WBR0143

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Edited by Will Gibson)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A 17-year-old male is brought by his parents for an annual checkup with a new physician. The patient’s face and jaw appear to be larger than normal. The patient is slow in answering the physician's questions and sounds younger than his age. A review of his past medical history reveals cognitive assessments consistent with mental retardation. The patient’s condition is most likely caused by trinucleotide repeats of which of the following?]]
Answer A	AnswerA::CAG
Answer A Explanation	[[AnswerAExp::CAG repeats are seen in Huntington’s disease. Wiki-mnemomic: You hunt animals and put them in the CAGe: Huntington Disease CAG]]
Answer B	AnswerB::GAA
Answer B Explanation	[[AnswerBExp::GAA repeats are seen in Friedreich's ataxia.]]
Answer C	AnswerC::GGA
Answer C Explanation	AnswerCExp::GGA repeats are not specific to a disease.
Answer D	AnswerD::CTG
Answer D Explanation	[[AnswerDExp::CTG repeats are seen in myotonic dystrophy.]]
Answer E	AnswerE::CGG
Answer E Explanation	[[AnswerEExp::CGG repeats are seen in Fragile X syndrome. The patient is suffering from mental retardation and enlarged face and jaw that are consistent with Fragile X syndrome.]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::The patient is suffering from mental retardation and enlarged face and jaw that are consistent with Fragile X syndrome. Fragile X syndrome is a syndrome of X-linked mental retardation. Boys with the syndrome may have large testicles (macroorchidism), prognathism, hypotonia and autism, and a characteristic but variable face with large ears, long face, high-arched palate, gynecomastia, and malocclusion. Additional abnormalities may include lordosis, heart defect, pectus excavatum, flat feet, shortening of the tubular bones of the hands, and joint laxity. Fragile X syndrome is caused by a trinucleotide expansion of CGG repeats in the FMR1 gene on the X chromosome. Expansion of CGG repeats beyond a certain threshold causes silencing of the FMR1 gene thereby leading to pathology. Example of facial characteristics of patient with Fragile X. Educational Objective: Fragile X syndrome is caused by expansion of CGG repeats in the FMR1 gene on the X Chromosome. It is the most common inherited cause of intellectual disability. References: First Aid 2014 page 89]]
Approved	Approved::Yes
Keyword	WBRKeyword::Mental retardation, WBRKeyword::Retardation, WBRKeyword::Repeat disorder, WBRKeyword::Nucleotide, WBRKeyword::Nucleotide repeat
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::