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

Tag: Biochemistry > Lysosomal & Glycogen Storage Diseases

USMLE-style questions on glycogen storage diseases (GSDs) are built to reward pattern recognition and punish shortcuts. Cori disease (GSD III) is a classic example: the vignette often feels like Von Gierke (GSD I) at first—until one or two details flip the diagnosis. This breakdown walks through a clinical stem, nails the correct answer, and then systematically dismantles the distractors the way NBME-style exams expect.

Clinical Vignette (USMLE-style)

A 14-month-old child is brought in for poor growth and a protuberant abdomen. Parents report episodes of irritability and sweating when meals are delayed. Exam shows hepatomegaly and mild hypotonia. Labs: fasting hypoglycemia, elevated AST/ALT, hyperlipidemia, and elevated creatine kinase. Lactate is normal. A liver biopsy shows increased glycogen with short outer branches (“limit dextrin-like” structures).

Question: Which enzyme is most likely deficient?

Correct Answer: Debranching enzyme deficiency (Glycogen Storage Disease type III, Cori disease)

Why this is Cori disease

Cori disease is due to deficiency of the glycogen debranching enzyme, which has two key activities:

4-α-glucanotransferase
α-1,6-glucosidase

Without debranching, glycogen breakdown stalls near branch points, creating limit dextrins.

High-yield clinical pattern

Cori (GSD III) = “Von Gierke-lite” + muscle involvement

Hepatomegaly (glycogen accumulation)
Fasting hypoglycemia (impaired glycogenolysis)
Normal lactate (this is the big differentiator from GSD I)
Muscle involvement → ↑ CK, weakness/hypotonia; may have cardiomyopathy in some cases

Key mechanistic takeaway

Glycogen phosphorylase can remove glucose residues until 4 units from a branch point
After that, debranching enzyme must:
- transfer 3 residues to another chain (transferase)
- cleave the remaining α-1,6 linked glucose (α-1,6-glucosidase)

How to “Lock In” the Diagnosis on Test Day

The two fastest discriminators

Lactate
- Cori: lactate typically normal
- Von Gierke: lactate high (due to blocked gluconeogenesis output)
Muscle findings
- Cori: muscle weakness, ↑ CK
- Von Gierke: primarily liver/kidney findings; muscle is not the classic focus

Distractor Breakdown: Why Each Wrong Answer Is Tempting (and Wrong)

Below are common answer choices that appear in the same question family.

Distractor 1: Glucose-6-phosphatase deficiency (GSD I, Von Gierke disease)

Why it’s tempting:
The stem has hepatomegaly + fasting hypoglycemia—very GSD I-coded.

Why it’s wrong here:
Von Gierke causes impaired conversion of G6P → glucose in liver/kidney, producing:

Severe fasting hypoglycemia
↑ Lactate (lactic acidosis)
↑ Uric acid
↑ Triglycerides
Hepatomegaly/renomegaly

Clue in the stem that rules it out: Normal lactate and muscle involvement (↑ CK).
Von Gierke is also associated with doll-like facies and thin extremities—often described explicitly.

USMLE pearl:
If the question highlights lactic acidosis + hyperuricemia, it’s nearly always GSD I.

Distractor 2: Lysosomal acid α-glucosidase deficiency (GSD II, Pompe disease)

Why it’s tempting:
Pompe is a glycogen storage disease and shows muscle symptoms.

Why it’s wrong here:
Pompe is a lysosomal storage problem, not a cytosolic glycogenolysis defect. It presents classically with:

Cardiomegaly / hypertrophic cardiomyopathy
Hypotonia (“floppy baby”)
Macroglossia
Often normal blood glucose (no prominent fasting hypoglycemia)

Clue in the stem that rules it out:
This patient has fasting hypoglycemia + hepatomegaly with limit dextrin-like glycogen, pointing to a glycogen breakdown pathway defect (Cori), not lysosomal accumulation (Pompe).

USMLE pearl:
Pompe = cardiomyopathy + hypotonia + normal glucose.

Distractor 3: Myophosphorylase deficiency (GSD V, McArdle disease)

Why it’s tempting:
McArdle also affects muscle and can show ↑ CK.

Why it’s wrong here:
McArdle is primarily a skeletal muscle glycogen phosphorylase deficiency:

Exercise intolerance
Muscle cramps
Myoglobinuria
“Second wind” phenomenon
No hepatomegaly and no fasting hypoglycemia (liver glucose output is intact)

Clue in the stem that rules it out:
A toddler with hepatomegaly and fasting hypoglycemia is not McArdle.

USMLE pearl:
McArdle is often tested with increased muscle glycogen and flat lactate curve during exercise testing, plus myoglobinuria.

Distractor 4: Phosphofructokinase-1 deficiency (Tarui disease, GSD VII)

Why it’s tempting:
Also causes exercise intolerance and muscle symptoms; can be lumped mentally with McArdle.

Why it’s wrong here:
PFK-1 deficiency impairs glycolysis in muscle (and RBCs):

Exercise intolerance, cramps
Possible hemolysis (RBC glycolysis defect)
No hepatomegaly and no fasting hypoglycemia as a primary feature

Clue in the stem that rules it out:
The presence of hepatomegaly + fasting hypoglycemia pushes you toward hepatic glycogenolysis defects (GSD I, III, VI, IX).

Distractor 5: Hepatic glycogen phosphorylase deficiency (GSD VI, Hers disease)

Why it’s tempting:
Hers causes hepatomegaly and fasting hypoglycemia—very close to Cori.

Why it’s wrong here:
Hers is generally milder and lacks the hallmark limit dextrin description because the block is earlier (phosphorylase step), not at branch points. Classically:

Mild fasting hypoglycemia
Hepatomegaly
Growth delay
Usually no significant muscle involvement

Clue in the stem that rules it out:
The biopsy description of short outer branches/limit dextrins points specifically to debranching problems (Cori). Also, ↑ CK suggests muscle involvement.

Distractor 6: Phosphorylase kinase deficiency (GSD IX)

Why it’s tempting:
Also presents with hepatomegaly and hypoglycemia; common pediatric liver GSD.

Why it’s wrong here:
GSD IX can resemble Hers (because phosphorylase kinase activates glycogen phosphorylase). Often:

Hepatomegaly
Growth delay
Mild hypoglycemia
Typically improves with age

Clue in the stem that rules it out:
Again, limit dextrin on biopsy is a debranching enzyme clue. Muscle symptoms and ↑ CK also fit Cori more cleanly in many question stems.

High-Yield Cori Disease Summary (Step 1/2)

Definition

GSD III (Cori) = debranching enzyme deficiency (transferase + α-1,6-glucosidase)

Findings

Hepatomegaly
Fasting hypoglycemia
Hyperlipidemia
↑ AST/ALT
Muscle weakness / hypotonia, ↑ CK
Normal lactate (helps differentiate from GSD I)

Pathology clue

Limit dextrin accumulation: glycogen with short outer chains

Management (high-yield, not niche)

Avoid fasting; maintain euglycemia (frequent feeds, complex carbs)
Some patients benefit from higher protein intake to support gluconeogenesis (often mentioned conceptually)

Rapid-Fire Differentiation Table (What Exam Writers Want)

Disease	Enzyme	Key Clue	Glucose	Lactate	Organ Pattern
Von Gierke (I)	Glucose-6-phosphatase	Lactic acidosis, hyperuricemia	↓↓↓	↑	Liver/kidney
Pompe (II)	Lysosomal acid α-glucosidase	Cardiomyopathy, hypotonia	~Normal	~Normal	Heart, muscle
Cori (III)	Debranching enzyme	Limit dextrin, ↑ CK	↓	Normal	Liver + muscle
McArdle (V)	Muscle glycogen phosphorylase	Second wind, myoglobinuria	Normal	Exercise lactate ↓	Muscle
Hers (VI)	Liver glycogen phosphorylase	Mild liver GSD	↓ (mild)	Normal	Liver
GSD IX	Phosphorylase kinase	Liver GSD, improves with age	↓ (mild)	Normal	Liver ± muscle

Exam-Style “One-Liners” to Memorize

Cori = debranching defect → limit dextrin + liver + muscle
Von Gierke = can’t export glucose → lactic acidosis + hyperuricemia
Pompe = lysosomal glycogen → cardiomyopathy + hypotonia, normal glucose
McArdle = can’t use muscle glycogen → cramps + myoglobinuria + second wind