Muscular Dystrophies

USMLE Step 1 trap: Attributes DMD and Becker to different genes rather than different mutation types in the same dystrophin gene. Both DMD and Becker MD are caused by mutations in the dystrophin gene; DMD has frameshift/nonsense mutations producing no functional dystrophin, while Becker has in-frame mutations producing reduced but partially functional dystrophin.

Muscular dystrophies are a group of inherited disorders characterized by progressive skeletal muscle weakness due to defects in proteins that maintain muscle fiber integrity. On USMLE Step 1, this topic clusters around three main diseases: Duchenne MD (DMD), Becker MD (BMD), and myotonic dystrophy — each tested from distinct angles. DMD and BMD are X-linked recessive disorders caused by dystrophin gene mutations, while myotonic dystrophy is autosomal dominant with a trinucleotide repeat expansion. Expect the exam to present a clinical vignette — a boy in early childhood who can't keep up with peers, has large calves, and struggles to stand from the floor — and ask you to diagnose, explain the underlying mechanism, or predict labs and genetic findings.

The trickiest part of this topic is that students treat DMD and Becker as caused by completely different genes, when in reality it's the same dystrophin gene with different mutation types. That single distinction — frameshift/nonsense (no functional protein, severe) versus in-frame (partial protein, milder) — explains the entire difference in clinical severity and is a favorite USMLE Step 1 testing angle. The other major trap is conflating myotonic dystrophy with DMD: myotonic dystrophy is autosomal dominant, shows anticipation via CTG repeat expansion in the DMPK gene, and has multisystem features (myotonia, cataracts, cardiac conduction defects, frontal balding, testicular atrophy) that DMD does not.

Diagnostically, the exam wants you to know that CK is markedly elevated early (before weakness is clinically apparent), genetic testing confirms the diagnosis, and muscle biopsy shows absent dystrophin staining in DMD versus reduced/patchy in Becker. Clinically, Gower sign — a child using their hands to 'walk up' their thighs to rise from the floor — reflects proximal lower limb weakness and is the canonical early exam finding. Calf pseudohypertrophy looks like muscle but is actually fat and fibrosis replacing dead muscle fibers, and the exam will test whether you understand that distinction.

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Common misconceptions

Common mistake

Wrong: Duchenne and Becker muscular dystrophy are caused by different genes.

Right: Both DMD and Becker MD are caused by mutations in the dystrophin gene; DMD has frameshift/nonsense mutations producing no functional dystrophin, while Becker has in-frame mutations producing reduced but partially functional dystrophin.

Both DMD and Becker MD are caused by mutations in the exact same gene — the dystrophin gene on the X chromosome. The difference is entirely about how the mutation disrupts the reading frame. Frameshift or nonsense mutations in DMD shift the reading frame, producing a truncated, nonfunctional protein that gets degraded — no dystrophin, severe disease. In-frame deletions in Becker preserve the reading frame, so a shorter but partially functional dystrophin is made — less severe disease. Remembering 'same gene, different mutation type, different severity' is the key mental model.

Common mistake

Wrong: Myotonic dystrophy follows the same X-linked recessive inheritance as Duchenne MD.

Right: Myotonic dystrophy is autosomal dominant with trinucleotide repeat expansion (CTG in DMPK gene) and shows anticipation, unlike the X-linked recessive DMD.

Myotonic dystrophy has nothing to do with X-linked inheritance — it is autosomal dominant, meaning one mutant allele is sufficient to cause disease and males and females are equally affected. The mutation is a CTG trinucleotide repeat expansion in the DMPK gene on chromosome 19, and it shows anticipation (worsening severity and earlier onset in successive generations). This is mechanistically completely different from DMD, which is a structural protein deficiency — myotonic dystrophy involves a toxic RNA gain-of-function mechanism that disrupts splicing of multiple proteins, explaining the multisystem involvement.

Common mistake

Gap: Misses Gower sign as a hallmark clinical finding reflecting proximal weakness in DMD

Gower sign (using hands to walk up the legs to stand from the floor) reflects proximal lower extremity weakness and is a classic early clinical finding in Duchenne muscular dystrophy.

Gower sign is the classic physical exam finding in DMD and reflects selective proximal lower extremity weakness — the hip extensors and knee extensors are too weak to push the torso upright from the floor. So the child compensates by using their hands to push on their thighs and essentially 'climb up' their own legs. On USMLE Step 1, if you see a young boy described as using his hands to rise from the floor, recognize this immediately as Gower sign and link it to proximal muscle weakness in DMD.

Common mistake

Wrong: Calf enlargement in DMD represents true muscle hypertrophy from compensatory use.

Right: Calf pseudohypertrophy in DMD results from replacement of muscle tissue with fat and fibrotic connective tissue, not true muscle growth.

The large calves in DMD look muscular but are not — this is a critical conceptual distinction. As dystrophic muscle fibers die, they are replaced by fat and fibrotic connective tissue, which physically enlarges the calf compartment but provides no contractile function. This is why it's called pseudohypertrophy. True muscle hypertrophy from compensatory overuse does not occur here; the muscle is being destroyed, not strengthened. Recognizing this helps you answer mechanism-based questions correctly and avoid conflating it with the adaptive hypertrophy seen in conditions like myotonia congenita.

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What the exam tests

Duchenne MD genetics and clinical course: Know that DMD is X-linked recessive, caused by frameshift or nonsense mutations in the dystrophin gene producing zero functional dystrophin, with onset in early childhood, loss of ambulation by teenage years, and death from cardiac or respiratory failure by the 20s-30s.
Becker MD mutation type and why it's milder: Know that Becker is also caused by dystrophin gene mutations, but in-frame deletions preserve partial dystrophin function, producing a milder, later-onset phenotype — patients often remain ambulatory into adulthood.
Myotonic dystrophy genetics and multisystem features: Know that DM1 is autosomal dominant with CTG repeat expansion in the DMPK gene, shows anticipation, and causes myotonia plus systemic features (cataracts, cardiac arrhythmias, frontal balding, testicular atrophy, insulin resistance) — a very different profile from DMD.
Diagnostic workup: Know that CK is markedly elevated and is often the first abnormality detected; genetic testing confirms the diagnosis; muscle biopsy with dystrophin immunostaining distinguishes absent (DMD) from reduced/abnormal (Becker); EMG shows myopathic pattern.
Management principles: Know that glucocorticoids (deflazacort, prednisone) slow DMD progression; exon-skipping therapies (e.g., eteplirsen) target specific mutations; cardiac and respiratory monitoring and support are essential as the disease progresses.

Can you avoid these mistakes?

A 5-year-old boy is noted to have difficulty climbing stairs and uses his hands to push off his thighs when rising from the floor. His calves appear enlarged. Serum CK is 10,000 U/L. Muscle biopsy shows complete absence of dystrophin staining. What type of mutation most likely explains his disease, and why does his CK not follow an X-linked dominant pattern?

A patient is diagnosed with Becker muscular dystrophy. His brother has Duchenne muscular dystrophy. Both have mutations in the dystrophin gene. Explain how two mutations in the same gene produce such different clinical severities, and predict what muscle biopsy dystrophin staining would show in each brother.

A 30-year-old man presents with difficulty releasing his grip after handshakes, frontal balding, bilateral cataracts, and a first-degree heart block on ECG. His father had similar but milder symptoms appearing at age 50. What is the diagnosis, what is the inheritance pattern and molecular mechanism, and why are his symptoms more severe than his father's?

A patient with DMD is being started on deflazacort. His parents ask why a steroid helps a genetic disease. What is the mechanism of benefit, and what two organ systems require progressive monitoring regardless of steroid use?

Muscular Dystrophies

Common misconceptions

What the exam tests

Can you avoid these mistakes?

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