WBR0143
| 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 disorder 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. |
| 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 |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |