Every IMG hits the same wall when glycogen storage diseases show up on UWorld. You memorize Von Gierke on Monday, confuse it with Cori by Wednesday, and by Friday you can’t tell Pompe from Andersen. The names rhyme. The enzymes overlap. The vignettes blur together.
We’ve taught hundreds of IMGs through this exact rotation, and we’ve watched the same five mistakes repeat themselves on every NBME assessment. By the end of this guide, you’ll never miscall a Von Gierke vs. Cori vignette again, you’ll spot a Pompe baby in the first sentence, and you’ll have a tissue anchored framework that holds even on test day under pressure.
This is the ultimate IMG friendly guide to glycogen storage diseases for USMLE Step 1 , built as high yield notes, not a textbook chapter. Mnemonics, flowcharts, comparison tables, NBME traps, and 10 vignette style MCQs at the end.
Under the current USMLE Step 1 content outline (2025–2026), biochemistry is no longer a standalone block. It’s woven into the Biochemistry & Nutrition category and integrated across Multisystem Processes & Disorders, contributing roughly 14–18% of total items when counted across organ systems
Glycogen storage diseases sit in a sweet spot the NBME loves: rare enough to feel intimidating, mechanistic enough to test pathway logic, and clinical enough to fit a vignette.
Expect at least 1–2 GSD flavored items on your real exam , sometimes disguised inside an endocrinology, hepatology, or pediatric stem.
The classic Step 1 stem follows a fixed pattern:
Vignette → enzyme defect → identify the disease, the labs, or the treatment.
On a pass/fail exam, missing a clean biochem block is exactly how borderline test takers slip into the fail zone.
High yield biochem is the cheapest territory on Step 1 : small surface area, predictable question style, massive return on memorization. Glycogen storage diseases are the single highest yield cluster inside that territory.
Before you memorize a single disease, lock down the pathway. Every GSD is just a knockout of one enzyme on this map.
Glucose → Glucose-6-P → Glucose-1-P → UDP-glucose → Glycogen (linear chain) → Branched glycogen
Key enzymes:
• Glycogen synthase : adds glucose units via α-1,4 bonds (rate-limiting step of glycogen synthesis)
• Branching enzyme (α-1,4 → α-1,6 transferase) — creates branch points
2.Glycogenolysis (breaking glycogen down)
Glycogen → Glucose-1-P → Glucose-6-P → Glucose (in liver only)
Key enzymes:
• Glycogen phosphorylase : cleaves α-1,4 bonds (rate-limiting step of glycogenolysis)
• Debranching enzyme has two activities: 4-α-glucanotransferase + α-1,6-glucosidase
• Glucose-6-phosphatase releases free glucose into blood (liver only)
• Lysosomal α-1,4-glucosidase (acid maltase) , degrades glycogen inside lysosomes
This one rule unlocks the entire topic:
Liver glycogen feeds the blood. Muscle glycogen feeds the muscle.
• Liver has glucose-6-phosphatase → can release free glucose into circulation → maintains blood sugar between meals.
• Muscle lacks glucose-6-phosphatase → glycogen serves only the muscle itself during exercise → never raises blood glucose.
Once you internalize this, you can predict which GSDs cause fasting hypoglycemia (liver enzymes) versus exercise intolerance (muscle enzymes) , without memorizing each disease.
Lock the first seven types in order with this single sentence:
“Very Poor Carbohydrate Metabolism Hurts Tarui”
| Letter | Disease | Type |
| Very | Von Gierke | I |
| Poor | Pompe | II |
| Carbohydrate | Cori | III |
| Metabolism | Andersen (M for “many branches”) | IV |
| Hurts | McArdle (use the A; see note below) | V |
| Tarui | Hers | VI |
Note: This is one of those mnemonics where you make it work for you.
The classic version most IMGs use is “Very Poor Carbohydrate Metabolism” for Types I–IV (Von Gierke, Pompe, Cori, Andersen), then add McArdle for V, Hers for VI, and Tarui for VII.
We recommend a second mnemonic layer (below) to anchor the muscle vs. liver split , that’s where most exam mistakes happen.
“Liver Loves V, Muscle Moves M, Lysosome Pomps.”
• Liver disease → Von Gierke (I), Cori (III), Hers (VI) → fasting hypoglycemia, hepatomegaly
• Muscle disease → McArdle (V), Tarui (VII) → exercise intolerance, cramps, myoglobinuria
• Lysosome disease → Pompe (II) → infantile cardiomegaly, hypotonia
• Both liver + muscle → Andersen (IV) → cirrhosis + muscle weakness
When the vignette gives you a baby with massive cardiomegaly, your brain should fire “Pompe” before you finish the sentence.
When it gives you a young adult with muscle cramps after exercise and burgundy urine, McArdle is automatic.
When it gives you a doll-faced infant with hepatomegaly, hypoglycemia, and high lactate, Von Gierke is the answer.
That’s the entire game.
• Deficient enzyme: Glycogen synthase (liver isoform)
• Pathway step blocked: UDP-glucose → glycogen
• Tissue: Liver
• Vignette: Infant with fasting hypoglycemia but NO hepatomegaly (because there’s no glycogen to accumulate). Postprandial hyperglycemia and lactic acidosis after meals.
• Labs: Low fasting glucose, high postprandial glucose, ketosis on fasting.
• Treatment: Frequent feeds, high-protein diet.
• Mnemonic: “Type Zero, zero glycogen” , the only GSD where the liver is not enlarged.
• NBME trap: Examiners pair “fasting hypoglycemia” with “no hepatomegaly” to trap students into picking Von Gierke. The absence of hepatomegaly is the giveaway.
• Deficient enzyme: Glucose-6-phosphatase (in the ER membrane)
• Pathway step blocked: Glucose-6-P → free glucose (the final exit step from both glycogenolysis AND gluconeogenesis)
• Tissue: Liver, kidney, intestine
• Inheritance: Autosomal recessive
• Classic vignette: Infant aged 3–6 months with “doll-like facies,” massive hepatomegaly, protuberant abdomen, fasting hypoglycemia, seizures, and growth failure.
• Labs (memorize this exact tetrad):
1. Hypoglycemia (severe, fasting)
2. Lactic acidosis (because pyruvate can’t exit gluconeogenesis as glucose, so it diverts to lactate)
3. Hyperuricemia (lactate competes with urate for renal excretion → gout like picture)
4. Hyperlipidemia / hypertriglyceridemia (excess acetyl-CoA shunts into fatty acid synthesis)
• Treatment: Frequent oral glucose / cornstarch, avoid fructose and galactose (their metabolism feeds into G6P which can’t exit).
• Mnemonic: “Von Gierke Gives 4 Highs” → Hypoglycemia, High lactate, High uric acid, High lipids.
• NBME trap: Both Von Gierke and Cori cause infantile hepatomegaly with hypoglycemia. The differentiator is lactate: Von Gierke = HIGH lactate, Cori = NORMAL lactate. Why? Because Von Gierke blocks gluconeogenesis (so pyruvate piles up as lactate), but Cori only blocks glycogenolysis (gluconeogenesis still works fine).
• Deficient enzyme: Lysosomal α-1,4-glucosidase (acid maltase)
• Pathway step blocked: Lysosomal degradation of glycogen
• Tissue: Lysosomes of every cell , but heart and skeletal muscle suffer most
• Inheritance: Autosomal recessive
• Classic vignette: Infant <1 year with severe cardiomegaly, hypotonia (“floppy baby”), macroglossia, hepatomegaly, and death by age 2 if untreated. ECG shows short PR interval, massive QRS voltages.
• Labs: Elevated CK; muscle biopsy shows glycogen-laden lysosomes (vacuoles on PAS stain).
• Treatment: Recombinant α-glucosidase (alglucosidase alfa) — enzyme replacement therapy.
• Mnemonic: “Pompe Pumps the heart” : Pompe = the only GSD that destroys the heart.
• NBME trap: The ONLY GSD that’s a lysosomal storage disease. If a question describes glycogen accumulating “inside membrane-bound vesicles” or shows “PAS-positive vacuoles in cardiac muscle,” the answer is Pompe even if they don’t name it.
Pearl: Pompe is the only GSD that does NOT cause hypoglycemia. The defect is lysosomal , cytosolic glucose handling is normal. Examiners exploit this constantly.
• Deficient enzyme: Debranching enzyme (α-1,6-glucosidase)
• Pathway step blocked: Cleavage of α-1,6 branch points during glycogenolysis
• Tissue: Liver + muscle
• Inheritance: Autosomal recessive
• Classic vignette: Infant with hepatomegaly, mild fasting hypoglycemia, ketosis, and mild muscle weakness. Labs look like a milder Von Gierke , but lactate is normal.
• Labs: Mild hypoglycemia, normal lactate, normal uric acid, mildly elevated CK. Glycogen accumulates with short outer branches (limit dextrin).
• Treatment: Frequent feeds, high protein diet.
• Mnemonic: “Cori Can’t Cut branches” , the debranching enzyme can’t trim the branch points, so glycogen accumulates as limit dextrin (short outer chains).
• NBME trap: The lab profile is “Von Gierke-lite.” Use lactate as your tiebreaker: high lactate = Von Gierke; normal lactate = Cori. The buzzword “limit dextrin” = Cori, always.
• Deficient enzyme: Branching enzyme (α-1,4 → α-1,6 transferase)
• Pathway step blocked: Branch formation during glycogenesis
• Tissue: Liver (primarily) + muscle, heart, brain
• Inheritance: Autosomal recessive
• Classic vignette: Infant with failure to thrive, progressive cirrhosis, hepatosplenomegaly, and death from liver failure by age 5. Glycogen is abnormal : long, unbranched chains that resemble amylopectin.
• Labs: Liver function tests deranged; biopsy shows PAS-positive amylopectin like inclusions.
• Treatment: Liver transplant is the only definitive option.
• Mnemonic: “Andersen = Abnormal, Awful Amylopectin” : branching enzyme defect → unbranched glycogen → cirrhosis.
• NBME trap: The buzzword is “amylopectin-like” glycogen. If you see that phrase, the answer is Andersen. Don’t confuse it with Cori’s “limit dextrin” (Cori = SHORT outer branches; Andersen = LONG unbranched chains).
• Deficient enzyme: Muscle glycogen phosphorylase (myophosphorylase)
• Pathway step blocked: Glycogenolysis in skeletal muscle (liver phosphorylase is unaffected)
• Tissue: Skeletal muscle ONLY
• Inheritance: Autosomal recessive
• Classic vignette: Young adult with painful muscle cramps after brief exercise, myoglobinuria (burgundy/cola-colored urine), and the pathognomonic “second-wind phenomenon” : symptoms improve after rest because the body switches to fatty acid oxidation.
• Labs: Elevated CK at baseline, no rise in venous lactate after forearm ischemic exercise test, ammonia rises normally.
• Treatment: Avoid strenuous exercise, oral sucrose before exercise, aerobic conditioning.
• Mnemonic: “McArdle = Muscle. Move it and it Aches.” Or: “McArdle’s Muscle Mocks Movement.”
• NBME trap: The single most tested fact is the flat venous lactate curve on the forearm exercise test. Normally, ischemic exercise raises lactate. In McArdle, glycogen can’t be broken down to lactate → lactate stays flat. Ammonia rises normally (it comes from the purine nucleotide cycle, not glycogen). If a question says “lactate didn’t rise but ammonia did” → McArdle.
• Deficient enzyme: Liver glycogen phosphorylase
• Pathway step blocked: Glycogenolysis in liver only
• Tissue: Liver
• Inheritance: Autosomal recessive
• Classic vignette: Child with mild hepatomegaly, mild fasting hypoglycemia, ketosis. Generally benign , patients improve with age.
• Labs: Mild hypoglycemia, normal lactate, normal uric acid, normal CK.
• Treatment: Often none; cornstarch if symptomatic.
• Mnemonic: “Hers is Hepatic, Harmless, and Hardly Hits” , the mildest liver GSD.
• NBME trap: Often confused with Cori. Differentiator: Hers has no muscle involvement (because muscle phosphorylase is a different gene); Cori involves both. Hers is also milder than Cori.
• Deficient enzyme: Muscle phosphofructokinase-1 (PFK-1)
• Pathway step blocked: Glycolysis in muscle and red blood cells
• Tissue: Muscle + RBCs
• Inheritance: Autosomal recessive
• Classic vignette: Young adult with exercise intolerance and cramps (looks like McArdle) PLUS hemolytic anemia (because RBCs depend on glycolysis for ATP).
• Labs: Elevated CK, hemolysis markers (low haptoglobin, elevated reticulocyte count, indirect hyperbilirubinemia), flat lactate on forearm test.
• Treatment: Exercise avoidance.
• Mnemonic: “Tarui = Two Tissues : muscle Tires and RBCs Tear.”
• NBME trap: McArdle vs. Tarui = both have exercise intolerance with flat lactate. Hemolysis = Tarui. No hemolysis = McArdle.
• Type IX — Liver phosphorylase kinase deficiency: Most common GSD overall but very mild. X-linked recessive in many cases. Mild hepatomegaly, mild hypoglycemia. Often resolves with age.
• Type XI — Fanconi-Bickel syndrome (GLUT2 deficiency): Hepatomegaly, renal Fanconi syndrome (glucosuria, phosphaturia, aminoaciduria), rickets.
• Type VIII: Largely reclassified into Type IX in modern literature.
These are recognition only. You will not see a deep vignette on these on Step 1.
| Type | Eponym | Deficient Enzyme | Tissue | Hypoglycemia | Lactate | Hepatomegaly | Muscle Sx | CK | Glycogen Structure | Hallmark Buzzword |
| 0 | — | Glycogen synthase | Liver | Yes (fasting) | Normal | No | None | Normal | Decreased glycogen | “No glycogen, no hepatomegaly” |
| I | Von Gierke | Glucose-6-phosphatase | Liver / kidney | Severe | High | Massive | None | Normal | Normal structure | “Doll-like facies, 4 Highs” |
| II | Pompe | Lysosomal α-1,4-glucosidase (acid maltase) | Lysosome (heart) | No | Normal | Yes | Severe hypotonia | High | Normal structure | “Cardiomegaly + floppy baby” |
| III | Cori | Debranching enzyme (α-1,6-glucosidase) | Liver + muscle | Mild | Normal | Yes | Mild | Mildly high | Limit dextrin | “Limit dextrin” |
| IV | Andersen | Branching enzyme (α-1,4 to α-1,6) | Liver + multi-organ | Mild | Normal | Cirrhosis | Mild | Mild | Long unbranched chains | “Amylopectin-like” |
| V | McArdle | Muscle phosphorylase (myophosphorylase) | Muscle only | No | Flat on exercise | No | Cramps, myoglobinuria | High | Normal structure | “Second-wind, flat lactate” |
| VI | Hers | Liver phosphorylase | Liver | Mild | Normal | Mild | None | Normal | Normal structure | “Mildest liver GSD” |
| VII | Tarui | Muscle PFK-1 | Muscle + RBC | No | Flat on exercise | No | Cramps + hemolysis | High | Normal structure | “McArdle + hemolysis” |
Memory Anchors:
Type I (Von Gierke): 4 Highs → high lactate, high uric acid, high triglycerides, high glucagon (with severe fasting hypoglycemia).
Type II (Pompe): pumps the heart → cardiomegaly is the giveaway.
Type V (McArdle): muscle only, flat lactate on ischemic exercise test, second-wind phenomenon.
Type VII (Tarui): McArdle picture plus hemolytic anemia (PFK-1 affects RBCs).
Bold rows = the five highest-yield diseases. Master these and you handle 90% of GSD questions on Step 1.
The NBME doesn’t test you on what you know , it tests you on what you can recognize under pressure. Here are the seven traps that catch IMGs every cycle.
TRAP #1: Von Gierke vs. Cori : use lactate as the tiebreaker. Both have hepatomegaly + hypoglycemia in infants. Von Gierke = HIGH lactate (gluconeogenesis blocked). Cori = NORMAL lactate (gluconeogenesis works). The buzzword “limit dextrin” = Cori.
TRAP #2: Pompe is NOT a typical GSD. Pompe doesn’t cause hypoglycemia, doesn’t follow the “liver vs. muscle” rule, and is the only lysosomal storage disease in the GSD family. Buzzwords: “floppy baby,” “infantile cardiomegaly,” “macroglossia,” “short PR interval, large QRS.”
TRAP #3: McArdle’s “second-wind phenomenon” is uniquely tested. After 8–10 minutes of rest, the patient can resume exercise as fatty acid oxidation kicks in. This phrase appears verbatim in NBME stems.
TRAP #4: Forearm ischemic exercise test : the flat lactate curve. McArdle and Tarui both flatten the lactate response. Ammonia still rises (it comes from the purine nucleotide cycle, not glycogen). If a question says “ammonia rose but lactate didn’t” → muscle GSD.
TRAP #5: Andersen vs. Cori glycogen morphology. Cori = short outer branches (limit dextrin). Andersen = long unbranched chains (amylopectin-like). Don’t flip these , examiners rotate the wording every cycle.
TRAP #6: Tarui = McArdle + hemolysis. Both look identical on exercise. The presence of hemolytic anemia (low haptoglobin, high reticulocytes, indirect hyperbilirubinemia) flips the diagnosis to Tarui. Why hemolysis? RBCs only have glycolysis : no PFK-1 = no ATP = membrane fragility.
TRAP #7: Galactose and fructose are forbidden in Von Gierke. Both sugars enter the pathway as G6P, which can’t exit because G6Pase is dead. Patients are placed on glucose and lactose-only diets with frequent cornstarch feeds. NBME loves to test diet management.
• GSD + endocrine: Von Gierke patients can develop pancreatitis from hypertriglyceridemia and gout from hyperuricemia. Both can be the “lead-in” finding.
• GSD + cardiology: Pompe ECG findings : short PR, massive QRS , often appear in pediatric cardiology vignettes.
• GSD + nephrology: Long standing Von Gierke causes renal disease and hepatic adenomas (which can transform into hepatocellular carcinoma).
GSD ULTRA-CHEAT SHEET — IMGHH
Tissue rule: Liver = blood sugar | Muscle = self-fuel
Master mnemonic: “Very Poor Carbohydrate Metabolism Hurts Tarui”
| Type | Eponym | Deficient Enzyme | Tissue Affected | Hallmark Feature |
| I | Von Gierke | Glucose-6-phosphatase | Liver | “Four Highs” (hypoglycemia, hyperlipidemia, hyperuricemia, lactic acidosis) |
| II | Pompe | Acid maltase (lysosomal) | Lysosome (heart, muscle) | Cardiomegaly with floppy infant |
| III | Cori | Debranching enzyme | Liver and muscle | Limit dextrin accumulation |
| IV | Andersen | Branching enzyme | Multiple tissues | Abnormal amylopectin-like glycogen |
| V | McArdle | Muscle phosphorylase | Muscle | Second-wind phenomenon, exercise intolerance |
| VI | Hers | Liver phosphorylase | Liver | Mildest form, hepatomegaly with mild hypoglycemia |
| VII | Tarui | Muscle phosphofructokinase-1 | Muscle | Exercise intolerance with hemolysis |
Lactate decoder:
HIGH lactate + hepatomegaly → Von Gierke
NORMAL lactate + hepatomegaly → Cori or Hers
FLAT lactate on exercise → McArdle or Tarui
Hypoglycemia decoder:
Severe + 4 Highs → Von Gierke
Severe + NO hepatomegaly → Type 0
Mild + hepatomegaly → Cori or Hers
QUICK NOTE:
The only GSD that destroys the heart → Pompe
The only GSD with hemolysis → Tarui
The only GSD that’s lysosomal → Pompe
The only GSD with cirrhosis → Andersen
The only GSD with renal Fanconi → Type XI (Fanconi-Bickel)
A 4-month-old male infant is brought to the emergency department for irritability and a seizure. His parents report that he has trouble going more than 3 hours between feedings. On examination, he has rounded “doll-like” facial features and a markedly protuberant abdomen. The liver is palpable 6 cm below the costal margin. Laboratory studies show:
• Glucose: 32 mg/dL
• Lactate: 9.2 mmol/L (elevated)
• Uric acid: 11 mg/dL (elevated)
• Triglycerides: 680 mg/dL (elevated)
Which of the following enzymes is most likely deficient?
A. Lysosomal α-1,4-glucosidase
B. Muscle glycogen phosphorylase
C. Glucose-6-phosphatase
D. Branching enzyme
E. Debranching enzyme
Correct Answer: C. Glucose-6-phosphatase
Explanation: This is the textbook tetrad of Von Gierke disease (GSD I): fasting hypoglycemia, lactic acidosis, hyperuricemia, and hyperlipidemia, plus the classic “doll-like facies” and massive hepatomegaly. Glucose-6-phosphatase is the final exit step shared by both glycogenolysis AND gluconeogenesis, when it’s dead, pyruvate piles up as lactate, lactate competes with urate for renal excretion (→ hyperuricemia), and excess acetyl-CoA shunts into triglyceride synthesis.
• A (acid maltase) = Pompe — would show cardiomegaly and hypotonia, not lactic acidosis.
• B (muscle phosphorylase) = McArdle — adult with exercise intolerance, no hepatomegaly.
• D (branching enzyme) = Andersen — cirrhosis and amylopectin-like glycogen, lactate normal.
• E (debranching enzyme) = Cori — hepatomegaly and mild hypoglycemia but NORMAL lactate.
Teaching point: The single fact that nails Von Gierke vs. Cori is lactate. High lactate = Von Gierke. Normal lactate = Cori.
A 6-month-old infant is brought to the pediatrician for poor feeding and lack of head control. On examination, the infant is hypotonic with macroglossia. A grade III/VI systolic murmur is heard. ECG shows a short PR interval with massive QRS voltages. Echocardiogram demonstrates severe biventricular hypertrophy. Muscle biopsy shows PAS-positive vacuoles within lysosomes containing glycogen.
Which of the following is the most appropriate treatment?
A. Cornstarch every 4 hours
B. Recombinant α-glucosidase
C. High-protein diet
D. Avoidance of strenuous exercise
E. Liver transplantation
Correct Answer: B. Recombinant α-glucosidase (alglucosidase alfa)
Explanation: This is Pompe disease (GSD II) , the only GSD that’s a lysosomal storage disorder. The hallmarks are infantile cardiomegaly, macroglossia, hypotonia (“floppy baby”), and the ECG signature (short PR, giant QRS). Treatment is enzyme replacement therapy with recombinant α-glucosidase.
• A (cornstarch) = treatment for Von Gierke and Cori (maintains blood sugar).
• C (high-protein diet) = treatment for Type 0 and Cori.
• D (exercise avoidance) = McArdle/Tarui management.
• E (liver transplant) = Andersen disease (only definitive cure for the cirrhosis).
Teaching point: Pompe is the ONLY GSD treated with enzyme replacement. It’s also the only GSD that doesn’t cause hypoglycemia and the only one that destroys the heart.
A 22-year-old man presents to the clinic after experiencing severe muscle cramps and dark, cola-colored urine following a basketball game. He reports that similar episodes have occurred since adolescence whenever he exercises strenuously. Notably, after resting for 10 minutes during a workout, he can often resume activity with less difficulty. A forearm ischemic exercise test is performed. Venous lactate fails to rise; venous ammonia rises appropriately.
The defective enzyme is most likely encoded by a gene expressed in which of the following tissues?
A. Liver only
B. Liver and skeletal muscle
C. Skeletal muscle only
D. Lysosomes of all cells
E. Erythrocytes and skeletal muscle
Correct Answer: C. Skeletal muscle only
Explanation: Classic McArdle disease (GSD V) , exercise intolerance, myoglobinuria, the “second-wind phenomenon,” and the diagnostic flat lactate curve with intact ammonia rise on forearm ischemic exercise. The defective enzyme is muscle glycogen phosphorylase (myophosphorylase), encoded by the PYGM gene, expressed exclusively in skeletal muscle. Liver phosphorylase is a separate gene (PYGL) and is unaffected.
• A (liver only) = Hers disease.
• B (liver + skeletal muscle) = Cori (debrancher is one enzyme expressed in both).
• D (lysosomes) = Pompe.
• E (erythrocytes + skeletal muscle) = Tarui (PFK-1 deficient in both — would also cause hemolysis).
Teaching point: Flat lactate + intact ammonia = muscle glycogen breakdown defect. No hemolysis distinguishes McArdle from Tarui.
A 9-month-old female is evaluated for failure to thrive. On examination, she has hepatosplenomegaly and mild ascites. Liver biopsy reveals abnormal glycogen with long, sparsely branched outer chains resembling amylopectin. By age 3, she develops progressive cirrhosis and is listed for liver transplantation.
Which of the following enzymes is most likely deficient?
A. α-1,4 → α-1,6 glucosyltransferase
B. α-1,6-glucosidase
C. Glycogen synthase
D. Glucose-6-phosphatase
E. Lysosomal acid maltase
Correct Answer: A. α-1,4 → α-1,6 glucosyltransferase (branching enzyme)
Explanation: Andersen disease (GSD IV) — branching enzyme deficiency produces amylopectin-like glycogen (long, unbranched outer chains). This abnormal glycogen is poorly soluble and triggers progressive cirrhosis, with death from liver failure typically by age 5 unless transplanted.
• B (α-1,6-glucosidase / debrancher) = Cori — produces “limit dextrin” (SHORT outer branches), not amylopectin.
• C (glycogen synthase) = Type 0 — no glycogen accumulates at all, no hepatomegaly.
• D (G6Pase) = Von Gierke — normal glycogen structure.
• E (acid maltase) = Pompe — heart involvement, not cirrhosis.
Teaching point: Cori = “limit dextrin” (short branches). Andersen = “amylopectin-like” (long unbranched). Andersen is the only GSD that causes cirrhosis and death by age 5.
A 19-year-old man presents with chronic exercise intolerance and dark urine after exertion. Laboratory studies show CK 3,400 U/L, hemoglobin 9.8 g/dL, reticulocyte count 8%, haptoglobin <10 mg/dL, and indirect bilirubin 2.4 mg/dL. A forearm ischemic exercise test shows no rise in venous lactate.
Which of the following enzymes is most likely deficient?
A. Muscle glycogen phosphorylase
B. Muscle phosphofructokinase-1
C. Carnitine palmitoyltransferase II
D. Pyruvate kinase
E. Glucose-6-phosphate dehydrogenase
Correct Answer: B. Muscle phosphofructokinase-1
Explanation: This is Tarui disease (GSD VII) , a McArdle-like exercise intolerance PLUS hemolytic anemia. PFK-1 is rate-limiting in glycolysis and is expressed in both skeletal muscle and erythrocytes. RBCs have no mitochondria and depend entirely on glycolysis for ATP , when PFK-1 is defective, RBC membranes become fragile → chronic hemolysis (low haptoglobin, high reticulocytes, indirect hyperbilirubinemia).
• A (muscle phosphorylase) = McArdle , same exercise picture but no hemolysis.
• C (CPT-II) = fatty acid oxidation defect , would NOT cause flat lactate (glycolysis is intact); rhabdomyolysis triggered by fasting/prolonged exercise, not brief exertion.
• D (pyruvate kinase) = hemolysis but no exercise intolerance; lactate would rise normally.
• E (G6PD) = oxidant-induced hemolysis (fava beans, primaquine), not exercise-related.
Teaching point: McArdle + hemolysis = Tarui. PFK-1 is the rate-limiting enzyme of glycolysis; RBCs depend on it for ATP.
A pediatric resident is reviewing the case of a 7-month-old with hepatomegaly and fasting hypoglycemia. The intern proposes a diagnosis of Von Gierke disease. The attending asks: “What single laboratory finding would most reliably distinguish Von Gierke (Type I) from Cori (Type III) disease?”
Which of the following is the best answer?
A. Serum glucose
B. Serum lactate
C. Serum CK
D. Serum ammonia
E. Urine ketones
Correct Answer: B. Serum lactate
Explanation: Von Gierke (G6Pase) blocks the final exit of BOTH glycogenolysis and gluconeogenesis → pyruvate diverts to lactate → lactic acidosis. Cori (debrancher) blocks only glycogenolysis → gluconeogenesis remains intact → lactate is normal. Both diseases share hepatomegaly and hypoglycemia, but only Von Gierke has the “4 Highs” tetrad.
• A (glucose): both have hypoglycemia.
• C (CK): Cori has mildly elevated CK; not as reliable a discriminator.
• D (ammonia): not characteristically deranged in either.
• E (urine ketones): both can show ketosis.
Teaching point: Lactate is the single most discriminating lab between the two infantile hepatomegaly GSDs.
A 5-month-old infant presents with seizures during early-morning feedings. Examination is unremarkable , no hepatomegaly, no cardiomegaly, normal tone. Fasting laboratory studies show glucose 38 mg/dL with appropriate ketosis and normal lactate. Postprandial studies after a meal show glucose 248 mg/dL and elevated lactate.
Which of the following best explains this presentation?
A. Glucose-6-phosphatase deficiency
B. Glycogen synthase deficiency
C. Debranching enzyme deficiency
D. Liver phosphorylase deficiency
E. Pyruvate kinase deficiency
Correct Answer: B. Glycogen synthase deficiency
Explanation: GSD Type 0 is unique because no glycogen is made → no hepatomegaly. Patients have fasting hypoglycemia (no glycogen to mobilize) but postprandial hyperglycemia and lactic acidosis (incoming glucose can’t be stored, so it floods glycolysis and overflows to lactate).
• A (G6Pase) = Von Gierke : would have hepatomegaly and fasting lactic acidosis.
• C (debrancher) = Cori : hepatomegaly present.
• D (liver phosphorylase) = Hers : mild hepatomegaly, normal post-meal labs.
• E (pyruvate kinase) = hemolytic anemia, not hepatic disease.
Teaching point: Type 0 is the ONLY GSD with fasting hypoglycemia AND no hepatomegaly. Postprandial hyperglycemia + lactic acidosis is the giveaway.
A 4-year-old boy with previously diagnosed Von Gierke disease is brought to the clinic for routine follow-up. He has been managed with overnight cornstarch feeds. His parents ask whether he can have apple juice, milk, and a banana with breakfast. Which of the following is the most appropriate dietary recommendation?
A. All three are safe in moderation
B. Apple juice is safe; avoid milk and banana
C. Milk is safe; avoid apple juice and banana
D. Avoid apple juice and milk; banana is safe
E. None of the three should be consumed
Correct Answer: D. Avoid apple juice and milk; banana is safe
Explanation: In Von Gierke, both fructose and galactose must be restricted because they enter the metabolic pathway as glucose-6-phosphate, which cannot exit as free glucose (G6Pase is dead) → worsens lactic acidosis and triglyceride accumulation.
• Apple juice = high fructose → AVOID.
• Milk = lactose (glucose + galactose) → AVOID.
• Banana = primarily glucose and starch → SAFE in moderation.
The recommended diet is glucose- and starch-based (cornstarch is the gold standard) with fructose and galactose restriction.
Teaching point: Von Gierke patients tolerate glucose and complex starch only. Fructose (juice, honey, sucrose) and galactose (milk products) bypass G6Pase but get stuck once they reach G6P. NBME loves dietary management questions.
A 16-year-old girl with a long-standing diagnosis of GSD I (Von Gierke) presents to the hepatology clinic for surveillance. Abdominal MRI reveals a 3.2 cm well-demarcated lesion in the right hepatic lobe with arterial enhancement. AFP is normal. Which of the following is the most likely diagnosis, and what is the principal long-term concern?
A. Focal nodular hyperplasia; no malignant potential
B. Hepatic adenoma; risk of hepatocellular carcinoma
C. Hemangioma; risk of rupture
D. Hepatoblastoma; metastatic spread
E. Cholangiocarcinoma; biliary obstruction
Correct Answer: B. Hepatic adenoma; risk of hepatocellular carcinoma
Explanation: Long standing Von Gierke disease is associated with hepatic adenomas in 70–80% of patients by adolescence. These adenomas carry a documented risk of malignant transformation into hepatocellular carcinoma , making lifelong imaging surveillance mandatory. The mechanism involves chronic metabolic derangement and hepatocyte stress.
• A (FNH) = unrelated to GSD.
• C (hemangioma) = unrelated.
• D (hepatoblastoma) = pediatric primary liver cancer, distinct from the GSD-adenoma-HCC pathway.
• E (cholangiocarcinoma) = biliary tree origin, not associated with Von Gierke.
Teaching point: Von Gierke complications grow with the patient : gout, pancreatitis, renal disease, hepatic adenomas, and HCC. Long-term surveillance is part of management. This is exactly the kind of integrated long-game question that NBME 30+ asks.
A 28-year-old man has been evaluated for fatigue and exercise intolerance since childhood. He reports that he can perform light, sustained activity (such as long walks) without difficulty, but brief intense exertion (sprinting up stairs) causes painful cramps that resolve after a few minutes of rest, after which he can resume activity. He has never had jaundice or anemia. His brother has identical symptoms. Forearm ischemic exercise test: lactate fails to rise.
Which of the following is most consistent with this patient’s underlying defect?
A. Glycogen accumulates in skeletal muscle as a structurally normal molecule
B. Glycogen accumulates in lysosomes throughout the body
C. Glycogen accumulates as amylopectin-like long unbranched chains
D. No glycogen accumulates because synthesis is impaired
E. Glycogen accumulates as limit dextrin in liver and muscle
Correct Answer: A. Glycogen accumulates in skeletal muscle as a structurally normal molecule
Explanation: McArdle disease: Muscle phosphorylase deficiency means muscle can synthesize glycogen normally but cannot break it down. The glycogen that accumulates is structurally normal (normal branching) it just can’t be mobilized. The flat lactate curve, second-wind phenomenon, and absence of hemolysis (rules out Tarui) seal the diagnosis. The hereditary pattern with an affected brother fits autosomal recessive inheritance.
• B (lysosomal accumulation) = Pompe.
• C (amylopectin-like) = Andersen.
• D (no glycogen) = Type 0.
• E (limit dextrin) = Cori.
Teaching point: McArdle = normal glycogen structure, just can’t be degraded. The structural form of accumulated glycogen is a high-yield differentiator across GSDs.
Yes. Biochemistry is integrated across the Biochemistry & Nutrition and Multisystem Processes & Disorders sections of the Step 1 content outline, contributing roughly 14–18% of items when counted across integrated organ systems. On a pass/fail exam, missing a single high yield biochem block can sink borderline test takers , and GSDs are one of the densest, most predictable scoring opportunities in that territory.
Anchor each disease to its tissue first (liver, muscle, lysosome, or both), attach a single mnemonic per disease, and drill yourself with NBME style vignettes. The trio of Von Gierke, Pompe, and McArdle covers most of what Step 1 will throw at you. Use the master comparison table in this guide as your single source of truth.
Von Gierke (Type I), Pompe (Type II), and McArdle (Type V) are the three most commonly tested. Cori (III) and Andersen (IV) are second tier but still expected. Tarui (VII) and Hers (VI) are buzzword recognition only.
No. Focus deeply on Types I, II, III, IV, V, VI, and VII. Types 0, VIII, IX, and XI are buzzword-level. Knowing the deficient enzyme, the affected tissue, and one classic vignette per disease is enough to clear any Step 1 question on the topic.
Both cause infantile hepatomegaly and fasting hypoglycemia. Von Gierke (G6Pase) shows the “4 Highs” , high lactate, high uric acid, high triglycerides, low glucose , because gluconeogenesis is also blocked. Cori (debranching enzyme) has normal lactate and milder hypoglycemia because gluconeogenesis is intact. The buzzword “limit dextrin” = Cori, always.
Glycogen storage diseases are not hard. They are just untaught , or taught badly. Most IMGs lose these questions on the real exam not because the content is complex, but because no one ever sat them down and showed them the pattern. You just read a guide that did.
That same gap exists across every biochem topic NBME tests. Urea cycle. Lysosomal storage. Rate-limiting enzymes. Glycolysis regulation. The whole metabolism integration block. Each one looks intimidating until someone breaks it down the way it actually appears on test day — and then it becomes free points.
That is exactly what we built the UIT Biochem Crash Course to do.
Every high-yield enzyme, pathway, and disease taught the way it shows up in vignettes. No fluff. No padding. No 40-hour video lectures. Just the patterns NBME tests, the mnemonics that actually stick, and the integration questions that separate a 240 from a 260.
If glycogen storage diseases finally clicked for you in this guide, the rest of biochem is going to feel the same way once you go through the full course.
IMG Helping Hands — USMLE Step 1 Prep
Glycogen storage diseases are one cluster. There are dozens more just like it, urea cycle, lysosomal storage, fatty acid oxidation, glycolysis regulation, each one predictable once someone breaks it down the right way. IMG Helping Hands has guided hundreds of IMGs through Step 1 prep built specifically for international medical graduates, not US med students.
High yield biochem is free points on Step 1. The IMGs who pass on their first attempt treat every topic cluster like this one — systematically, not randomly.
References
1. USMLE Step 1 Content Outline, current edition
2. First Aid for the USMLE Step 1, latest edition : Glycogen Storage Diseases section
3. Lippincott’s Illustrated Reviews: Biochemistry : Glycogen Metabolism chapter
4. StatPearls: Glycogen Storage Disease entries
5. NIH OMIM: entries for GSD Types I through VI
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