Q-Bank Breakdown: Hurler/Hunter syndrome — Why Every Answer Choice Matters
Tag: Biochemistry > Lysosomal & Glycogen Storage Diseases
Lysosomal storage diseases are classic USMLE territory because they blend biochemistry + genetics + clinical pattern recognition. Hurler and Hunter syndromes are especially high-yield: both are mucopolysaccharidoses (MPS) with coarse facial features and developmental issues—but the exam loves the differences.
The Clinical Vignette (Q-Bank Style)
A 3-year-old child is brought to clinic for progressive developmental delay and frequent respiratory infections. Parents report the child has become increasingly “stiff” with limited joint mobility. Physical exam shows coarse facial features, hepatosplenomegaly, and clouding of the corneas. X-ray shows dysostosis multiplex.
Which enzyme is most likely deficient?
Stepwise Reasoning (What the Stem is Telling You)
This stem screams MPS:
- Coarse facies
- Hepatosplenomegaly
- Skeletal abnormalities (dysostosis multiplex)
- Recurrent respiratory infections (GAG deposition in airway tissues)
- Neurodevelopmental delay (common in severe MPS)
- Corneal clouding → key discriminator
Corneal clouding points to Hurler (MPS I) over Hunter (MPS II).
Correct Answer: Hurler Syndrome (MPS I)
✅ Deficient enzyme
α-L-iduronidase
✅ What accumulates
- Dermatan sulfate
- Heparan sulfate
✅ Inheritance
Autosomal recessive
✅ Classic clinical features (USMLE high-yield)
- Corneal clouding (big differentiator)
- Coarse facies
- Developmental delay
- Hepatosplenomegaly
- Dysostosis multiplex
- Often fatal in childhood (severe forms) due to cardiorespiratory complications
Memory hook: Hurler is “Hazy” → Hurler = Hazy corneas.
Why Every Answer Choice Matters: Systematic Distractor Breakdown
Below are common distractors in MPS/GSD question sets—and exactly how to eliminate them.
Distractor 1: Iduronate sulfatase deficiency → Hunter syndrome (MPS II)
Why it’s tempting: Hunter looks similar to Hurler (both accumulate dermatan + heparan sulfate).
Why it’s wrong here: Hunter does NOT cause corneal clouding.
Hunter syndrome key facts
- Enzyme: Iduronate sulfatase
- Inheritance: X-linked recessive (testable!)
- Findings: Coarse facies, developmental delay, hepatosplenomegaly, dysostosis multiplex
- Unique differentiator: No corneal clouding
- Sometimes remembered as “Hunter can’t see the clouds” → no clouding
USMLE tip: If the vignette highlights a boy + X-linked pattern + no corneal clouding, think Hunter.
Distractor 2: Hexosaminidase A deficiency → Tay-Sachs disease
Why it’s tempting: Neurodegeneration is prominent in both Tay-Sachs and severe MPS.
Why it’s wrong here: Tay-Sachs has no hepatosplenomegaly and is characterized by GM2 ganglioside accumulation, not GAGs.
Tay-Sachs key facts
- Enzyme: Hexosaminidase A
- Accumulation: GM2 ganglioside
- Inheritance: Autosomal recessive
- Findings: Progressive neurodegeneration, cherry-red macula, hyperacusis, seizures
- Absent: Hepatosplenomegaly
Elimination clue: If you see organomegaly, move away from Tay-Sachs.
Distractor 3: Sphingomyelinase deficiency → Niemann-Pick disease
Why it’s tempting: Hepatosplenomegaly + neuro symptoms can mimic MPS.
Why it’s wrong here: Niemann-Pick is a sphingolipidosis, not a mucopolysaccharidosis, and you’d expect foam cells and often cherry-red macula (types A/B).
Niemann-Pick key facts
- Enzyme: Sphingomyelinase
- Accumulation: Sphingomyelin
- Findings: Neurodegeneration, hepatosplenomegaly, foam cells, cherry-red macula (esp. type A)
Elimination clue: MPS tends to have skeletal abnormalities (dysostosis multiplex) and GAG-related features; Niemann-Pick is more “lipid storage + foam cells.”
Distractor 4: Glucocerebrosidase deficiency → Gaucher disease
Why it’s tempting: Hepatosplenomegaly and bone disease are common.
Why it’s wrong here: Gaucher causes bone crises and pancytopenia with Gaucher cells, but not classic coarse facies + corneal clouding + GAG storage pattern.
Gaucher key facts
- Enzyme: β-glucocerebrosidase
- Accumulation: Glucocerebroside
- Findings: Hepatosplenomegaly, pancytopenia, bone pain/crises, Erlenmeyer flask deformity, “crumpled tissue paper” macrophages
Elimination clue: Think Gaucher when the vignette emphasizes bone pain + cytopenias rather than coarse facies and airway issues.
Distractor 5: Acid α-glucosidase (acid maltase) deficiency → Pompe disease (GSD II)
Why it’s tempting: “Storage disease” + pediatric presentation.
Why it’s wrong here: Pompe is a glycogen storage disease, not MPS. It presents with cardiomegaly/hypertrophic cardiomyopathy, hypotonia, and weakness—rather than coarse facies + corneal clouding.
Pompe key facts
- Enzyme: Lysosomal acid α-1,4-glucosidase
- Accumulation: Glycogen in lysosomes
- Findings: Cardiomegaly, hypertrophic cardiomyopathy, hypotonia, muscle weakness
- Can cause macroglossia, but not the MPS constellation of GAG features (coarse facies + dysostosis multiplex + corneal clouding).
Elimination clue: Cardiac + muscle weakness points to Pompe.
Distractor 6: Debranching enzyme deficiency → Cori disease (GSD III)
Why it’s tempting: Hepatomegaly can overlap.
Why it’s wrong here: Cori presents with milder fasting hypoglycemia and hepatomegaly, not coarse facies, corneal clouding, or dysostosis multiplex.
Cori key facts
- Enzyme: Debranching enzyme (α-1,6-glucosidase)
- Findings: Hepatomegaly, mild hypoglycemia, muscle weakness; “limit dextrin” accumulation
Elimination clue: GSDs are about fasting intolerance/hypoglycemia and muscle symptoms—not GAG deposition.
High-Yield Comparison Table: Hurler vs Hunter
| Feature | Hurler (MPS I) | Hunter (MPS II) |
|---|---|---|
| Enzyme | α-L-iduronidase | Iduronate sulfatase |
| Inheritance | Autosomal recessive | X-linked recessive |
| GAGs accumulated | Dermatan sulfate, heparan sulfate | Dermatan sulfate, heparan sulfate |
| Corneal clouding | Yes | No |
| Classic clues | Coarse facies, developmental delay, dysostosis multiplex, hepatosplenomegaly | Similar findings + no corneal clouding |
Exam-Ready Takeaways (Memorize These)
- Hurler = AR + α-L-iduronidase deficiency + corneal clouding
- Hunter = XLR + iduronate sulfatase deficiency + no corneal clouding
- Both can cause:
- Coarse facies
- Hepatosplenomegaly
- Dysostosis multiplex
- Developmental delay
- Recurrent respiratory infections
Fast elimination strategy:
If the question stem includes cloudy corneas, do not pick Hunter.
Mini Drill: 3 Rapid-Fire Vignette Clues
- “Boy with coarse facies, hepatosplenomegaly, no corneal clouding” → Hunter
- “Child with corneal clouding + dysostosis multiplex” → Hurler
- “Neurodegeneration + cherry-red macula + no HSM” → Tay-Sachs