Q-Bank Breakdown: Andersen disease — Why Every Answer Choice Matters

Tag: Biochemistry > Lysosomal & Glycogen Storage Diseases

Andersen disease (Glycogen Storage Disease type IV) is a classic “don’t-miss” USMLE vignette because it tests whether you understand glycogen structure, not just enzyme names. In this Q-bank-style breakdown, we’ll walk through a representative clinical stem, lock in the correct diagnosis, then systematically eliminate the distractors the way test writers expect.

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The Clinical Vignette (USMLE Style)

A 10-month-old infant is evaluated for failure to thrive, hepatosplenomegaly, and progressive liver dysfunction. The parents report poor feeding and increasing abdominal distension. Labs show elevated AST/ALT and signs of early cirrhosis. A liver biopsy demonstrates accumulation of abnormally structured glycogen with long, unbranched chains.

Which enzyme is most likely deficient?

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Correct Answer: Branching enzyme (α-1,4 → α-1,6 transglucosidase)

Why this is Andersen Disease (GSD IV)

Andersen disease = deficiency of the glycogen branching enzyme, which normally creates α-1,6 branch points. Without branching:

• Glycogen becomes poorly branched (long outer chains)
• It resembles amylopectin-like structure (plant starch)
• The abnormal glycogen is less soluble and more “foreign,” triggering hepatic inflammation and fibrosis
• Clinical course: progressive hepatosplenomegaly → cirrhosis → liver failure (often in early childhood)

High-yield clinical features

• Hepatomegaly, failure to thrive
• Progressive liver disease, portal hypertension, cirrhosis
• Can have muscle hypotonia/cardiomyopathy in some variants (less emphasized than hepatic failure)

High-yield pathology clue

• “Long, unbranched glycogen chains” on biopsy = think branching enzyme defect (GSD IV)

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Why Every Distractor Matters (and How to Eliminate Them)

Below are the common answer choices that show up next to Andersen—and what clinical/lab clues should steer you away.

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Distractor 1: Debranching enzyme deficiency (GSD III, Cori disease)

What it is: Defect in α-1,6-glucosidase (and 4-α-glucanotransferase activities) used to remove branches during glycogen breakdown.

Why it’s tempting: Also causes hepatomegaly and abnormal glycogen.

Key differences from Andersen:

• Cori causes “limit dextrin” accumulation (glycogen with short outer chains because breakdown stops near branch points)
• Often presents with:
  • Mild fasting hypoglycemia (less severe than von Gierke)
  • Muscle weakness (skeletal ± cardiac involvement)
• Does not classically cause early, rapidly progressive cirrhosis like Andersen

Buzzwords for GSD III:

• Limit dextrin
• Hepatomegaly + muscle symptoms
• Milder hypoglycemia than GSD I

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Distractor 2: Glucose-6-phosphatase deficiency (GSD I, von Gierke disease)

What it is: Inability to convert G6P → glucose in liver/kidney → impaired final step of glycogenolysis and gluconeogenesis.

Why it’s tempting: Hepatomegaly + glycogen accumulation is a shared theme.

Clues that argue against it in this vignette:

• von Gierke is defined by severe fasting hypoglycemia
• Prominent metabolic derangements:
  • Lactic acidosis
  • Hyperuricemia
  • Hypertriglyceridemia
• Classic phenotype: “Doll-like face,” protuberant abdomen
• The vignette’s key histology is abnormally structured glycogen (long unbranched chains)—that’s a branching problem, not a glucose release problem.

Buzzwords for GSD I:

• Severe hypoglycemia, lactic acidosis, hyperuricemia, hyperlipidemia
• Increased glycogen in liver
• No abnormal branching pattern as the central clue

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Distractor 3: Lysosomal acid α-glucosidase deficiency (GSD II, Pompe disease)

What it is: Impaired breakdown of glycogen in lysosomes.

Why it’s tempting: It’s a glycogen storage disease and shows up in the same topic block.

How to eliminate it quickly:

• Pompe is primarily cardiac and muscle, not liver failure:
  • Cardiomegaly, hypertrophic cardiomyopathy
  • Hypotonia, macroglossia
• Normal blood glucose (key testable point)
• Liver may be enlarged, but progressive cirrhosis is not the headline

Buzzwords for Pompe:

• Cardiomyopathy + hypotonia
• Lysosomal accumulation
• Normal glucose

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Distractor 4: Muscle glycogen phosphorylase deficiency (GSD V, McArdle disease)

What it is: Impaired glycogen breakdown in skeletal muscle.

Why it’s tempting: Another high-yield GSD name.

Why it’s wrong here:

• McArdle is an exercise intolerance disease, not infantile liver failure.
• Hallmarks:
  • Muscle cramps with exercise
  • Myoglobinuria
  • Second-wind phenomenon
  • Elevated CK
• Liver findings (hepatomegaly/cirrhosis) would be atypical.

Buzzwords for McArdle:

• Exercise-induced cramps, myoglobinuria
• Second wind
• No hepatomegaly/cirrhosis

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Distractor 5: Acid α-glucosidase vs. branching enzyme (classic “name trap”)

Test writers love pairing these because both include “glucosidase/glucosidase-like” language.

• Pompe (GSD II) = lysosomal acid α-glucosidase → cardiomyopathy, hypotonia
• Andersen (GSD IV) = branching enzyme → abnormal glycogen structure, cirrhosis

If the stem screams liver failure + abnormal branching, pick branching enzyme.

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Rapid-Fire High-Yield Table (Exam Day Recall)

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How Q-Banks Test Andersen (Patterns to Recognize)

Look for stems that combine:

• Infant/young child
• Hepatosplenomegaly
• Progressive hepatic dysfunction/cirrhosis
• Biopsy: abnormal glycogen architecture (often described as “unbranched,” “amylopectin-like,” or “polyglucosan bodies”)

The “gotcha” is that many GSDs cause hepatomegaly—but early cirrhosis + abnormal branching is the Andersen fingerprint.

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Take-Home Points (USMLE High-Yield)

• Andersen (GSD IV) = branching enzyme deficiency → poorly branched glycogen → progressive cirrhosis.
• Cori (GSD III) = debranching enzyme deficiency → limit dextrin + liver and muscle involvement.
• Pompe (GSD II) is lysosomal and presents with cardiomyopathy + hypotonia, usually normal glucose.
• von Gierke (GSD I) screams severe hypoglycemia and metabolic derangements (lactate, uric acid, triglycerides).
• When a stem emphasizes glycogen structure, think branching/debranching, not glucose-6-phosphatase.