WBR0303
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Author | [[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D.)]] |
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Exam Type | ExamType::USMLE Step 1 |
Main Category | MainCategory::Genetics |
Sub Category | SubCategory::Neurology, SubCategory::General Principles |
Prompt | [[Prompt::A 26-year-old man presents to his primary care physician with complaints of muscle weakness. The patient used to partake in vigorous exercise, but over the past year has noted a gradual decline in his strength. He explains that he can no longer hold his body weight on the pull-up bar at the gym. Upon further questioning, the patient also reports he is sometimes unable to release his grip from a doorknob. On physical examination, the physician notes the patient has dysarthric speech and is unable to reverse the contraction of his thumb following percussion of the thenar eminence. What is the mode of inheritance of this patient's condition?]] |
Answer A | AnswerA::Autosomal recessive |
Answer A Explanation | AnswerAExp::Myotonic dystrophy is not an autosomal recessive disorder. An example of an autosomal recessive disease is phenylketonuria. |
Answer B | AnswerB::Autosomal dominant |
Answer B Explanation | AnswerBExp::Myotonic dystrophy is an autosomal dominant disorder. |
Answer C | AnswerC::X-linked dominant |
Answer C Explanation | AnswerCExp::Myotonic dystrophy is not an X-linked dominant disorder. An example of an X-linked dominant disorder is Rett syndrome. |
Answer D | AnswerD::X-linked recessive |
Answer D Explanation | AnswerDExp::Myotonic dystrophy is not an X-linked recessive disorder. An example of an X-linked recessive disorder is Duchenne muscular dystrophy (DMD) |
Answer E | AnswerE::Mitochondrial inheritance |
Answer E Explanation | AnswerEExp::Myotonic dystrophy is not a mitochondrial disorder. An example of a mitochondrial disorder is Leber’s hereditary optic neuropathy. |
Right Answer | RightAnswer::B |
Explanation | [[Explanation::Myotonic dystrophy subtype 1 (DM1 or MMD1 or Steinert's disease) is a autosomal dominant genetic disorder characterized by progressive distal muscle weakness of the upper and lower extremities. Patients often complain of foot drop or inability to perform physical activities that require intricate use of the hands (use tools or doorknobs). In addition, patients with myotonic dystrophy have frontal balding and unique facial features (temporal wasting and hatchet-appearance) caused by the wasting of the facial muscles and also resulting in ptosis and dysarthria. Patients may exhibit a "warm-up phenomenon", characterized by the improvement in strength of the handgrip myotonia upon repeated contractions (e.g. inability to release hand from doorknob following contraction). On physical examination, percussion myotonia may also be evaluated by percussion of the thenar eminence using a tendon hammer. Patients with myotonic dystophy are at-risk of several complications, including subcapsular cataracts, which eventually develop in almost all patients, cardiac conduction abnormalities (potentially fatal tachyarrhythmias), aspiration pneumonia and diaphragmatic weakness, endocrinopathies, intellectual deficits, cholecystitis and decreased esophageal peristalsis (increased tonicity of gallbladder sphincter and esophageal muscles, respectively), slow gastric emptying, constipation (may be due to pseudo-obstruction), or diarrhea and fecal incontinence.
Myotonic dystophy has 2 subtypes:
Educational Objective: Myotonic dystrophy subtype 1 (DM1 or MMD1) is a autosomal dominant genetic disorder characterized by progressive distal muscle weakness of the upper and lower extremities. Patients often complain of foot drop or inability to perform physical activities that require intricate use of the hands (use tools or doorknobs). |
Approved | Approved::Yes |
Keyword | WBRKeyword::Myotonic dystrophy, WBRKeyword::Genetics, WBRKeyword::Trinucleotide repeat disorders, WBRKeyword::CTG repeats, WBRKeyword::Steinert's disease, WBRKeyword::DM1, WBRKeyword::MMD1, WBRKeyword::Autosomal dominant, WBRKeyword::Temporal wasting, WBRKeyword::Hatchet appearance, WBRKeyword::Myotonic handgrip, WBRKeyword::Myotonic percussion |
Linked Question | Linked:: |
Order in Linked Questions | LinkedOrder:: |