WBR1020

Author	[[PageAuthor::Ogheneochuko Ajari, MB.BS, MS [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	Prompt::A boy with Duchenne Muscular Dystrophy (DMD) was born to parents with no family history of the disease. The most likely mechanism for this occurrence would be as a result of?
Answer A	AnswerA::A CGG expansion that resulted in the disruption of the promoter of the dystrophin gene
Answer A Explanation	AnswerAExp::A CGG expansion results in Fragile X syndrome due to an X-linked defect affecting methylation and expression of the FMR1 gene. It is not associated with the dystrophin gene.
Answer B	AnswerB::A point mutation in the dystrophin gene
Answer B Explanation	AnswerBExp::DMD results from a frameshift mutation and not from a point mutation.
Answer C	AnswerC::A translocation that resulted in the disruption of the dystrophin gene
Answer C Explanation	AnswerCExp::A frameshift mutation, and not translocation causes a deletion in the dystrophin gene.
Answer D	AnswerD::A recombination event in the dystrophin gene that gave rise to a frameshift mutation
Answer D Explanation	AnswerDExp::Correct. See explanation.
Answer E	AnswerE::A CTG expansion that resulted in the disruption of the dystrophin gene
Answer E Explanation	AnswerEExp::A CTG expansion causes myotonic dystrophy and not associated with disruption of the dystrophin gene.
Right Answer	RightAnswer::D
Explanation	[[Explanation::Duchenne Muscular Dystrophy (DMD) is an X-linked frameshift mutation due to the deletion of the dystrophin gene. It leads to accelerated muscle breakdown with onset before the age of 5 years. A less severe form of the disease is called Becker’s muscular dystrophy, with onset in adolescence or early childhood. Reference: First AID for the USMLE Step 1 2013 pg 86 Educational Objective: References: ]]
Approved	Approved::Yes
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