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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor={{Rim}} (Reviewed by {{YD}})
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|MainCategory=Biochemistry, Genetics
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|Prompt=A 28-year-old female is having a genetic counseling for metabolic disorders before her pregnancy. She is very worried that her future baby will carry the “bad gene” that her husband’s family has. Her husband’s brother has died when he was 7 due to coma resulting from an elevated level of ammonia in his blood. After reviewing the records, the lady is informed that her husband’s brother had the most common form of urea cycle disorder. What is the mode of inheritance of this disease?
|Prompt=A young couple are referred for genetic counseling. The couple are planning to conceive a child, and they are very concerned about the health of their child. The man's brother died at the age of 7 years due to coma that was caused by high serum concentration of ammonia. Following a thorough revision of the family records, the couple are informed that the man's brother suffered from the most common form of urea cycle disorder. What is the mode of inheritance of the disease the caused the death of this man's brother?
|Explanation=[[OTC|Ornithine transcarmylase]] (OTC) deficiency is the most common form of urea metabolism disorders.  The [[urea cycle]] is composed of five enzymes, the deficiency in any of which might lead to a urea metabolism disorder. OTC deficiency is X-linked, whereas other urea metabolism disorders are autosomal recessive.  Shown below is a diagram depicting the urea cycle.
|Explanation=The [[urea cycle]] is composed of five enzymes, and deficiency of any of these enzymes results in a urea metabolism disorder. [[OTC|Ornithine transcarmylase]] (OTC) deficiency is the most common disorder of urea metabolism. It is characterized by hyperammonemia and an increased concentration of blood and urine orotic acid concentrations. In converse to other urea metabolism disorders (autosomal recessive pattern of inheritance), OTC deficiency is commonly inherited in an X-linked recessive pattern.
 
[[File:Urea Cycle Diagram.png|500px]]
 
'''Educational objective:'''
OTC deficiency is X-linked, whereas other urea metabolism disorders are autosomal recessive.
 
'''Reference:'''
First aid for USMLE step 1, 2013. Page 107.
|AnswerA=Autosomal recessive
|AnswerA=Autosomal recessive
|AnswerAExp=OTC deficiency is X-linked, whereas other urea metabolism disorders are autosomal recessive.
|AnswerAExp=OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
|AnswerB=Autosomal dominant
|AnswerB=Autosomal dominant
|AnswerBExp=OTC deficiency is X-linked, whereas other urea metabolism disorders are autosomal recessive.
|AnswerBExp=While OTC deficiency is usually inherited in an X-linked pattern, other urea metabolism disorders are usually inherited in an autosomal recessive pattern.
|AnswerC=X-linked recessive
|AnswerC=X-linked recessive
|AnswerCExp=OTC deficiency is X-linked, whereas other urea metabolism disorders are autosomal recessive.
|AnswerCExp=OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
|AnswerD=X-linked dominant
|AnswerD=X-linked dominant
|AnswerDExp=OTC deficiency is X-linked, whereas other urea metabolism disorders are autosomal recessive.
|AnswerDExp=OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
|AnswerE=Y-linked
|AnswerE=Y-linked
|AnswerEExp=OTC deficiency is X-linked, whereas other urea metabolism disorders are autosomal recessive.
|AnswerEExp=OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
|EducationalObjectives=OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
|References=First Aid 2014 page 109
|RightAnswer=C
|RightAnswer=C
|WBRKeyword=OTC, Urea cycle,
|WBRKeyword=OTC, Urea cycle, OTC deficiency, Ornithine transcarbamylase deficiency, Hyperammonemia, Orotic acid, X-linked recessive, X-linked, Mode of inheritance, Recessive
|Approved=No
|Approved=Yes
}}
}}

Revision as of 19:44, 10 March 2015
Author [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biochemistry
Sub Category SubCategory::General Principles
Prompt [[Prompt::A young couple are referred for genetic counseling. The couple are planning to conceive a child, and they are very concerned about the health of their child. The man's brother died at the age of 7 years due to coma that was caused by high serum concentration of ammonia. Following a thorough revision of the family records, the couple are informed that the man's brother suffered from the most common form of urea cycle disorder. What is the mode of inheritance of the disease the caused the death of this man's brother?]]
Answer A AnswerA::Autosomal recessive
Answer A Explanation AnswerAExp::OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
Answer B AnswerB::Autosomal dominant
Answer B Explanation AnswerBExp::While OTC deficiency is usually inherited in an X-linked pattern, other urea metabolism disorders are usually inherited in an autosomal recessive pattern.
Answer C AnswerC::X-linked recessive
Answer C Explanation AnswerCExp::OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
Answer D AnswerD::X-linked dominant
Answer D Explanation AnswerDExp::OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
Answer E AnswerE::Y-linked
Answer E Explanation AnswerEExp::OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
Right Answer RightAnswer::C
Explanation [[Explanation::The urea cycle is composed of five enzymes, and deficiency of any of these enzymes results in a urea metabolism disorder. Ornithine transcarmylase (OTC) deficiency is the most common disorder of urea metabolism. It is characterized by hyperammonemia and an increased concentration of blood and urine orotic acid concentrations. In converse to other urea metabolism disorders (autosomal recessive pattern of inheritance), OTC deficiency is commonly inherited in an X-linked recessive pattern.

Educational Objective: OTC deficiency is a genetic disorder that is usually inherited in an X-linked recessive pattern.
References: First Aid 2014 page 109]]

Approved Approved::Yes
Keyword WBRKeyword::OTC, WBRKeyword::Urea cycle, WBRKeyword::OTC deficiency, WBRKeyword::Ornithine transcarbamylase deficiency, WBRKeyword::Hyperammonemia, WBRKeyword::Orotic acid, WBRKeyword::X-linked recessive, WBRKeyword::X-linked, WBRKeyword::Mode of inheritance, WBRKeyword::Recessive
Linked Question Linked::
Order in Linked Questions LinkedOrder::
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