Question
A 26-year-old woman comes to the physician because she has not had a menstrual period for 5 weeks. Menarche was at the age of 14 years and menses occurred at regular 30-day intervals. She reports having unprotected sexual intercourse 3 weeks ago. A urine pregnancy test is positive. Which of the following best describes the stage of development of the embryo at this time?
| A. |
Sexual differentiation has begun, but fetal movement has not started
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| B. |
Fetal heart is beating, but cardiac activity is not yet visible on ultrasound
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| C. |
Neural crest has formed, but limb buds have not yet formed
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| D. |
Limb buds have formed, but fetal movements have not begun
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Show Answer
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Correct Answer � C
Explanation
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Answer C) Neural crest has formed, but limb buds have not yet formed
Ovulation occurs 12–16 days before the menstrual period, which coincides with the last time this patient had unprotected sexual intercourse. The embryo is, therefore, most likely in week 3 of development.
Neural crest has formed, but limb buds have not yet formed
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Neurulation occurs during week 3 of embryonic development. During this process, the neural crest forms from the dorsal portion of the folded neural tube.
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Limb buds are structures of undifferentiated mesodermal cells that are sheathed by ectoderm.
They form during week 4 of development and are unlikely to be present in this patient’s embryo, which is most likely in week 3 of development.
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