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. |
| 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:: |