WBR0974

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A previously healthy 6-year-old boy is brought by his mother for lethargy and hallucinations. The mother reports that her son has always been healthy until few days ago when he had flu symptoms for which he received supportive care. Since then he has been sleepy and vomited several times. Today, the mother heard him hallucinating about tigers and lions present in his bedroom. The laboratory tests results revealed an elevated ammonia level, normal anion gap and normal glucose level. A urea cycle disorder is suspected and more extensive laboratory tests and genetic studies are ordered. Shown below is an image depicting the urea cycle. Which of the following steps of the urea cycle is directly responsible for the release of the urea molecules?]]
Answer A	AnswerA::A
Answer A Explanation	[[AnswerAExp::A corresponds to the enzyme carbamoyl phosphate synthetase I which catalyzes the synthesis of carbamoyl phosphate from ammonia and carbon dioxide.]]
Answer B	AnswerB::B
Answer B Explanation	[[AnswerBExp::B corresponds to ornithine transcarbamoylase which catalyzes the formation of citrulline from ornithine and carbamoyl phosphate.]]
Answer C	AnswerC::C
Answer C Explanation	AnswerCExp::C corresponds to arginosuccinate synthetase which catalyzes the conversion of citrulline into arginosuccinate.
Answer D	AnswerD::D
Answer D Explanation	[[AnswerDExp::D corresponds to arginosuccinase which catalyzes the conversion of arginosuccinate to arginine.]]
Answer E	AnswerE::E
Answer E Explanation	[[AnswerEExp::E corresponds to arginase which is responsible for the formation of urea and ornithine as a result of the hydrolysis of arginine.]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::Urea cycle disorders are a spectrum of metabolic disorders ranging from very severe to mild. They can be present at birth or later in life depending on the complete or partial deficiency of a particular enzyme in the urea cycle. Episodes of hyperammonia can be triggered by stress or infections as seen in this boy. Symptoms of hyperammonia include nausea, vomiting, lethargy, seizures, hallucinations, somnolence and even coma. The urea cycle involves a set of 5 enzymes that help in the conversion of ammonia into urea which is excreted in the urine. A part of the reactions in the urea cycle occurs in the mitochondria whereas the remaining reactions occur in the cytoplasm of the hepatocytes. The mitochondrial enzymes are: carbamoyl phosphate synthetase I and ornitithine transcarbamoylase (OTC). The cytoplasmic enzymes are: arginosuccinate synthetase, arginosuccinase and arginase. Urea is released as a result of the conversion of arginine into ornithine by the enzyme arginase. Shown below is a diagram depicting the different steps of the urea cycle. Education objective: Urea is released as a result of the conversion of arginine into ornithine by the enzyme arginase. Reference: First Aid for the USMLE step 1. Page 106. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::Urea cycle, WBRKeyword::urea, WBRKeyword::arginase
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::