WBR0332

Author	[[PageAuthor::Rim Halaby, M.D. [1], Alison Leibowitz [2] (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Genetics
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A researcher inactivates the components of the ectoderm rimming of the distal tip of a limb bud, from a mouse, at an early stage. He finds that only the proximal skeletal segment develops with a complete absence of the middle and the distal segments. Then, he applies beads soaked in recombinant proteins at the tip of the limb buds and observes that normal limb development occurs successfully. Protein is most likely expressed by which of the following genes in the recombinant-soaked beads?]]
Answer A	AnswerA::FGF
Answer A Explanation	AnswerAExp::The fibroblast growth factor (FGF), produced at the apical ectodermal ridge (AER), induces mitosis of the mesoderm and allows for limb lengthening.
Answer B	AnswerB::Sonic hedgehog
Answer B Explanation	AnswerBExp::The sonic hedgehog gene, produced at the base of the limbs, is involved in CNS development.
Answer C	AnswerC::Wnt-7
Answer C Explanation	AnswerCExp::The wnt-7 gene, produced at apical ectodermal ridge (AER), is involved in the organization along dorso-ventral axis.
Answer D	AnswerD::Homeobox
Answer D Explanation	AnswerDExp::The homeobox (Hox) gene is involved in segmental organization along the cranio-caudal direction.
Answer E	AnswerE::SRY
Answer E Explanation	AnswerEExp::The SRY gene, located on the Y chromosome, is involved in the production of the testis-determining factor.
Right Answer	RightAnswer::A
Explanation	[[Explanation::Embryologically, the fibroblast growth factor (FGF) gene induces mitosis of the mesoderm and 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 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, limb development occurs normally. Educational Objective: The FGF gene is located at the apical ectodermal ridge (AER) and is responsible for the lengthening of limbs. References: Sun X, Mariani FV, Martin GR. Functions of FGF signaling from the apical ectodermal ridge in limb development. Nature. 2002; 418:501-8]]
Approved	Approved::
Keyword	WBRKeyword::embryogenesis, WBRKeyword::FGF, WBRKeyword::genes, WBRKeyword::proteins, WBRKeyword::apical, WBRKeyword::ectodermal, WBRKeyword::ridge, WBRKeyword::AER, WBRKeyword::lengthening, WBRKeyword::mesoderm, WBRKeyword::mitosis
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::