WBR0907

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 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 to galacticol and glucose to sorbitol. It is not involved in the synthesis of galactose or lactose.
Answer B	AnswerB::Galactokinase
Answer B Explanation	AnswerBExp::Galactokinase catalyzes the conversion of galactose to galactokinase 1 phosphate. The enzyme is already deficient in this patient and it is not directly involved in the synthesis of lactose.
Answer C	AnswerC::Galactose 1 phosphate uridyl transferase
Answer C Explanation	AnswerCExp::Galactose 1 phosphate uridyl transferase (GALT) catalyzes the reversible conversion of galactose 1 phosphate to UDP galactose. This enzyme is deficient in patients with classic galactosemia.
Answer D	AnswerD::UDP galactose epimerase
Answer D Explanation	[[AnswerDExp::UDP galactose 4-epimerase (GALE), also known as UDP glucose 4 epimerase, catalyzing the reversible conversion of UDP-galactose to UDP-glucose. GALE tightly binds nicotinamide adenine dinucleotide (NAD+), a co-factor required for catalytic activity. 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 pyrophosphate
Answer E Explanation	AnswerEExp::UDP glucose pyrophosphate 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 the galactose metabolism. Patients with galactokinase deficiency are required to have a galactose free diet to avoid the accumulation of galacticol. Even in the absence of dietary galactose, the endogenous synthesis of galactose occurs in the body. Galactose is produced in the form of UDP galactose through the activity of the epimerase enzyme on UDP glucose. The synthesis of lactose (milk) in the mammary glands results from the activity of lactose synthase on UDP galactose and glucose. Learning objective: Even in the absence of dietary galactose, the endogenous synthesis of galactose occurs in the body through the activity of the epimerase enzyme on UDP glucose. Reference: Shcwarz V. Disorders of galactose metabolism. Journal of clinical pathology (1969). Educational Objective: References: ]]
Approved	Approved::No
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