WBR0310

Author	PageAuthor::William J Gibson
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry, MainCategory::Genetics
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A 25 year old girl presents to her primary care physician complaining of generalized weakness. She reports that the weakness is particularly pronounced in her hands, feet and neck. Furthermore, she reports some difficulty swallowing the multivitamins she takes daily. On further questioning, she admits to greater than normal daytime sleepiness. Past medical history significant is only for occasional gastrointestinal distress over the past 5 years. The young woman reports 3 sexual partners in the last six months. The physician notes that the patient was unable to release her grip after shaking hands. Which of the following techniques would allow a pathologist to confirm the diagnosis from a sample of the patient’s DNA?]]
Answer A	[[AnswerA::Northern blot; 5’(CAG)n3’ probe]]
Answer A Explanation	AnswerAExp::Incorrect: Norther blots are used to detect RNA, not DNA. The patient’s RNA is subjected to electrophoresis and probed with a labeled DNA probe.
Answer B	[[AnswerB::Northern blot; 5’(CTG)n3’ probe]]
Answer B Explanation	AnswerBExp::Incorrect: Norther blots are used to detect RNA, not DNA. The patient’s RNA is subjected to electrophoresis and probed with a labeled DNA probe.
Answer C	[[AnswerC::Southern blot; 5’(CAG)n3’ probe]]
Answer C Explanation	AnswerCExp::Correct: Southern blots are used to detect the presence of certain sequences of DNA. The probe 5’(CAG) n 3’ is the reverse complement of the 5‘CTG 3’ repeat.
Answer D	[[AnswerD::Southern blot; 5’(CTG)n3’ probe]]
Answer D Explanation	AnswerDExp::Incorrect: While 5’ CTG 3’ is the repeat observed in myotonic dystrophy, the reverse complement of 5’ CTG 3’ is 5’ CAG 3’.
Answer E	[[AnswerE::Southern blot; 5’(CAA)n3’ probe]]
Answer E Explanation	AnswerEExp::Incorrect : While Sounther blots are used to detect DNA, 5’ CAA 3’ is the repeat responsible for Friedrich’s ataxia.
Right Answer	RightAnswer::C
Explanation	[[Explanation:: Educational Objective: References: ]]
Approved	Approved::Yes
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