WBR0972: Difference between revisions
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|MainCategory=Biochemistry | |MainCategory=Biochemistry | ||
|SubCategory=General Principles | |SubCategory=General Principles | ||
|Prompt=A newborn | |Prompt=A 35-year-old woman complains that her newborn is feeding poorly. The newborn was born one day ago by an uneventful vaginal delivery. His mother received all her prenatal vitamins during pregnancy and followed up with her obstetrician regularly. On physical examination, the newborn is hypothermic and appears lethargic. Further investigation demonstrates an elevated concentration of serum ammonia and deficiency in N-acetylglutamate synthase activity. The physician explains to the newborn's mother that his condition is caused by abnormal functioning of an enzyme of the urea cycle in hepatocytes. Which of the following reactions of the urea cycle is directly involved in this patient's condition?<br> | ||
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|Explanation=The urea cycle involves a set of 5 hepatic enzymes that collectively convert ammonia into urea. Two of the reactions involved in urea synthesis occur within the [[mitochondria]], whereas the remaining reactions occur in the [[cytoplasm]].<br> | |Explanation=The urea cycle involves a set of 5 hepatic enzymes that collectively convert ammonia into urea. Two of the reactions involved in urea synthesis occur within the [[mitochondria]], whereas the remaining reactions occur in the [[cytoplasm]].<br> | ||
*The [[mitochondria|mitochondrial]] enzymes are carbamoyl phosphate synthetase I (requires ATP) and [[OTC|ornithine transcarbamylase]] (OTC)<br> | *The [[mitochondria|mitochondrial]] enzymes are carbamoyl phosphate synthetase I (requires ATP) and [[OTC|ornithine transcarbamylase]] (OTC)<br> | ||
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|RightAnswer=A | |RightAnswer=A | ||
|WBRKeyword=Urea cycle, Carbamoyl phosphate synthetase I, N acetylglutamate, Urea synthesis, Ammonia, Rate limiting step, Reaction, Enzyme | |WBRKeyword=Urea cycle, Carbamoyl phosphate synthetase I, N acetylglutamate, Urea synthesis, Ammonia, Rate limiting step, Reaction, Enzyme | ||
|Approved= | |Approved=Yes | ||
}} | }} |
Revision as of 16:54, 10 March 2015
Author | [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Yazan Daaboul, M.D.)]] |
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Exam Type | ExamType::USMLE Step 1 |
Main Category | MainCategory::Biochemistry |
Sub Category | SubCategory::General Principles |
Prompt | [[Prompt::A 35-year-old woman complains that her newborn is feeding poorly. The newborn was born one day ago by an uneventful vaginal delivery. His mother received all her prenatal vitamins during pregnancy and followed up with her obstetrician regularly. On physical examination, the newborn is hypothermic and appears lethargic. Further investigation demonstrates an elevated concentration of serum ammonia and deficiency in N-acetylglutamate synthase activity. The physician explains to the newborn's mother that his condition is caused by abnormal functioning of an enzyme of the urea cycle in hepatocytes. Which of the following reactions of the urea cycle is directly involved in this patient's condition? |
Answer A | AnswerA::A |
Answer A Explanation | AnswerAExp::A corresponds to the enzyme carbamoyl phosphate synthetase I, which is the rate-limitng step of the urea cycle. Carbamoyl phosphate synthetase I activation requires ATP and the cofactor N-acetylglutamate. |
Answer B | AnswerB::B |
Answer B Explanation | [[AnswerBExp::B corresponds to ornithine transcarbamylase, which catalyzes the reaction that produces citrulline.]] |
Answer C | AnswerC::C |
Answer C Explanation | AnswerCExp::C corresponds to argininosuccinate synthetase, which is an ATP-requiring enzyme that catalyzes the reaction that produces argininosuccinate. |
Answer D | AnswerD::D |
Answer D Explanation | AnswerDExp::D corresponds to arginosuccinase, which catalyzes the reaction that produces arginine and fumarate. |
Answer E | AnswerE::E |
Answer E Explanation | [[AnswerEExp::E corresponds to arginase, which catalyzes the reaction that produces ornithine.]] |
Right Answer | RightAnswer::A |
Explanation | [[Explanation::The urea cycle involves a set of 5 hepatic enzymes that collectively convert ammonia into urea. Two of the reactions involved in urea synthesis occur within the mitochondria, whereas the remaining reactions occur in the cytoplasm.
The rate limiting step in the urea cycle is the reaction catalyzed by mitochondrial enzyme carbamoyl phosphate synthetase I that produces carbamoyl phosphate from carbon dioxide and ammonia. is the conversion of ammonia and carbon dioxide into carbamoyl phosphate. The enzyme of this rate limiting step requires ATP and the cofactor N-acetylglutamate. N-acetylglutamate synthetase deficiency in this newborn results in a reduced activation of carbamoyl phosphate synthetase I enzyme and consequently, a reduced concentration of carbamoyl phosphate. Shown below is an illustration of the reactions of the urea cycle, all of which occur in hepatocytes in the liver |
Approved | Approved::Yes |
Keyword | WBRKeyword::Urea cycle, WBRKeyword::Carbamoyl phosphate synthetase I, WBRKeyword::N acetylglutamate, WBRKeyword::Urea synthesis, WBRKeyword::Ammonia, WBRKeyword::Rate limiting step, WBRKeyword::Reaction, WBRKeyword::Enzyme |
Linked Question | Linked:: |
Order in Linked Questions | LinkedOrder:: |