on almost all cell surfaces as well as in the basement membrane, which separates epithelial cells from the connective tissue that lies beneath.

When mucopolysaccharides like heparin sulfate and dermatan sulfate need to get degraded, they are taken over to a lysosome, which contains enzymes needed to break down the mucopolysaccharides.

Each mucopolysaccharide requires multiple enzymes to fully degrade, and some mucopolysaccharides share certain enzymes in common.

For example, both heparan sulfate and dermatan sulfate need the iduronate sulfatase and alpha-L-iduronidase enzymes to get broken down.

Hunter syndrome is caused by a deficiency in iduronate sulfatase, and the result is that heparan sulfate and dermatan sulfate can’t be degraded, so they build up in various tissues.

Hunter Syndrome is an X-linked recessive disorder so it’s more common in males.


Mucopolysaccharide storage disease type 2, also known as Hunter syndrome, is an inherited disorder caused by a deficiency in the iduronate sulfatase enzyme. This enzyme is responsible for breaking down glycosaminoglycans (GAGs) which are large molecules that include heparan sulfate and dermatan sulfate.

Deficiency in iduronate sulfatase results in GAGs accumulating in organs and tissues throughout the body, leading to a wide range of symptoms, such as developmental delay, hearing loss, respiratory difficulties, enlarged organs, and skin abnormalities. Treatment focuses on managing symptoms, and may include physical, occupational, and speech therapies, enzyme replacement therapy, and bone marrow transplantation.


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