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 male presents to his primary care physician complaining of muscle weakness. The patient used to partake in vigorous exercise, but over the past year has noted a gradual decline in his strength. In particular he notes that he can no longer hold his bodyweight on the pullup 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. During the interview, the physician notes the patient has mildly dysarthric speech. On neurologic exam, the patient was unable to reverse the contraction of his thumb following percussion of the thenar eminence. What is the mode of inheritance of the most likely disorder?]] |
| Answer A | AnswerA::Autosomal recessive |
| Answer A Explanation | AnswerAExp::'''Incorrect:''' 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::'''Correct:''' Myotonic dystrophy is an autosomal dominant disorder. |
| Answer C | AnswerC::X-linked dominant |
| Answer C Explanation | AnswerCExp::'''Incorrect:''' 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::'''Incorrect:''' 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::'''Incorrect:''' 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::The patient in this vignette is suffering from myotonic dystrophy. Myotonic dystrophy is an autosomal dominant disorder characterized by progressive muscle weakness and hypotonia eventually leading to cardiopulmonary involvement and death in the majority of patients by the age of 65 (Brain,1998[1]). 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. References: First Aid 2012 page 92 |
| Approved | Approved::Yes |
| Keyword | |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |