Q-Bank Breakdown: Pompe disease — Why Every Answer Choice Matters
Tag: Biochemistry > Lysosomal & Glycogen Storage Diseases
Pompe disease is a classic USMLE favorite because it blends biochemistry (enzyme defects), pathology (glycogen accumulation), and clinical medicine (cardiomyopathy + hypotonia). In this Q-bank breakdown, we’ll walk through a vignette, nail the correct answer, and then dissect each distractor so you understand why every option matters.
The Clinical Vignette (Pompe in the Wild)
A 3-month-old infant is brought to the clinic for poor feeding and “floppiness.” The parents report the baby has been increasingly lethargic and has difficulty breathing when lying flat. On exam, the infant is hypotonic with macroglossia and hepatomegaly. Cardiac exam reveals a gallop; echocardiogram shows hypertrophic cardiomyopathy. Labs show elevated creatine kinase (CK). A muscle biopsy reveals vacuolated myocytes with glycogen accumulation.
Question: Which enzyme is deficient?
Correct Answer: Acid α-glucosidase (acid maltase)
Why this is Pompe disease
Pompe disease (Glycogen Storage Disease type II) is caused by deficiency of lysosomal acid α-glucosidase, leading to glycogen accumulation in lysosomes, especially in:
- Cardiac muscle → hypertrophic cardiomyopathy
- Skeletal muscle → hypotonia, weakness
- Sometimes liver → hepatomegaly (but hypoglycemia is not a defining feature)
High-yield clinical features
Pompe (infantile form) often presents with:
- Cardiomegaly / hypertrophic cardiomyopathy
- Hypotonia (“floppy baby”)
- Macroglossia
- Respiratory distress (from muscle weakness)
- Elevated CK
- Can be fatal early due to cardiorespiratory failure if untreated
High-yield biochem/path tie-in
- Pompe is a lysosomal storage disease and a glycogen storage disease.
- The problem is lysosomal glycogen degradation, not cytosolic glycogen breakdown.
- Histology often shows vacuoles in myocytes packed with glycogen.
Treatment (USMLE-relevant)
- Enzyme replacement therapy (ERT): alglucosidase alfa
- Early treatment improves survival and cardiac outcomes.
Why Each Distractor Matters (and What It Really Points To)
Below are the most common answer choices seen alongside Pompe. Learn the “pattern recognition hooks” so you can eliminate quickly.
Distractor 1: Glucose-6-phosphatase deficiency → Von Gierke (GSD I)
Why it’s wrong here
Von Gierke causes problems with maintaining blood glucose, not primarily cardiomyopathy.
Key clues for Von Gierke
- Severe fasting hypoglycemia
- Lactic acidosis
- Hyperuricemia
- Hypertriglyceridemia
- Hepatomegaly/renomegaly
- Often described as “doll-like face”
Board distinction:
Pompe = cardiomyopathy + hypotonia
Von Gierke = hypoglycemia + metabolic derangements
Distractor 2: Debranching enzyme deficiency (α-1,6-glucosidase) → Cori (GSD III)
Why it’s wrong here
Cori can cause hepatomegaly and muscle weakness, but the hallmark is milder hypoglycemia and a distinctive glycogen structure, not massive lysosomal storage with infantile cardiomyopathy as the signature.
Key clues for Cori
- Milder fasting hypoglycemia than Von Gierke
- Hepatomegaly
- Muscle weakness
- Glycogen with short outer chains (“limit dextrin” accumulation)
Board distinction:
Cori often looks like a “milder Von Gierke,” while Pompe is a lysosomal defect with prominent cardiac involvement.
Distractor 3: Branching enzyme deficiency → Andersen (GSD IV)
Why it’s wrong here
Andersen is primarily a liver disease with progressive cirrhosis and failure—different timeline and organ focus.
Key clues for Andersen
- Hepatosplenomegaly
- Progressive cirrhosis → liver failure in childhood
- Abnormal glycogen that resembles amylopectin (poorly branched, less soluble)
- Can have failure to thrive, hypotonia—but liver failure dominates
Board distinction:
Andersen = cirrhosis and liver failure
Pompe = cardiomyopathy + skeletal muscle weakness
Distractor 4: Muscle glycogen phosphorylase deficiency → McArdle (GSD V)
Why it’s wrong here
McArdle is an exercise intolerance disease in older children/adolescents—not infantile cardiomyopathy.
Key clues for McArdle
- Exercise-induced muscle cramps
- Myoglobinuria (dark urine) after exertion
- Second-wind phenomenon
- Typically no hepatomegaly
- Flat lactate curve on ischemic forearm exercise test
Board distinction:
McArdle = pain with exercise
Pompe = floppy infant + cardiomyopathy
Distractor 5: Hexosaminidase A deficiency → Tay-Sachs
Why it’s wrong here
Tay-Sachs is a lysosomal storage disease, but it involves GM2 ganglioside, not glycogen—and it’s neurodegenerative without hepatosplenomegaly.
Key clues for Tay-Sachs
- Neurodegeneration
- Cherry-red spot on macula
- No hepatosplenomegaly
- Increased startle response
Board distinction:
Pompe has muscle + heart issues due to glycogen.
Tay-Sachs is primarily CNS.
Distractor 6: Sphingomyelinase deficiency → Niemann-Pick
Why it’s wrong here
Niemann-Pick features organomegaly and neurodegeneration with foam cells, not cardiomyopathy with glycogen-filled lysosomes.
Key clues for Niemann-Pick
- Hepatosplenomegaly
- Neurodegeneration
- Foam cells
- Cherry-red spot (in some types)
Distractor 7: α-galactosidase A deficiency → Fabry
Why it’s wrong here
Fabry presents later with pain crises and vascular skin findings, not floppy infant cardiomyopathy.
Key clues for Fabry
- X-linked recessive
- Angiokeratomas
- Acroparesthesias
- Hypohidrosis
- Progressive renal failure and cardiovascular disease
Ultra High-Yield Comparison Table (Rapid USMLE Sorting)
| Disease | Enzyme Defect | Big Clues | Hypoglycemia? | Key Organs |
|---|---|---|---|---|
| Pompe (GSD II) | Acid α-glucosidase (lysosomal) | Floppy baby, cardiomyopathy, macroglossia | Usually no | Heart, skeletal muscle |
| Von Gierke (GSD I) | Glucose-6-phosphatase | Severe fasting hypoglycemia, lactic acidosis | Yes | Liver, kidney |
| Cori (GSD III) | Debranching enzyme | Milder hypoglycemia, hepatomegaly | Sometimes | Liver, muscle |
| Andersen (GSD IV) | Branching enzyme | Cirrhosis, liver failure | Variable | Liver |
| McArdle (GSD V) | Muscle glycogen phosphorylase | Exercise intolerance, myoglobinuria, second wind | No | Skeletal muscle |
USMLE “Lock-In” Pearls for Pompe
- Think Pompe when you see: infant + cardiomyopathy + hypotonia.
- Enzyme: acid maltase (lysosomal acid α-glucosidase).
- Path: glycogen in lysosomes → vacuolated myocytes.
- Treatment: enzyme replacement therapy can be life-changing.
- Don’t get baited by hepatomegaly: many storage diseases enlarge the liver; cardiac hypertrophy is your big differentiator.
Quick Practice: One-Line Elimination Strategy
If the vignette screams floppy infant + hypertrophic cardiomyopathy, eliminate options involving:
- Fasting hypoglycemia pathways (Von Gierke/Cori) unless hypoglycemia is prominent
- Exercise intolerance (McArdle)
- Neurodegeneration + cherry-red spot (Tay-Sachs/Niemann-Pick)
…and land on acid α-glucosidase deficiency.