McArdle disease is one of those Step questions that looks like vague “exercise intolerance,” but it actually has a crisp biochemical signature. If you can picture a muscle cell trying to sprint with its glycogen locked in a safe, you’ll never miss it again.
The Visual Hack (Mnemonic You Can Draw in 5 Seconds)
“McArdle = Myo-locked glycogen”
Picture this:
- A muscle (MYO) holding a glycogen “vault/safe”
- The key is labeled “Muscle phosphorylase (PYGM)”
- The key is missing/broken → glycogen can’t be opened during exercise
- A runner starts sprinting → legs cramp, then slows down and somehow feels better (second wind)
One-liner:
McArdle = muscle glycogen can’t be broken down (↓ myophosphorylase) → early exercise cramps + myoglobinuria + “second wind.”
What It Is (In One High-Yield Sentence)
McArdle disease (GSD V) is AR deficiency of muscle glycogen phosphorylase (myophosphorylase; gene: PYGM) → impaired glycogenolysis in skeletal muscle.
The Classic USMLE Presentation
Symptoms & triggers
- Exercise intolerance (especially brief, intense activity: sprinting, lifting)
- Painful muscle cramps
- Weakness/fatigue early in exertion
- Myoglobinuria after strenuous exercise (tea-colored urine)
The buzzy hallmark: Second-wind phenomenon
After stopping briefly or switching to lower intensity, symptoms improve because muscle can rely more on:
- Blood glucose
- Free fatty acids / oxidative metabolism
Step-friendly phrasing: “Patient gets cramps early in exercise but improves after resting briefly and resuming at lower intensity.”
The Biochem That Pays Rent on Exams
What’s blocked?
Normally in muscle during exercise:
- Glycogen → glucose-1-P via glycogen phosphorylase
- Glucose-1-P → glycolysis → ATP + lactate
In McArdle:
- Muscle can’t access glycogen for quick ATP
- So you get early ATP depletion → cramps, muscle injury
Lab pattern (very testable)
- ↓ Lactate rise after ischemic forearm exercise test
- ↑ Ammonia (from AMP deamination during energy stress)
- ↑ CK (rhabdomyolysis risk)
Ischemic forearm exercise test: expected curves
| Condition | Lactate after exercise | Ammonia after exercise |
|---|---|---|
| Normal | ↑ | ↑ (mild) |
| McArdle (GSD V) | Flat / ↓ rise | ↑ (often marked) |
How to Differentiate from “Similar” Step Traps
McArdle vs Pompe (very common compare/contrast)
| Feature | McArdle (GSD V) | Pompe (GSD II) |
|---|---|---|
| Primary tissue | Skeletal muscle (exercise-related) | Cardiac + skeletal muscle (lysosomal) |
| Enzyme | Muscle glycogen phosphorylase | Acid α-glucosidase (acid maltase) |
| Key symptom | Exercise intolerance, cramps, myoglobinuria, second wind | Cardiomyopathy, hypotonia, macroglossia |
| Lactate after exercise | No rise | Not the classic hook |
| Organelle | Cytosol (glycogenolysis) | Lysosome |
McArdle vs Von Gierke (if the stem mentions fasting hypoglycemia)
- Von Gierke (GSD I): severe fasting hypoglycemia, lactic acidosis, hepatomegaly
- McArdle: no primary fasting hypoglycemia; problem is muscle during exercise
High-Yield “If You See This, Think McArdle” Clues
- “Painful muscle cramps during exercise” + “myoglobin in urine”
- “Symptoms improve after a brief rest” (second wind)
- “No increase in lactate after exercise”
- Elevated CK in a patient with exertional rhabdomyolysis
Treatment (What USMLE Usually Wants)
- Avoid intense, isometric bursts (sprinting, heavy lifting)
- Gentle aerobic conditioning can help
- Pre-exercise oral sucrose/glucose may improve tolerance (fuel from blood)
- Prevent rhabdo complications: hydration; recognize risk with extreme exertion
15-Second Memory Anchor (Shareable)
McArdle = “MYO can’t UNLOCK glycogen.”
Early cramps + myoglobinuria, no lactate rise, and second wind when you slow down and use blood glucose/FFA.