WBR0907

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::A pregnant woman known to have classic galactokinase deficiency is worried she will not be able to breastfeed her baby because she has been on a strict galactose free diet since her birth. The physician comforts her and tells her that she will be having normal breast milk for lactation. The ability of the mammary glands to synthesize milk in the absence of dietary galactose in this patient is due to the activity of which of the following enzymes?]]
Answer A	AnswerA::Aldose reductase
Answer A Explanation	AnswerAExp::Aldose reductase catalyzes the conversion of galactose into galacticol and glucose to sorbitol.
Answer B	AnswerB::Galactokinase
Answer B Explanation	AnswerBExp::Galactokinase catalyzes the conversion of galactose to galactose-1-phosphate. The enzyme is deficient in this patient, but it is not directly involved in the synthesis of lactose.
Answer C	AnswerC::Galactose-1-phosphate uridyltransferase
Answer C Explanation	AnswerCExp::Galactose-1-phosphate uridyltransferase (GALT) catalyzes the reversible conversion of galactose-1-phosphate into UDP-galactose. This enzyme is deficient in patients with classic galactosemia.
Answer D	AnswerD::UDP-galactose-4-epimerase
Answer D Explanation	[[AnswerDExp::UDP-galactose-4-epimerase (GALE), also known as UDP-glucose-4-epimerase, catalyzes the reversible conversion of UDP-galactose to UDP-glucose. In the absence of an exogenous source of galactose, GALE catalyzes the formation of UDP-galactose necessary for the synthesis of lactose.]]
Answer E	AnswerE::UDP-glucose-pyrophosphorylase
Answer E Explanation	AnswerEExp::UDP-glucose-pyrophosphorylase catalyzes the conversion of UDP-glucose to glucose-1-phosphate. The enzyme is not directly involved in the synthesis of lactose.
Right Answer	RightAnswer::D
Explanation	[[Explanation::The patient suffers from galactokinase deficiency, a genetic disease of galactose metabolism. Patients with galactokinase deficiency are required to have a galactose free diet to avoid the accumulation of galacticol in tissues most commonly manifested as early onset cataracts. Even in the absence of dietary galactose, the endogenous synthesis of galactose can occur. Galactose is produced as UDP-galactose by the activity of the epimerase enzyme on UDP-glucose. The synthesis of the disaccharide lactose (the primary milk sugar) in the mammary glands occurs via lactose synthase with UDP-galactose and UDP-glucose as substrates. Educational Objective: Even in the absence of dietary galactose, the endogenous synthesis of galactose occurs via the epimerase enzyme using UDP-glucose. References: Shcwarz V. Disorders of galactose metabolism. Journal of clinical pathology (1969).]]
Approved	Approved::Yes
Keyword	WBRKeyword::Galactose, WBRKeyword::Lactose, WBRKeyword::Glucose, WBRKeyword::Galactokinase deficiency, WBRKeyword::Breastfeeding, WBRKeyword::Epimerase
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