WBR0332

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::Using mice, a researcher inactivates the components of the ectoderm rimming the distal tip of the limb bud at an early stage. He finds that only the proximal skeletal segment develops with complete absence of the middle and the distal segments. Then, he applies beads soaked in recombinant proteins at the tip of the mice limb buds and finds that normal limb development occurs successfully. Protein expressed by which gene is most likely present in the recombinant-soaked beads?]]
Answer A	AnswerA::''FGF''
Answer A Explanation	AnswerAExp::''Fibroblast growth factor'' (''FGF'') gene is produced at the apical ectodermal ridge (AER). It induces mitosis of the mesoderm and allows for limb lengthening.
Answer B	AnswerB::''Sonic hedgehog''
Answer B Explanation	AnswerBExp::''Sonic hedgehog'' gene is produced at the base of the limbs. It is involved in CNS development.
Answer C	AnswerC::''Wnt-7''
Answer C Explanation	AnswerCExp::''Wnt-7'' gene is produced at apical ectodermal ridge (AER). It is involved in the organization along dorso-ventral axis.
Answer D	AnswerD::''Homeobox''
Answer D Explanation	AnswerDExp::''Homeobox'' (''Hox'') gene is involved in segmental organization along the cranio-caudal direction.
Answer E	AnswerE::''SRY''
Answer E Explanation	AnswerEExp::''SRY'' gene is located on the Y chromosome. It is involved in the production of testis-determining factor.
Right Answer	RightAnswer::A
Explanation	[[Explanation::FGF gene is important embryologically for the induction of mitosis of the mesoderm and for the lengthening of the limbs. It is produced by the apical ectodermal ridge (AER), which is the ectoderm rimming the distal tip of the limb bud. In this experiment, the AER is lost. Consequently, the FGF protein produced by the FGF gene will no longer be present to induce the lengthening of the limbs. The mice limbs thus only developed the proximal segment but had misising middle and distal segments. When beads soaked in recombinant FGF protein are introduced to the tip of the limbs that are missing AER, FGF is present and limb development occurs normally. Educational Objective: FGF gene is located at the apical ectodermal ridge (AER) and is responsible for the lengthening of limbs. Reference: Sun X, Mariani FV, Martin GR. Functions of FGF signaling from the apical ectodermal ridge in limb development. Nature. 2002; 418:501-8 Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::embryogenesis, WBRKeyword::FGF, WBRKeyword::gene, WBRKeyword::protein, WBRKeyword::apical, WBRKeyword::ectodermal, WBRKeyword::ridge, WBRKeyword::AER, WBRKeyword::limb, WBRKeyword::length, WBRKeyword::lengthening, WBRKeyword::mesoderm, WBRKeyword::mitosis, WBRKeyword::bead, WBRKeyword::soak, WBRKeyword::soaked, WBRKeyword::recombinant
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