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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}} (Edited by Will Gibson)
|QuestionAuthor={{Rim}} (Reviewed by Will Gibson and {{YD}})
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Genetics
|MainCategory=Genetics
Line 21: Line 21:
|MainCategory=Genetics
|MainCategory=Genetics
|SubCategory=General Principles
|SubCategory=General Principles
|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?
|Prompt=A 17-year-old adolescent male is brought by his parents to a new physician for an annual check-up. The patient’s face and jaw appear to be larger than normal. The patient is very slow to answer the physician's questions and sounds younger than his age. A review of his past medical history reveals cognitive assessments consistent with mental retardation along with delayed language development. The physician suspects the patient might have a genetic disease. The patient’s condition is most likely caused by the presence of trinucleotide repeats of which of the following?
|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.
|Explanation=Fragile X syndrome is an X-linked genetic disorder that affects boys. It is caused by a trinucleotide expansion of CGG repeats that affect the methylation and expression of the ''FMR1'' gene on the X chromosome. Expansion of CGG repeats beyond a certain threshold causes silencing of the ''FMR1'' gene thereby leading to the manifestation of the disease. It is the second most common cause of intellectual disability following Down syndrome. Fragile X syndrome is characterized by hypotonia and delayed language development and motor milestones during early childhood. Also, patients have characteristic hand-flapping movements with poor eye contact. Behavioral characteristics suggestive of fragile X syndrome include ADHD-like features, impulsivity, repetitive movements, and easy irritability, which makes the autism spectrum disoder a differential diagnosis of fragile X syndrome. Additionally, boys with the syndrome may have post-pubertal [[macroorchidism]] (enlarged testes), [[prognathism]], and a characteristic but variable face with large everted ears, long face, high-arched [[palate]], [[gynecomastia]], and teeth [[malocclusion]]. Additional abnormalities may include [[lordosis]], heart defects such as mitral valve prolapse, [[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.
 
[[File:202px-Fragile_x_syndrom.png|center|200px]]
 
Example of facial characteristics of patient with Fragile X.
|AnswerA=CAG
|AnswerA=CAG
|AnswerAExp=CAG repeats are seen in [[Huntington’s disease]]. '''Wiki-mnemomic:''' You hunt animals and put them in the '''CAG'''e: Huntington Disease '''CAG'''
|AnswerAExp=CAG repeats are observed among patients with [[Huntington’s disease]].
|AnswerB=GAA
|AnswerB=GAA
|AnswerBExp=GAA repeats are seen in [[Friedreich's ataxia]].
|AnswerBExp=GAA repeats are observed among patients with [[Friedreich's ataxia]].
|AnswerC=GGA
|AnswerC=GGA
|AnswerCExp=GGA repeats are not specific to a disease.
|AnswerCExp=GGA repeats are not specific to a disease.
|AnswerD=CTG
|AnswerD=CTG
|AnswerDExp=CTG repeats are seen in [[myotonic dystrophy]].
|AnswerDExp=CTG repeats are observed among patients with [[myotonic dystrophy]].
|AnswerE=CGG
|AnswerE=CGG
|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.
|AnswerEExp=CGG repeats are observed among patients with [[Fragile X syndrome]]. The patient is suffering from [[mental retardation]] with signs and symptoms consistent with Fragile X syndrome.
|EducationalObjectives=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.
|EducationalObjectives=Fragile X syndrome is caused by expansion of CGG repeats the affect the methylation and expression of the ''FMR1'' gene on the X chromosome.
|References=First Aid 2014 page 89
|References=Bagni C, Tassone F, Neri G, et al. Fragile X syndrome: causes, diagnosis, mechanisms, and therapeutics. J Clin Invest. 2012;122(12):4314:4322.<br>
First Aid 2014 page 89
|RightAnswer=E
|RightAnswer=E
|WBRKeyword=Mental retardation, Retardation, Repeat disorder, Nucleotide, Nucleotide repeat
|WBRKeyword=Mental retardation, Retardation, Repeat disorder, Nucleotide, Nucleotide repeat, Fragile X syndrome, Fragile X, Intellectual disability, CGG,
|Approved=Yes
|Approved=Yes
}}
}}

Revision as of 15:36, 9 October 2014

 
Author [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Will Gibson and Yazan Daaboul, M.D.)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Genetics
Sub Category SubCategory::General Principles
Prompt [[Prompt::A 17-year-old adolescent male is brought by his parents to a new physician for an annual check-up. The patient’s face and jaw appear to be larger than normal. The patient is very slow to answer the physician's questions and sounds younger than his age. A review of his past medical history reveals cognitive assessments consistent with mental retardation along with delayed language development. The physician suspects the patient might have a genetic disease. The patient’s condition is most likely caused by the presence of trinucleotide repeats of which of the following?]]
Answer A AnswerA::CAG
Answer A Explanation [[AnswerAExp::CAG repeats are observed among patients with Huntington’s disease.]]
Answer B AnswerB::GAA
Answer B Explanation [[AnswerBExp::GAA repeats are observed among patients with 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 observed among patients with myotonic dystrophy.]]
Answer E AnswerE::CGG
Answer E Explanation [[AnswerEExp::CGG repeats are observed among patients with Fragile X syndrome. The patient is suffering from mental retardation with signs and symptoms consistent with Fragile X syndrome.]]
Right Answer RightAnswer::E
Explanation [[Explanation::Fragile X syndrome is an X-linked genetic disorder that affects boys. It is caused by a trinucleotide expansion of CGG repeats that affect the methylation and expression of the FMR1 gene on the X chromosome. Expansion of CGG repeats beyond a certain threshold causes silencing of the FMR1 gene thereby leading to the manifestation of the disease. It is the second most common cause of intellectual disability following Down syndrome. Fragile X syndrome is characterized by hypotonia and delayed language development and motor milestones during early childhood. Also, patients have characteristic hand-flapping movements with poor eye contact. Behavioral characteristics suggestive of fragile X syndrome include ADHD-like features, impulsivity, repetitive movements, and easy irritability, which makes the autism spectrum disoder a differential diagnosis of fragile X syndrome. Additionally, boys with the syndrome may have post-pubertal macroorchidism (enlarged testes), prognathism, and a characteristic but variable face with large everted ears, long face, high-arched palate, gynecomastia, and teeth malocclusion. Additional abnormalities may include lordosis, heart defects such as mitral valve prolapse, pectus excavatum, flat feet, shortening of the tubular bones of the hands, and joint laxity.

Educational Objective: Fragile X syndrome is caused by expansion of CGG repeats the affect the methylation and expression of the FMR1 gene on the X chromosome.
References: Bagni C, Tassone F, Neri G, et al. Fragile X syndrome: causes, diagnosis, mechanisms, and therapeutics. J Clin Invest. 2012;122(12):4314:4322.
First Aid 2014 page 89]]

Approved Approved::Yes
Keyword WBRKeyword::Mental retardation, WBRKeyword::Retardation, WBRKeyword::Repeat disorder, WBRKeyword::Nucleotide, WBRKeyword::Nucleotide repeat, WBRKeyword::Fragile X syndrome, WBRKeyword::Fragile X, WBRKeyword::Intellectual disability, WBRKeyword::CGG
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Order in Linked Questions LinkedOrder::