WBR0910

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 1-year-old boy is brought by his mother to the pediatrician for a scheduled vaccine appointment and general check up. As the pediatrician is examining the baby, he holds a toy in his hand in order to get the baby’s attention. The pediatrician notices that the baby is unable to follow the toy with his eyes. Upon questioning the mother, she reports that her son is unable to follow with his eyes the baby hanging bell toys on top of his beds. She also tells the pediatrician that her son does not interact with people around him as if he does not see them. Physical exam reveals no other abnormalities and the growth chart depicts a normal development of the baby. The pediatrician suspects a metabolic disorder and requests a radiolabeled 14C galactose analysis. Enzyme deficiency in which of the following steps of galactose metabolism is most likely? File:Galactose metabolism question.png]]
Answer A	AnswerA::A
Answer A Explanation	[[AnswerAExp::Galactokinase deficiency is characterized by the accumulation of galactose in the blood and urine and elevated levels of galacticol. The accumulation of galacticol is associated with the occurrence of cataracts in the absence of mental retardation, failure to thrive and severe symptoms of jaundice and vomiting.]]
Answer B	AnswerB::B
Answer B Explanation	[[AnswerBExp::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 diagnosis of this patient is galactokinase deficiency and not classic galactosemia. Galactokinase deficiency is different from the more severe classic galactosemia and is not associated with mental retardation, failure to thrive and severe symptoms of jaundice and vomiting.]]
Answer C	AnswerC::C
Answer C Explanation	[[AnswerCExp::UDP glactose 4 epimerase (GALE) catalyzes the conversion of UDP galactose to UDP glucose. Deficiency in GALE results in a spectrum of symptoms, the most severe of which are a constellation of symptoms similar to classic galactosemia. The presentation of this patient is mild and is only characterized by the presence of visual deficit. the visual deficit is suggestive of the presence of cataract resulting from galacticol accumulation due to galactokinase deficiency.]]
Answer D	AnswerD::D
Answer D Explanation	AnswerDExp::Aldose reductase converts galactose to galacticol. Its deficiency does not lead to cataracts. On the opposite, overproduction of galacticol lead to cataract.
Answer E	AnswerE::E
Answer E Explanation	AnswerEExp::UDP glucose pyrophosphate converts UDP glucose to glucose 1 phosphate. The patient's presentation is more suggestive of galactokinase deficiency.
Right Answer	RightAnswer::A
Explanation	[[Explanation::Galactokinase deficiency is a mild autosomal recessive galactose metabolism disorder. It is characterized by the accumulation of galactose in the blood and urine and elevated levels of galacticol. The accumulation of galacticol is associated with the occurrence of cataracts resulting in decreased vision that can be suspected by the inability of the patients to track objects with their eyes and absence of social smiling. Galactokinase deficiency is different from the more severe classic galactosemia and is not associated with mental retardation, failure to thrive and severe symptoms of jaundice and vomiting. Shown below is a diagram depicting galactose metabolism. Educational objective: Galactokinase deficiency is a mild galactose metabolism disorder leading to accumulation of galacticol and subsequent formation of cataract. It is different from the more severe classic galactosemia that is associated with mental retardation, failure to thrive and severe symptoms of jaundice and vomiting. Reference: V. Schwarz. Disorders of galactose metabolism. J Clin Pathol Suppl (Assoc Clin Pathol). 1969; 2: 72–75. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::Galactose, WBRKeyword::Galactosemia, WBRKeyword::galactokinase, WBRKeyword::galactokinase deficiency
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::