WBR0144

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::Musculoskeletal/Rheumatology
Prompt	[[Prompt::A 37-year-old male presented to the emergency department for severe sudden chest pain that is tearing in nature and radiating to the back. The patient is tall, has abnormal joints laxity and long extremities and limbs. A medical student is suggesting that this patient has Marfan syndrome, whereas the resident in the ER is arguing that these features of long extremities and joint laxity could be found in other inherited conditions. Which of the following can explain the resident’s argument?]]
Answer A	AnswerA::Mosaicism
Answer A Explanation	[[AnswerAExp::Mosaicism denotes the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg. Different types of mosaicism exist, such as gonadal mosaicism (restricted to the gametes) or tissue or somatic mosaicism.]]
Answer B	AnswerB::Heteroplasmy
Answer B Explanation	[[AnswerBExp::Heteroplasmy is the presence of a mixture of more than one type of an organellar genome (mitochondrial DNA or plastid DNA) within a cell or individual. It is a factor for the severity of mitochondrial diseases.]]
Answer C	AnswerC::Pleiotropy
Answer C Explanation	AnswerCExp::Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously.
Answer D	AnswerD::Locus heterogeneity
Answer D Explanation	[[AnswerDExp::Locus heterogeneity is a single disorder, trait, or pattern of traits caused by mutations in genes at different chromosomal loci. This patient’s marfanoid habitus can be caused by Marfan syndrome, homocysteinuria or MEN 2 B syndrome.]]
Answer E	AnswerE::Variable expression
Answer E Explanation	AnswerEExp::Variable expression occurs when a phenotype is expressed to a different degree among individuals with the same genotype.
Right Answer	RightAnswer::D
Explanation	[[Explanation::Patient with marfanoid habitus are usually tall, have abnormal joints laxity and long extremities and limbs. Marfanoid habitus can be caused by Marfan syndrome, homocysteinuria or MEN 2 B syndrome. This is an example of locus heterogeneity. Educational objective: Locus heterogeneity is a single disorder, trait, or pattern of traits caused by mutations in genes at different chromosomal loci. Marfanoid habitus is an example of locus heterogeneity. Educational Objective: References: ]]
Approved	Approved::No
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