WBR0078
| Author | [[PageAuthor::Lauren E Klinker (Reviewed by Yazan Daaboul, M.D.)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Embryology |
| Sub Category | SubCategory::General Principles |
| Prompt | [[Prompt::A 32-year-old woman presents to her fertility physician for pregnancy evaluation. She reports that the first day of her most recent menstrual period occurred exactly 5 weeks ago, and her period lasted for 7 days. Her test results reveal beta-hCG levels at 30 mIU/mL. If the patient currently follows a typical menstruation cycle following her most recent menstrual period, and fertilization occurs on the expected date of ovulation, which of the following stages of embryonic development is expected to have occurred at the time of her office visit?]] |
| 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. |
| 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. 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. 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 by the trophoblast cells begins within the first week after fertilization and can be detected in the blood 11 days after fertilization and in the urine 14 days after fertilization.
In this vignette, the first day of the patient's most recent menstrual period, which is considered day 1 of her current cycle, was 5 weeks ago. Although the patient reports her last menstrual period persisted for 7 days, counting the number of days of the new menstrual cycle always starts at the first day of her previous menstruation. Based on a typical menstrual cycle, ovulation in this patient most likely occurred 14 days or 2 weeks after the first day of menstruation. Because fertilization occurs at ovulation, the patient's embryo is expected to to be at week-3 of development, which is marked by gastrulation. Gastrulation is the process by which the trilaminar germ disc forms from the epiblast layer of the bilaminar germ disc. 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 that are involved in organogenesis: ectoderm, mesoderm, and endoderm.
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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:: |
