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. 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 documented decreased cognition function. 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::GGG |
| 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. References: First Aid 2012 page 92 |
| 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:: |