Q-Bank Breakdown: McArdle Disease — Why Every Answer Choice Matters

Tag: Biochemistry > Lysosomal & Glycogen Storage Diseases

McArdle disease (GSD V) is a classic USMLE test favorite because it rewards pattern recognition and punishes shallow memorization. The key is learning how to distinguish it from other glycogen storage diseases and common look-alike distractors—especially those involving muscle cramps, exercise intolerance, and myoglobinuria.

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Clinical Vignette (USMLE-Style)

A 19-year-old man reports painful muscle cramps and fatigue that begin within the first few minutes of exercise. He notes that after briefly resting, he can often continue exercising at a lower intensity (“second wind”). After intense activity, his urine sometimes turns dark. Physical exam is normal between episodes. Labs show elevated creatine kinase (CK).

Most likely underlying defect?

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The Correct Answer: McArdle Disease (Glycogen Storage Disease Type V)

✅ Enzyme Deficiency

Muscle glycogen phosphorylase (myophosphorylase) deficiency

✅ Pathophysiology (What’s actually happening?)

Skeletal muscle can’t effectively break down glycogen → glucose-1-phosphate, so it can’t rapidly generate ATP via glycolysis during anaerobic exercise.

• Early exercise → ATP depletion → cramps, pain, fatigue
• Muscle injury → rhabdomyolysis → myoglobinuria (dark urine)
• Compensatory shift to oxidative metabolism after rest → “second wind”

✅ High-Yield Clinical Clues

• Exercise intolerance (especially short, high-intensity exercise)
• Muscle cramps
• Myoglobinuria after exertion
• Elevated CK
• Second wind phenomenon (very testable)

✅ High-Yield Biochem/Lab Findings

• Flat/absent rise in lactate during ischemic forearm exercise test
  • Because glycolysis can’t ramp up without glycogen breakdown
• Increased ammonia during exercise (AMP deamination pathway ramps up)

✅ Where does glycogen accumulate?

In skeletal muscle (not liver), because the defect is in muscle phosphorylase.

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Why Every Answer Choice Matters (Systematic Distractor Breakdown)

Below are the most common distractors paired against McArdle—and exactly how to eliminate them.

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Distractor 1: Pompe Disease (GSD II) — Lysosomal acid α-glucosidase deficiency

Why it tempts you: It involves muscle and glycogen.
Why it’s wrong here: Pompe is a lysosomal glycogen storage disease with prominent cardiomyopathy.

Key differentiators

• Enzyme: Acid α-glucosidase (acid maltase)
• Organs: Heart + muscle (lysosomal accumulation)
• Presentation:
  • Infantile: cardiomegaly, hypertrophic cardiomyopathy, hypotonia, macroglossia
  • Can present later with limb-girdle weakness
• Not classic for: “second wind” or exercise-induced myoglobinuria

USMLE tip: If you see cardiomyopathy + hypotonia + glycogen in lysosomes, think Pompe.

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Distractor 2: Cori Disease (GSD III) — Debranching enzyme deficiency

Why it tempts you: Glycogen breakdown defect; can cause muscle symptoms.
Why it’s wrong here: Cori is primarily a fasting hypoglycemia + hepatomegaly disease.

Key differentiators

• Enzyme: Debranching enzyme (α-1,6-glucosidase)
• Organ involvement: Liver ± muscle
• Presentation:
  • Mild fasting hypoglycemia
  • Hepatomegaly
  • Growth delay
• Lab: Increased limit dextrin (“abnormal glycogen”)

USMLE tip: If the stem includes hepatomegaly + fasting hypoglycemia, McArdle is unlikely.

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Distractor 3: Von Gierke Disease (GSD I) — Glucose-6-phosphatase deficiency

Why it tempts you: It’s the “big name” GSD and often appears as a default choice.
Why it’s wrong here: Von Gierke is a liver/kidney disease with severe metabolic derangements—not isolated exercise intolerance.

Key differentiators

• Enzyme: Glucose-6-phosphatase
• Organs: Liver and kidney
• Presentation: Severe fasting hypoglycemia, hepatomegaly, seizures/irritability
• Labs:
  • ↑ lactate (lactic acidosis)
  • ↑ uric acid (gout risk)
  • ↑ triglycerides

USMLE tip: “Lactic acidosis + hepatomegaly + severe hypoglycemia” = Von Gierke.

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Distractor 4: Andersen Disease (GSD IV) — Branching enzyme deficiency

Why it tempts you: Another glycogen processing enzyme—easy to confuse.
Why it’s wrong here: Andersen causes progressive liver disease due to abnormal glycogen structure.

Key differentiators

• Enzyme: Branching enzyme
• Glycogen: Less branched → insoluble → tissue damage
• Presentation:
  • Hepatomegaly
  • Progressive cirrhosis
  • Failure to thrive
• Not classic for: Exercise-induced cramps + “second wind”

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Distractor 5: Tarui Disease (GSD VII) — Phosphofructokinase (PFK-1) deficiency

Why it tempts you: It looks very similar to McArdle (exercise intolerance + cramps).
Why it’s wrong here: Tarui typically includes hemolysis because RBCs rely on glycolysis and PFK is essential.

Key differentiators

• Enzyme: PFK-1
• Organs: Muscle and RBCs
• Presentation:
  • Exercise intolerance, cramps, myoglobinuria (can occur)
  • Hemolytic anemia (key clue)
• Unique concept: Because glycolysis is blocked, glucose ingestion may worsen symptoms (less consistent “second wind” benefit)

USMLE tip: If they add hemolytic anemia to an exercise intolerance stem, think Tarui.

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Distractor 6: CPT II Deficiency (Fatty acid oxidation disorder, not a GSD)

Why it tempts you: Also causes exercise-induced myoglobinuria.
Why it’s wrong here: FAO disorders often present with symptoms triggered by prolonged exercise, fasting, or illness—not within the first minutes of exertion with a “second wind.”

Key differentiators

• Mechanism: Impaired long-chain fatty acid entry into mitochondria (or use thereof)
• Triggers: Prolonged exercise, fasting, cold exposure, infection
• Labs: Hypoketotic hypoglycemia can appear in FAO disorders (more common in systemic forms)

USMLE tip:

• Early, intense exercise symptoms → glycogen/glycolysis problem (McArdle/Tarui)
• Prolonged exercise or fasting symptoms → fatty acid oxidation problem

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Rapid-Fire High-Yield McArdle Facts (Step 1 & Step 2)

• Defect: Muscle glycogen phosphorylase (myophosphorylase)
• Core triad: Exercise intolerance + muscle cramps + myoglobinuria
• Second wind: Symptoms improve after rest due to increased blood-borne fuels (glucose/FFA) and oxidative metabolism
• Labs: ↑ CK; no rise in lactate with ischemic forearm exercise; ↑ ammonia
• Organ involvement: Skeletal muscle (no hepatomegaly, no fasting hypoglycemia)

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One-Table Differentiation (What to Recognize in 10 Seconds)

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Exam-Writer Mindset: How This Question Is Built

USMLE loves McArdle because it tests whether you can:

• Identify muscle-only glycogenolysis failure
• Use timing of symptoms (early intense exercise)
• Recognize the second wind phenomenon
• Separate it from:
  • Liver hypoglycemia syndromes (Von Gierke/Cori/Andersen)
  • Lysosomal disease (Pompe)
  • Glycolysis block with hemolysis (Tarui)
  • FAO disorders (CPT II)