WBR0303
| Author | PageAuthor::William J Gibson |
|---|---|
| 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 notes that he can no longer hold his body weight on the pull-up bar at the gym. The patient also reports feeling substantial neck soreness at the end of the work day after looking at his computer monitor for hours. 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. |
| Answer E | AnswerE::Mitochondrial |
| 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 is an autosomal dominant 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 unique pattern of frontal balding and facial features (hatchet-appearance) caused by the wasting of the facial muscles and 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 (inability to release hand from doorknob). 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, 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.
and hypotonia eventually leading to cardiopulmonary involvement and death in the majority of patients by the age of 65 [1]. Early signs of the disease include loss of grip strength (indicated by the patient’s difficulty with the pullup bar), and weakness in the neck, feet and hands. Speech and swallowing may become difficult for patient’s due to loss of muscle tone in the tongue and the esophagus. Myotonic dystrophy is caused by expansion of a CTG triplet repeat in the DMPK gene*. Like other triplet repeat diseases such as Huntington’s, myotonic dystrophy can cause earlier and more severe symptoms in successive generations due to anticipation. A useful clinical clue for diagnosis is the failure of spontaneous letting go of the hands following strong handshakes due to myotonia (delayed relaxation of muscles after contraction) which accompanies muscle weakness.There is currently no cure for or treatment specific to myotonic dystrophy.
Educational Objective: Myotonic dystrophy is an autosomal dominant disorder. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Myotonic dystrophy, WBRKeyword::Genetics, WBRKeyword::Trinucleotide repeat disorders |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |