WBR0908

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
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 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?]]
Answer A	[[AnswerA::Absence of any detectable UDP 14C 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 accumulation of galactose 1 phosphate, galactose and galacticol. The enzyme assay will more likely reveal an absence of any detectable UDP 14C galactose after the addition of 14C galactose 1 phosphate.]]
Answer B	[[AnswerB::Absence of any detectable 14C galactose 1 phosphate after the addition of 14C galactose]]
Answer B Explanation	[[AnswerBExp::Absence of any detectable 14C galactose 1 phosphate after the addition of 14 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 cataratcs but not mental retardation, failure to thrive and severe symptoms of jaundice and vomiting.]]
Answer C	[[AnswerC::Accumulation of UDP 14C 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 will more likely reveal an absence , not accumulation, of any detectable UDP 14C galactose after the addition of 14C galactose 1 phosphate.]]
Answer D	[[AnswerD::Accumulation of 14C galactose 1 phosphate after the addition of 14C galactose]]
Answer D Explanation	[[AnswerDExp::Classic galactosemia is characterized by the absence of galactose 1 phosphate uridyltransferase (GALT). Accumulation of 14C galactose 1 phosphate after the addition of 14C galactose is not characteristic of the absence of this enzyme.]]
Answer E	[[AnswerE::Absence of any detectable UDP 14C glucose 1 phosphate after the addition of 14C UDP glucose]]
Answer E Explanation	[[AnswerEExp::Absence of any detectable UDP 14C glucose 1 phosphate after the addition of 14C 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).]]
Right Answer	RightAnswer::A
Explanation	[[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 14C galactose after the addition of 14C galactose 1 phosphate. Shown below is a diagram depicting galactose metabolism. Learning objective: 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. Reference: Cuthbert C. Diagnosis of inherited disorders of galactose metabolism. Curr Protoc Hum Genet. 2008 Jan;Chapter 17:Unit 17.5. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::galactose, WBRKeyword::classic galactosemia, WBRKeyword::galactosemia
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::