WBR0078
| Author | PageAuthor::Lauren E Klinker |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Embryology |
| Sub Category | SubCategory::General Principles |
| Prompt | [[Prompt::A 32-year-old female patient presents to her fertility physician for pregnancy evaluation. She reports that the last day of her most recent menstrual period occurred exactly 5 weeks ago. Her test results reveal beta-hCG levels at 30 mIU/mL. Which of the following stages of embryonic development is expected to have occurred at the time of her office visit? Assume that this patient follows a typical menstruation cycle and that fertilization would occur on the expected date of ovulation.]] |
| Answer A | AnswerA::None. These test results are not indicative of pregnancy at this point in time. |
| Answer A Explanation | [[AnswerAExp::Beta-hCG levels above 25 mIU/mL are considered positive for pregnancy. Beta-hCG levels at 5 mIU/mL or lower are considered negative for pregnancy. Secretion of hCG begins within the first week after fertilization and can be detected in the blood 11 days after fertilization.]] |
| Answer B | AnswerB::Implantation on the uterine wall |
| Answer B Explanation | [[AnswerBExp::Migration of the embryo from the uterine tubes to the uterus begins at Day 4 after fertilization, and implantation of the fluid-filled blastocyst on the uterine wall is completed by Day 10. Intrauterine devices (IUDs) are a form of contraception that function by inhibiting implantation of the embryo in the uterus.]] |
| Answer C | AnswerC::Formation of the bilaminar germ disc |
| Answer C Explanation | [[AnswerCExp::The bilaminar germ disc consists of two layers: the epiblast and the hypoblast. These layers differentiate from the embryoblast at Day 8 after fertilization. The epiblast will further differentiate into the ectoderm, mesoderm, and endoderm to make up the trilaminar germ disc. The hypoblast will form the extraembryonic membrane and the yolk sac.]] |
| Answer D | AnswerD::Gastrulation |
| Answer D Explanation | [[AnswerDExp::Gastrulation is the process by which the trilaminar germ disc forms from the epiblast layer of the bilaminar germ disc during Week 3 after fertilization. Gastrulation begins with the invagination of the epiblast layer to form the primitive node and the primitive streak. Epiblast cells then migrate along the primitive streak and settle between the epiblast and hypoblast, forming three distinct layers that make up the trilaminar disc. At the end of gastrulation, the trilaminar germ disc consists of three germ layers involved in organogenesis: the ectoderm, mesoderm, and endoderm.]] |
| Answer E | AnswerE::Limb bud formation |
| Answer E Explanation | AnswerEExp::The upper and lower limbs begin to bud from the mesoderm layer during Week 4 after fertilization. A helpful mnemonic is that Week 4 corresponds with 4 limbs. Week 4 is also marked by initiation of the embryonic heart beating. |
| Right Answer | RightAnswer::D |
| Explanation | [[Explanation::Based on her beta-hCG levels, this patient has tested positive for pregnancy. The last day of her most recent menstrual period was 5 weeks ago, and based on a typical menstrual cycle, we can expect that ovulation occurred 2 weeks later. Because we assumed that fertilization occurred at ovulation, we can estimate the embryo to be at Week 3 of development. Week 3 is marked by gastrulation, or the development of the trilaminar germ disc into the 3 embryonic layers: the endoderm, mesoderm, and ectoderm. A helpful mnemonic is that Week 3 corresponds with 3 embryonic layers.
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| Approved | Approved::Yes |
| Keyword | WBRKeyword::Gastrulation, WBRKeyword::Embryology, WBRKeyword::Embryogenesis, WBRKeyword::Development, WBRKeyword::Trilaminar disc, WBRKeyword::Bilaminar disc, WBRKeyword::Pregnancy, WBRKeyword::Embryo |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
