WBR0909

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 40-year-old woman presents for complaints of recurrent episodes of abdominal pain, bloating and diarrhea. She reports that the symptoms started several months ago and have been getting progressively worse, and seem to be related to the consumption of dairy products. Acquired deficiency of which of the following explains the symptoms of the patient?]]
Answer A	AnswerA::Galactokinase
Answer A Explanation	AnswerAExp::Galactokinase catalyzes the conversion of galactose to galactose-1-phosphate. The main presentation of patients with galactokinase deficiency is early-onset cataracts.
Answer B	AnswerB::Galactose-1-phosphate uridyl transferase
Answer B Explanation	AnswerBExp::Galactose-1-phosphate uridyl transferase catalyzes the reversible conversion of galactose-1-phosphate to UDP galactose. This enzyme is deficient in patients with classic galactosemia.
Answer C	AnswerC::UDP-galactose epimerase
Answer C Explanation	[[AnswerCExp::UDP-galactose-4-epimerase (GALE), also known as UDP-glucose-4-epimerase, catalyzes the reversible conversion of UDP-galactose to UDP-glucose. The deficiency of this enzyme may result in a spectrum of symptoms, the most severe of which is a constellation of symptoms similar to classic galactosemia.]]
Answer D	AnswerD::UDP-glucose-pyrophosphorylase
Answer D Explanation	AnswerDExp::UDP-glucose-pyrophosphorylase catalyzes the conversion of UDP-glucose to glucose-1-phosphate.
Answer E	AnswerE::β-D-galactosidase
Answer E Explanation	AnswerEExp::Lactase, or β-D-galactosidase, hydrolyzes lactose into its monosaccharide components: galactose and glucose.
Right Answer	RightAnswer::E
Explanation	[[Explanation::Lactase, also known as β-D-galactosidase, is a glycoside hydrolase found predominantly along the brush border membrane of enterocytes that breaks down the disaccharide lactose into its two monosaccharide components: galactose and glucose. Lactase deficiency is a major cause of lactose intolerance presenting with gastrointestinal discomfort, bloating, and diarrhea following the consumption of any food products that contain lactose (primarily milk but also other dairy products). Lactase deficiency may be either congenital or acquired. Although the acquired form is more common, the congenital form may be more severe preventing newborns from having human milk. Diagnosis is made using the hydrogen breath test, which consists of administering up to 25g of lactose orally after an overnight fast and measuring the amount of hydrogen in the exhaled air. As lactose cannot be digested, it is metabolized by the gut bacteria releasing gases, such as hydrogen gas and methane, that are responsible for the symptoms of lactose intolerance. Educational Objective: β-D-galactosidase (Lactase) is a intestinal brush border enzyme responsible for the hydrolysis of the disaccharide lactose into its monosaccharide components: galactose and glucose. References: Swajerty DL Jr. Lactose intolerance. Am Fam Physician. 2002 May 1;65(9):1845-50.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Lactase deficiency, WBRKeyword::Lactase, WBRKeyword::Galactosidase, WBRKeyword::Lactose intolerance, WBRKeyword::β-D-galactosidase, WBRKeyword::Galactose, WBRKeyword::Glucose
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