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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}} {{Alison}}
|QuestionAuthor={{YD}} (Reviewed by {{YD}} and {{AJL}})
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Biochemistry
|MainCategory=Biochemistry
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|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=Gastrointestinal
|SubCategory=Gastrointestinal
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
|MainCategory=Biochemistry
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|MainCategory=Biochemistry
|MainCategory=Biochemistry
|SubCategory=Gastrointestinal
|SubCategory=Gastrointestinal
|Prompt=A 6-month-old male is brought by his mother to the physician's office for failure to thrive and steatorrhea. Upon appropriate work-up, you diagnose him with abetalipoproteinemia. Which of the following genetic mutations is the primary defect in abetalipoproteinemia?
|Prompt=An 6-month-old boy is brought by his mother to the pediatrician's office for failure to thrive, persistent diarrhea, and steatorrhea. Peripheral blood smear reveals acanthocytosis with undetectable plasma cholesterol and triglyceride. Which of the following disorders is the primary defect in this patient's condition?
|Explanation=[[Abetalipoproteinemia]], an inherited disorder characterized by fat accumulation in enterocytes, results in malabsorption of dietary fat and fat-soluble vitamins. [[Microsomal transfer protein]]s (MTP) are required for the creation of [[lipoproteins]], such as VLDL, LDL, and [[chylomicrons]]. A defect of the MTP gene, located on the long arm of chromosome 4, results in [[abetalipoproteinemia]].
|Explanation=[[Abetalipoproteinemia]] is a rare autosomal recessive genetic disease characterized by fat accumulation in enterocytes, fat malabsorption, acanthocytosis, and hypocholesterolemia. Abetalipoproteinemia is caused by mutations in the ''MTP'' gene (long arm of chromosome 4) that encodes microsomal triglyceride transfer protein (MTP or MTTP). MTPs are required for the synthesis and post-translational processing of apo-B. As such, abetalipoproteinemia is marked by the complete absence of apo-B-containing [[lipoproteins]], namely VLDL, LDL, and [[chylomicrons]]. At older age, [[abetalipoproteinemia]] frequently manifests with clinical signs and symptoms associated with fat-soluble vitamin deficiency. Patients often present with complaints related to the GI (steatorrhea and fat malabsorption), ocular (atypical retinitis pigmentosa), hematological (coagulopathy), and neurological (posterior column neuropathy) systems.
 
|AnswerA=Defect in LDL receptors
[[Abetalipoproteinemia]] frequently manifests with GI, ocular, hematological, and neurological symptoms.
 
|EducationalObjectives=
[[Abetalipoproteinemia]] results from a genetic mutation of the MTP gene.
|References= Aggerbeck LP, Bouma ME, Eisenberg C, et al. Absence of microsomal transfer protein in individuals with abetalipoproteinemia. Science. 1992;258(5084):999-1001
 
|AnswerA=Defect of LDL receptors
|AnswerAExp=A defect in LDL receptors frequently results in [[familial hypercholesterolemia]] (FH).
|AnswerAExp=A defect in LDL receptors frequently results in [[familial hypercholesterolemia]] (FH).
|AnswerB=Defect of apolipoprotein B
|AnswerB=Defect in apolipoprotein B
|AnswerBExp=Although apoB synthesis frequently decreases in individuals with [[abetalipoproteinema]], an apoB defect is not the direct mutation that leads to [[abetalipoproteinema]].
|AnswerBExp=Although apo-B synthesis is affected in individuals with [[abetalipoproteinema]], an apo-B defect is not the primary detect that leads to [[abetalipoproteinema]].
|AnswerC=Defect of microsomal triglyceride transfer protein
|AnswerC=Defect in microsomal triglyceride transfer protein
|AnswerCExp=A defect of the MTP gene (or MTTP) frequently results in [[abetalipoproteinemia]].
|AnswerCExp=A defect in the ''MTP'' gene results in [[abetalipoproteinemia]].
|AnswerD=Defect of lipoprotein lipase
|AnswerD=Defect in lipoprotein lipase
|AnswerDExp=A defect in lipoprotein lipase (LPL) frequently results in [[chylomicronemia]]
|AnswerDExp=A defect in lipoprotein lipase (LPL) results in [[chylomicronemia]]
|AnswerE=Defect of chylomicron
|AnswerE=Defect in chylomicron
|AnswerEExp=A defect in [[chylomicron]] is not the primary defect in individuals with [[abetalipoproteinemia]], but often it occurs secondary to an MTP gene defect.
|AnswerEExp=A defect in [[chylomicron]] is not the primary defect in individuals with [[abetalipoproteinemia]], but often it occurs secondary to an ''MTP'' gene defect.
|EducationalObjectives=[[Abetalipoproteinemia]] results from a genetic mutation in the ''MTP'' gene.
|References=Aggerbeck LP, Bouma ME, Eisenberg C, et al. Absence of microsomal transfer protein in individuals with abetalipoproteinemia. Science. 1992;258(5084):999-1001.<br>
Zamel R, Khan R, Pollex RL, et al. Abetalipoproteinemia: two case reports and literature review. Orphanet J Rare Dis. 2008;3:19.<br>
First Aid 2014 page 353
|RightAnswer=C
|RightAnswer=C
|WBRKeyword=abetalipoproteinemia, malabsorption, diarrhea, microsomal triglyceride transfer protein, enzymes, chylomicron, apolipoprotein, genetics, inheritance, mutation
|WBRKeyword=Abetalipoproteinemia, Malabsorption, Diarrhea, Microsomal triglyceride transfer protein, Apolipoprotein, MTP, MTTP, Apo-B, Acanthocytosis
|Approved=Yes
|Approved=Yes
}}
}}

Revision as of 19:35, 23 October 2014
Author [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Yazan Daaboul, M.D. and Alison Leibowitz [1])]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Biochemistry
Sub Category SubCategory::Gastrointestinal
Prompt [[Prompt::An 6-month-old boy is brought by his mother to the pediatrician's office for failure to thrive, persistent diarrhea, and steatorrhea. Peripheral blood smear reveals acanthocytosis with undetectable plasma cholesterol and triglyceride. Which of the following disorders is the primary defect in this patient's condition?]]
Answer A AnswerA::Defect in LDL receptors
Answer A Explanation [[AnswerAExp::A defect in LDL receptors frequently results in familial hypercholesterolemia (FH).]]
Answer B AnswerB::Defect in apolipoprotein B
Answer B Explanation [[AnswerBExp::Although apo-B synthesis is affected in individuals with abetalipoproteinema, an apo-B defect is not the primary detect that leads to abetalipoproteinema.]]
Answer C AnswerC::Defect in microsomal triglyceride transfer protein
Answer C Explanation [[AnswerCExp::A defect in the MTP gene results in abetalipoproteinemia.]]
Answer D AnswerD::Defect in lipoprotein lipase
Answer D Explanation [[AnswerDExp::A defect in lipoprotein lipase (LPL) results in chylomicronemia]]
Answer E AnswerE::Defect in chylomicron
Answer E Explanation [[AnswerEExp::A defect in chylomicron is not the primary defect in individuals with abetalipoproteinemia, but often it occurs secondary to an MTP gene defect.]]
Right Answer RightAnswer::C
Explanation [[Explanation::Abetalipoproteinemia is a rare autosomal recessive genetic disease characterized by fat accumulation in enterocytes, fat malabsorption, acanthocytosis, and hypocholesterolemia. Abetalipoproteinemia is caused by mutations in the MTP gene (long arm of chromosome 4) that encodes microsomal triglyceride transfer protein (MTP or MTTP). MTPs are required for the synthesis and post-translational processing of apo-B. As such, abetalipoproteinemia is marked by the complete absence of apo-B-containing lipoproteins, namely VLDL, LDL, and chylomicrons. At older age, abetalipoproteinemia frequently manifests with clinical signs and symptoms associated with fat-soluble vitamin deficiency. Patients often present with complaints related to the GI (steatorrhea and fat malabsorption), ocular (atypical retinitis pigmentosa), hematological (coagulopathy), and neurological (posterior column neuropathy) systems.

Educational Objective: Abetalipoproteinemia results from a genetic mutation in the MTP gene.
References: Aggerbeck LP, Bouma ME, Eisenberg C, et al. Absence of microsomal transfer protein in individuals with abetalipoproteinemia. Science. 1992;258(5084):999-1001.
Zamel R, Khan R, Pollex RL, et al. Abetalipoproteinemia: two case reports and literature review. Orphanet J Rare Dis. 2008;3:19.
First Aid 2014 page 353]]

Approved Approved::Yes
Keyword WBRKeyword::Abetalipoproteinemia, WBRKeyword::Malabsorption, WBRKeyword::Diarrhea, WBRKeyword::Microsomal triglyceride transfer protein, WBRKeyword::Apolipoprotein, WBRKeyword::MTP, WBRKeyword::MTTP, WBRKeyword::Apo-B, WBRKeyword::Acanthocytosis
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