Muscular dystrophy

00:00 / 00:00



Muscular dystrophy

Musculoskeletal system


Muscular dystrophy


0 / 26 complete

USMLE® Step 1 questions

0 / 5 complete

High Yield Notes

3 pages


Muscular dystrophy

of complete


USMLE® Step 1 style questions USMLE

of complete

A study is carried out to evaluate X-linked recessive disorders in female carriers. The clinical features of eight female gene carriers of Duchenne muscular dystrophy were noted, and a relationship was established between the genotype and phenotype. Five of the eight cases were symptomatic and showed mild muscle weakness, difficulty walking, mild to moderate elevations of the serum creatinine kinase level, or myopathic changes on an electromyogram. Upon immunofluorescence, a mosaic pattern of dystrophin deficiency was found. Only two carriers were clinically asymptomatic. Which of the following genetic terms best describes the underlying cause for the difference in these patients’ symptoms?

External References

First Aid








Becker muscular dystrophy p. 59

presentation p. 714


Content Reviewers

With muscular dystrophy, “dys” means bad or difficult, and “troph” means nourish; so muscular dystrophy basically refers to the muscle appearing poorly nourished because of degeneration, which leads to muscle weakness.

Under a microscope, a biopsy of the tissue shows changes in the muscle itself but not in the nerve or neuromuscular junction; this distinguishes muscular dystrophy from other problems that cause muscle weakness as a result of nerve damage, like neuropathies.

Muscular dystrophy is actually a group of disorders, all of which are caused by genetic mutations.

Within that group, dystrophinopathies are the most common, which includes Duchenne muscular dystrophy, or DMD, and Becker muscular dystrophy, both of which result from mutations in the dystrophin gene.

In addition to those two, genetic mutations in other genes are responsible for several dozen other muscular dystrophies, some of which code for proteins that form a protein complex with dystrophin protein.

These other muscular dystrophies, therefore end up causing a lot of the same symptoms as the dystrophinopathies.

Now, the fact that both Duchenne and Becker muscular dystrophy result from mutations in the same dystrophin gene means that they are “allelic disorders,” and when a mutation occurs in dystrophin that’s severe enough to result in no protein at all, for example a nonsense or a frameshift mutation, the result is Duchenne muscular dystrophy, which ends up being the more severe of the two, with symptoms usually presenting by age 5.

On the other hand, mutations that allow for a misshapen protein to form, like missense mutations, lead to Becker muscular dystrophy which is basically a milder form of Duchenne muscular dystrophy that presents later on, usually between age 10 to 20.

Alright so the dystrophin gene is a huge gene on the X-chromosome, that has 79 exons and is over 2 million base pairs in length.

By comparison, most genes have only about 10 exons and are 50 thousand base pairs in length.


Muscular dystrophy is a group of inherited diseases that cause progressive weakness and degeneration of the skeletal muscles that control movement (e.g. Duchenne and Becker muscular dystrophy). Muscular dystrophy is caused by genetic defects that interfere with the production of proteins needed to form healthy muscle. There is currently no cure for muscular dystrophy, physical and occupational therapy, and medications can help manage the symptoms and slow the progression of the disease.


Copyright © 2023 Elsevier, its licensors, and contributors. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Cookies are used by this site.

USMLE® is a joint program of the Federation of State Medical Boards (FSMB) and the National Board of Medical Examiners (NBME). COMLEX-USA® is a registered trademark of The National Board of Osteopathic Medical Examiners, Inc. NCLEX-RN® is a registered trademark of the National Council of State Boards of Nursing, Inc. Test names and other trademarks are the property of the respective trademark holders. None of the trademark holders are endorsed by nor affiliated with Osmosis or this website.