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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor= {{Rim}} (Reviewed by Serge Korjian)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Biochemistry
|MainCategory=Biochemistry
Line 21: Line 21:
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=General Principles
|SubCategory=General Principles
|Prompt=Elevated levels of galactose are detected in a newborn screening test and prompt further laboratory tests to confirm the presence of a galactose metabolism disorder and determine its specific type. Enzyme assays monitor the accumulation of products of the galactose metabolic pathway following the addition of radiolabeled substrates. The enzyme assay confirmed the diagnosis of a galactose metabolism disorder that leads to mental retardation, failure to thrive and severe symptoms of jaundice and vomiting if galactose is not eliminated from the diet. Which of the following is the most likely result obtained by the enzyme assay?
|Prompt=Elevated levels of galactose are detected in a newborn screening test and prompt further investigation for a galactose metabolism disorder. An erythrocyte enzyme assay that monitors the accumulation of products of the galactose metabolic pathway following the addition of a radiolabeled substrate is carried out. The enzyme assay confirms the diagnosis of a galactose metabolism disorder that leads to mental retardation, failure to thrive, and jaundice if galactose is not eliminated from the diet. Which of the following is most likely the result of the enzyme assay in this patient?
|Explanation=The enzyme assay in this patient reveals the diagnosis of a [[galactose]] metabolism disorder that leads to mental retardation, failure to thrive and severe symptoms of jaundice and vomiting if galactose is not eliminated from the diet. The most likely diagnosis in this case is [[classic galactosemia]] which is a severe galactose metabolism disorder. Classic galactosemia is an autosomal recessive disease characterized by the absence of galactose 1 phosphate uridyltransferase (GALT) resulting in the accumulation of galactose 1 phosphate, galactose and galacticol. The enzyme assay will more likely reveal an absence of any detectable UDP <sup>14</sup>C galactose after the addition of <sup>14</sup>C galactose 1 phosphate.
|Explanation=This patient most likely suffers from classic galactosemia, a autosomal recessive disorder of galactose metabolism characterized by the absence of galactose-1-phosphate uridyltransferase (GALT) resulting in the accumulation of galactose-1-phosphate, galactose and galacticol. Classic galactosemia leads to severe manifestations early on in life if galactose is not eliminated from the diet. Patients present with failure to thrive and mental retardation. The radiolabeled enzyme assay will most likely demonstrate complete absence of <sup>14</sup>C UDP-galactose after the addition of <sup>14</sup>C galactose-1-phosphate. Infants in the United States are routinely screened for galactosemia as part of their newborn screening. Prenatal testing with chorionic villus sampling or amniocentesis is also possible. Despite early diagnosis and diet restriction a significant portion of patients have serious complications including neurological deficits and learning disabilities.


[[ File:GalactoseMetabolism.png]]
[[File:Galactose_metabolism_disorders.png|700px]]
|AnswerA=Absence of any detectable UDP <sup>14</sup>C galactose after the addition of <sup>14</sup>C galactose 1 phosphate
|AnswerA=Absence of <sup>14</sup>C UDP-galactose after the addition of <sup>14</sup>C galactose-1-phosphate
|AnswerAExp=[[Classic galactosemia]] is characterized by the absence of galactose 1 phosphate uridyltransferase (GALT) resulting in the accumulation of galactose 1 phosphate, [[galactose]] and [[galacticol]]. The enzyme assay will more likely reveal an absence of any detectable UDP <sup>14</sup>C galactose after the addition of <sup>14</sup>C galactose 1 phosphate.
|AnswerAExp=Classic galactosemia is characterized by the absence of galactose-1-phosphate uridyltransferase (GALT) resulting in the absence of detectable UDP <sup>14</sup>C galactose after the addition of <sup>14</sup>C galactose-1-phosphate as a substrate.
|AnswerB=Absence of any detectable <sup>14</sup>C galactose 1 phosphate after the addition of <sup>14</sup>C galactose
|AnswerB=Absence of <sup>14</sup>C galactose-1-phosphate after the addition of <sup>14</sup>C galactose
|AnswerBExp=Absence of any detectable <sup>14</sup>C galactose 1 phosphate after the addition of <sup>14</sup> C galactose will be obtained in the case of [[galactokinase deficiency]]The patient’s condition is inconsistent with the diagnosis of galactokinase deficiency because this condition is mild and is associated with the occurrence of [[cataratc]]s but not mental retardation, failure to thrive and severe symptoms of [[jaundice]] and vomiting.
|AnswerBExp=Absence of <sup>14</sup>C galactose-1-phosphate after the addition of <sup>14</sup> C galactose is observed in galactokinase deficiency.  Galactokinase deficiency is a relatively mild disorder associated with early-onset cataracts with no effect on early development.
|AnswerC=Accumulation of UDP <sup>14</sup>C galactose after the addition of <sup>14</sup>C galactose 1 phosphate
|AnswerC=Accumulation of <sup>14</sup>C UDP-galactose after the addition of <sup>14</sup>C galactose-1-phosphate
|AnswerCExp=Classic galactosemia characterized by the absence of galactose 1 phosphate uridyltransferase (GALT).  The enzyme assay will more likely reveal an absence , not accumulation, of any detectable UDP <sup>14</sup>C galactose after the addition of <sup>14</sup>C galactose 1 phosphate.
|AnswerCExp=Classic galactosemia characterized by the absence of galactose-1-phosphate uridyltransferase (GALT).  The enzyme assay would reveal an absence , not accumulation, of <sup>14</sup>C UDP-galactose after the addition of <sup>14</sup>C galactose-1-phosphate.
|AnswerD=Accumulation of <sup>14</sup>C galactose 1 phosphate after the addition of <sup>14</sup>C galactose
|AnswerD=Accumulation of <sup>14</sup>C UDP-galactose after the addition of <sup>14</sup>C galactose
|AnswerDExp=Classic galactosemia is characterized by the absence of galactose 1 phosphate uridyltransferase (GALT).  Accumulation of <sup>14</sup>C galactose 1 phosphate after the addition of <sup>14</sup>C galactose is not characteristic of the absence of this enzyme.
|AnswerDExp=Accumulation of <sup>14</sup>C UDP-galactose after the addition of <sup>14</sup>C galactose occurs in the absence of the UDP-glucose 4-epimerase (GALE) enzyme.
|AnswerE=Absence of any detectable UDP <sup>14</sup>C glucose 1 phosphate after the addition of <sup>14</sup>C UDP glucose
|AnswerE=Absence of <sup>14</sup>C glucose-1-phosphate or <sup>14</sup>C galactose-1-phosphate after the addition of <sup>14</sup>C lactose.
|AnswerEExp=Absence of any detectable UDP <sup>14</sup>C glucose 1 phosphate after the addition of <sup>14</sup>C UDP glucose can be present in the absence of the enzyme UDP glucose pyrophosphate and not in the absence of galactose 1 phosphate uridyltransferase (GALT).
|AnswerEExp=This enzyme assay finding is observed in patients with lactase deficiency, i.e. lactose intolerance.
|EducationalObjectives=Classic galactosemia is characterized by the absence of galactose 1 phosphate uridyltransferase (GALT) which is the enzyme needed to convert gaalctose 1 phosphate to UDP galactose.
|EducationalObjectives=Classic galactosemia is characterized by the absence of galactose-1-phosphate uridyltransferase (GALT) which is the enzyme needed to convert galactose-1-phosphate to UDP-galactose.
|References=Cuthbert C. Diagnosis of inherited disorders of galactose metabolism. Curr Protoc Hum Genet. 2008 Jan;Chapter 17:Unit 17.5.
|References=Cuthbert C. Diagnosis of inherited disorders of galactose metabolism. Curr Protoc Hum Genet. 2008 Jan;Chapter 17:Unit 17.5.
|RightAnswer=A
|RightAnswer=A
|WBRKeyword=Galactose, Classic galactosemia, Galactosemia
|WBRKeyword=Galactose, Classic galactosemia, Galactosemia, Galactokinase deficiency, Lactose, Lactase deficiency, Lactose intolerance, Enzyme, Enzyme assay
|Approved=Yes
|Approved=Yes
}}
}}

Latest revision as of 02:02, 28 October 2020

 
Author [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Serge Korjian)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biochemistry
Sub Category SubCategory::General Principles
Prompt [[Prompt::Elevated levels of galactose are detected in a newborn screening test and prompt further investigation for a galactose metabolism disorder. An erythrocyte enzyme assay that monitors the accumulation of products of the galactose metabolic pathway following the addition of a radiolabeled substrate is carried out. The enzyme assay confirms the diagnosis of a galactose metabolism disorder that leads to mental retardation, failure to thrive, and jaundice if galactose is not eliminated from the diet. Which of the following is most likely the result of the enzyme assay in this patient?]]
Answer A [[AnswerA::Absence of 14C UDP-galactose after the addition of 14C galactose-1-phosphate]]
Answer A Explanation [[AnswerAExp::Classic galactosemia is characterized by the absence of galactose-1-phosphate uridyltransferase (GALT) resulting in the absence of detectable UDP 14C galactose after the addition of 14C galactose-1-phosphate as a substrate.]]
Answer B [[AnswerB::Absence of 14C galactose-1-phosphate after the addition of 14C galactose]]
Answer B Explanation [[AnswerBExp::Absence of 14C galactose-1-phosphate after the addition of 14 C galactose is observed in galactokinase deficiency. Galactokinase deficiency is a relatively mild disorder associated with early-onset cataracts with no effect on early development.]]
Answer C [[AnswerC::Accumulation of 14C UDP-galactose after the addition of 14C galactose-1-phosphate]]
Answer C Explanation [[AnswerCExp::Classic galactosemia characterized by the absence of galactose-1-phosphate uridyltransferase (GALT). The enzyme assay would reveal an absence , not accumulation, of 14C UDP-galactose after the addition of 14C galactose-1-phosphate.]]
Answer D [[AnswerD::Accumulation of 14C UDP-galactose after the addition of 14C galactose]]
Answer D Explanation [[AnswerDExp::Accumulation of 14C UDP-galactose after the addition of 14C galactose occurs in the absence of the UDP-glucose 4-epimerase (GALE) enzyme.]]
Answer E [[AnswerE::Absence of 14C glucose-1-phosphate or 14C galactose-1-phosphate after the addition of 14C lactose.]]
Answer E Explanation AnswerEExp::This enzyme assay finding is observed in patients with lactase deficiency, i.e. lactose intolerance.
Right Answer RightAnswer::A
Explanation [[Explanation::This patient most likely suffers from classic galactosemia, a autosomal recessive disorder of galactose metabolism characterized by the absence of galactose-1-phosphate uridyltransferase (GALT) resulting in the accumulation of galactose-1-phosphate, galactose and galacticol. Classic galactosemia leads to severe manifestations early on in life if galactose is not eliminated from the diet. Patients present with failure to thrive and mental retardation. The radiolabeled enzyme assay will most likely demonstrate complete absence of 14C UDP-galactose after the addition of 14C galactose-1-phosphate. Infants in the United States are routinely screened for galactosemia as part of their newborn screening. Prenatal testing with chorionic villus sampling or amniocentesis is also possible. Despite early diagnosis and diet restriction a significant portion of patients have serious complications including neurological deficits and learning disabilities.


Educational Objective: Classic galactosemia is characterized by the absence of galactose-1-phosphate uridyltransferase (GALT) which is the enzyme needed to convert galactose-1-phosphate to UDP-galactose.
References: Cuthbert C. Diagnosis of inherited disorders of galactose metabolism. Curr Protoc Hum Genet. 2008 Jan;Chapter 17:Unit 17.5.]]

Approved Approved::Yes
Keyword WBRKeyword::Galactose, WBRKeyword::Classic galactosemia, WBRKeyword::Galactosemia, WBRKeyword::Galactokinase deficiency, WBRKeyword::Lactose, WBRKeyword::Lactase deficiency, WBRKeyword::Lactose intolerance, WBRKeyword::Enzyme, WBRKeyword::Enzyme assay
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