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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor={{Rim}} (Edited by Will Gibson)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Genetics
|MainCategory=Genetics
Line 20: Line 20:
|MainCategory=Genetics
|MainCategory=Genetics
|SubCategory=Musculoskeletal/Rheumatology
|SubCategory=Musculoskeletal/Rheumatology
|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?
|Prompt=A 37-year-old male presents to the emergency department for severe sudden "tearing" chest pain that radiates to the back.  The patient is tall, has abnormal joints laxity and long extremities and limbs.  A medical student suggests that this patient has Marfan syndrome, whereas the resident in the ER argues that long extremities and joint laxity are features of other inherited conditions.  Which of the following can explain the resident’s argument?
|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 [[Multiple endocrine neoplasia type 2|MEN 2 B syndrome]].  This is an example of locus heterogeneity.
|Explanation=The patient in this vignette has most likely suffered aortic dissection due to Marfan syndrome.  Patients 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 [[Multiple endocrine neoplasia type 2|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:''' 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.
 
'''References:''' First Aid 2012 page 87
|AnswerA=Mosaicism
|AnswerA=Mosaicism
|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]].  
|AnswerAExp='''Incorrect:''' [[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.
Different types of [[mosaicism]] exist, such as gonadal mosaicism (restricted to the [[gametes]]) or tissue or somatic mosaicism.
|AnswerB=Heteroplasmy
|AnswerB=Heteroplasmy
|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 disease]]s.
|AnswerBExp='''Incorrect:''' [[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 disease]]s.
|AnswerC=Pleiotropy
|AnswerC=Pleiotropy
|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.
|AnswerCExp='''Incorrect:''' 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.  While the Fibrillin mutations of Marfan syndrome are illustrations of pleiotropy, pleiotropy is not the most accurate descriptor of the resident's argument.
|AnswerD=Locus heterogeneity
|AnswerD=Locus heterogeneity
|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 [[Multiple endocrine neoplasia type 2|MEN 2 B syndrome]].
|AnswerDExp='''Correct:''' 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 [[Multiple endocrine neoplasia type 2|MEN 2 B syndrome]].
|AnswerE=Variable expression
|AnswerE=Variable expression
|AnswerEExp=Variable expression occurs when a phenotype is expressed to a different degree among individuals with the same genotype.
|AnswerEExp='''Incorrect:''' Variable expression occurs when a phenotype is expressed to a different degree among individuals with the same genotype.
|RightAnswer=D
|RightAnswer=D
|Approved=No
|WBRKeyword=Marfan, Marfan syndrome, Marfan's syndrome, Pleiotropy,
|Approved=Yes
}}
}}

Revision as of 01:32, 2 October 2013
Author [[PageAuthor::Rim Halaby, M.D. [1] (Edited by Will Gibson)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Genetics
Sub Category SubCategory::Musculoskeletal/Rheumatology
Prompt [[Prompt::A 37-year-old male presents to the emergency department for severe sudden "tearing" chest pain that radiates to the back. The patient is tall, has abnormal joints laxity and long extremities and limbs. A medical student suggests that this patient has Marfan syndrome, whereas the resident in the ER argues that long extremities and joint laxity are features of other inherited conditions. Which of the following can explain the resident’s argument?]]
Answer A AnswerA::Mosaicism
Answer A Explanation [[AnswerAExp::Incorrect: 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::Incorrect: 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::Incorrect: 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. While the Fibrillin mutations of Marfan syndrome are illustrations of pleiotropy, pleiotropy is not the most accurate descriptor of the resident's argument.]]
Answer D AnswerD::Locus heterogeneity
Answer D Explanation [[AnswerDExp::Correct: 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::'''Incorrect:''' Variable expression occurs when a phenotype is expressed to a different degree among individuals with the same genotype.
Right Answer RightAnswer::D
Explanation [[Explanation::The patient in this vignette has most likely suffered aortic dissection due to Marfan syndrome. Patients 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.

References: First Aid 2012 page 87
Educational Objective:
References: ]]

Approved Approved::Yes
Keyword WBRKeyword::Marfan, WBRKeyword::Marfan syndrome, WBRKeyword::Marfan's syndrome, WBRKeyword::Pleiotropy
Linked Question Linked::
Order in Linked Questions LinkedOrder::
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