WBR0513
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| Author | [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian and Yazan Daaboul, M.D.)]] |
|---|---|
| Exam Type | USMLE Step 1 |
| Main Category | Physiology |
| Sub Category | Reproductive |
| Prompt | A couple has been trying to conceive a child for 5 years with no success. They decide to explore in-vitro fertilization (IVF) and ask their primary care physician for advice. The physician describes the process explaining that after fertilization the embyro is directly introduced into the uterus. Assuming fertilization occurs at day 0, at which day post-fertilization does an embryo undergoes implantation in the uterus? |
| Answer A | Day 2 |
| Answer A Explanation | On day 2, the zygote is still within the fallopian tubes and has not descended to the level of the endometrium. |
| Answer B | Day 4 |
| Answer B Explanation | On day 4, the morula is still within the fallopian tubes and has not descended to the level of the endometrium. |
| Answer C | Day 6 |
| Answer C Explanation | On day 6, the blastocyst is in the endometrium and implantation starts. |
| Answer D | Day 10 |
| Answer D Explanation | On day 10, implantation has occured and hCG is secreted by the syncytiotrophoblast. |
| Answer E | Day 12 |
| Answer E Explanation | On day 12, implantation has occured and hCG is secreted by the syncytiotrophoblast. |
| Right Answer | C |
| Explanation | During a normal pregnancy, fertilization is considered to occur at day 0. The fertilization process typically occurs in the ampullary-ischmic junction of the fallopian tubes. After fertilization, the oocyte proceeds with meiosis II, which was previously arrested following ovulation. The fertilized egg continues to descend along the fallopian tube as mitotic division of the embryo proceeds and gives rise to the morula on day 3 and the blastocyst on day 5. On day 6, the blastocyst reaches the endometrium, and implantation starts. Soon after implantation takes place, hCG is secreted and pregnancy then becomes detectable by serum and urinary concentrations of hCG within 10-14 days post-fertilization. Educational Objective: Blastocyst implantation occurs on day 6 post-fertilization. |
| Approved | Yes |
| Keyword | Implantation, Blastocyst, Morula, hCG, Zygote, Fertilization, Ampulla, Oocyte, Sperm, IVF |
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