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|Explanation=[[Lactase]], a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into its constituent galactose and glucose monomers. Lactase is present predominantly along the brush border membrane of the differentiated [[enterocyte]]s lining the villi of the small intestine. | |Explanation=[[Lactase]], a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into its constituent galactose and glucose monomers. Lactase is present predominantly along the brush border membrane of the differentiated [[enterocyte]]s lining the villi of the small intestine. | ||
Acquired lactase deficiency is a cause of [[lactose intolerance]] characterized by episodes of gastrointestinal discomfort, bloating and diarrhea. | Acquired lactase deficiency is a cause of [[lactose intolerance]] characterized by episodes of gastrointestinal discomfort, bloating and diarrhea. | ||
Shown below is a diagram depicting galactose metabolism. | |||
[[ File:Galactose metabolism.png]] | |||
Educational objective: | Educational objective: | ||
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Reference: Swajerty DL Jr. Lactose intolerance. Am Fam Physician. 2002 May 1;65(9):1845-50. | Reference: Swajerty DL Jr. Lactose intolerance. Am Fam Physician. 2002 May 1;65(9):1845-50. | ||
|AnswerA=Galactokinase | |AnswerA=Galactokinase | ||
|AnswerAExp=[[Galactokinase]] catalyzes the conversion of galactose to galactokinase 1 phosphate. The main presentation of patients with galactokinase deficiency is cataracts. The enzyme is not involved in the breakdown of lactose into galactose and glucose. | |AnswerAExp=[[Galactokinase]] catalyzes the conversion of galactose to galactokinase 1 phosphate. The main presentation of patients with galactokinase deficiency is cataracts. The enzyme is not involved in the breakdown of lactose into galactose and glucose. | ||
|AnswerB=Galactose 1 phosphate uridyl transferase | |AnswerB=Galactose 1 phosphate uridyl transferase | ||
|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]]. The enzyme is not involved in the breakdown of lactose into galactose and glucose. | |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]]. The enzyme is not involved in the breakdown of lactose into galactose and glucose. | ||
|AnswerC=UDP galactose epimerase | |AnswerC=UDP galactose epimerase | ||
|AnswerCExp=UDP galactose 4-epimerase ([[GALE]]), also known as UDP glucose 4 epimerase, catalyzing 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]]. The enzyme is not involved in the breakdown of lactose into galactose and glucose. | |AnswerCExp=UDP galactose 4-epimerase ([[GALE]]), also known as UDP glucose 4 epimerase, catalyzing 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]]. The enzyme is not involved in the breakdown of lactose into galactose and glucose. | ||
|AnswerD=UDP glucose pyrophosphate | |AnswerD=UDP glucose pyrophosphate | ||
|AnswerDExp=UDP glucose pyrophosphate catalyzes the conversion of UDP glucose to glucose 1 phosphate. The enzyme is not involved in the breakdown of [[lactose]] into [[galactose]] and [[glucose]]. | |AnswerDExp=UDP glucose pyrophosphate catalyzes the conversion of UDP glucose to glucose 1 phosphate. The enzyme is not involved in the breakdown of [[lactose]] into [[galactose]] and [[glucose]]. | ||
|AnswerE=β-D-galactosidase | |AnswerE=β-D-galactosidase | ||
|AnswerEExp=[[Lactase]], a part of the [[β-galactosidase]] family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent [[galactose]] and [[glucose]] monomers. | |AnswerEExp=[[Lactase]], a part of the [[β-galactosidase]] family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent [[galactose]] and [[glucose]] monomers. | ||
Revision as of 15:46, 16 December 2013
| 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 40-year-old female complains of recurrent episodes of abdominal pain, bloating and diarrhea. These symptoms have started few months ago and have been getting progressively more severe. The patient has been noticing that these episodes mainly occur when she ingests milk and 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 galactokinase 1 phosphate. The main presentation of patients with galactokinase deficiency is cataracts. The enzyme is not involved in the breakdown of lactose into galactose and glucose.]] |
| 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. The enzyme is not involved in the breakdown of lactose into galactose and glucose.]] |
| Answer C | AnswerC::UDP galactose epimerase |
| Answer C Explanation | [[AnswerCExp::UDP galactose 4-epimerase (GALE), also known as UDP glucose 4 epimerase, catalyzing 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. The enzyme is not involved in the breakdown of lactose into galactose and glucose.]] |
| Answer D | AnswerD::UDP glucose pyrophosphate |
| Answer D Explanation | [[AnswerDExp::UDP glucose pyrophosphate catalyzes the conversion of UDP glucose to glucose 1 phosphate. The enzyme is not involved in the breakdown of lactose into galactose and glucose.]] |
| Answer E | AnswerE::β-D-galactosidase |
| Answer E Explanation | [[AnswerEExp::Lactase, a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers.]] |
| Right Answer | RightAnswer::E |
| Explanation | [[Explanation::Lactase, a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into its constituent galactose and glucose monomers. Lactase is present predominantly along the brush border membrane of the differentiated enterocytes lining the villi of the small intestine.
Acquired lactase deficiency is a cause of lactose intolerance characterized by episodes of gastrointestinal discomfort, bloating and diarrhea. Shown below is a diagram depicting galactose metabolism.
Educational objective: Lactase, a part of the β-galactosidase family of enzymes, is a brush border enzyme in the gastrointestinal wall involved in the hydrolysis of the disaccharide lactose into its constituent galactose and glucose monomers. Reference: Swajerty DL Jr. Lactose intolerance. Am Fam Physician. 2002 May 1;65(9):1845-50. |
| Approved | Approved::No |
| Keyword | WBRKeyword::lactase deficiency, WBRKeyword::lactase, WBRKeyword::galactosidase |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
