WBR0332
| Author | [[PageAuthor::Rim Halaby, M.D. [1]]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | |
| Sub Category | |
| Prompt | [[Prompt::In an experiment that uses chicks, a researcher removes the apical ectodermal ridge (AER) and finds that only the proximal skeletal segment develops with complete absence of the middle and the distal segments. The researcher then applies beads soaked in recombinant proteins at the tip of the chick limbs that are missing AERs and finds that normal limb development occurs. Protein expressed by which gene is most likely present in the recombinant-soaked beads?]] |
| Answer A | AnswerA::FGF |
| Answer A Explanation | AnswerAExp::FGF 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 CNC 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::Homeobos (Hox) |
| Answer D Explanation | AnswerDExp::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). 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 chick limbs. The chick 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 will be present and limb development will occur 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 |
| 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 |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |