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 [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. A second form of myotonic dystrophy is caused by expansion of a CCTG repeat in the ZNF9 gene. This is not tested on the USMLE. Educational Objective: Myotonic dystrophy is an autosomal dominant disorder. References: First Aid 2012 page 92 Educational Objective: References: ]]
Approved	Approved::Yes
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