Muscular dystrophies, myasthenia gravis, and other neuromuscular junction disorders with their hallmark clinical and lab findings tested on USMLE Step 1.
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- What is the inheritance pattern of Duchenne muscular dystrophy?
- X-linked recessive
- What gene mutation causes Duchenne muscular dystrophy?
- A frameshift mutation in the dystrophin gene causing little to no dystrophin protein
- At what age do symptoms of Duchenne muscular dystrophy typically begin?
- Early childhood, usually before age 5
- What classic exam sign is seen in Duchenne muscular dystrophy when a child rises from the floor using their hands to push up their legs?
- Gower sign
- What calf finding is characteristic of Duchenne muscular dystrophy?
- Pseudohypertrophy of the calves from fibrofatty replacement of muscle
- What lab value is markedly elevated in Duchenne muscular dystrophy?
- Creatine kinase (CK)
- How does Becker muscular dystrophy differ from Duchenne muscular dystrophy in terms of dystrophin production?
- Becker produces partially functional dystrophin, causing a milder, later-onset course
- What is the most common adult-onset form of muscular dystrophy?
- Myotonic dystrophy
- What genetic mechanism causes myotonic dystrophy type 1?
- A CTG trinucleotide repeat expansion in the DMPK gene
- What clinical feature gives myotonic dystrophy its name?
- Myotonia, sustained muscle contraction with delayed relaxation after voluntary contraction
- What facial finding is classic in myotonic dystrophy?
- A long, thin "hatchet" face with frontal balding and ptosis
- What autoimmune disorder involves antibodies against the postsynaptic acetylcholine receptor at the neuromuscular junction?
- Myasthenia gravis
- What is the characteristic pattern of weakness in myasthenia gravis?
- Weakness that worsens with repeated use and improves with rest
- What ocular findings are common early symptoms of myasthenia gravis?
- Ptosis and diplopia
- What bedside test can improve ptosis in myasthenia gravis by resting the eye muscles?
- The ice pack test
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