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