WBR0976

Author	[[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry, MainCategory::Genetics
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::