Approximately 3 days after fertilization, what is the term for the developing zygote, which is a 16-cell mass?

Correct Answer: B

Rationale: The correct answer is B: Morula. At approximately 3 days post-fertilization, the zygote undergoes multiple cell divisions, forming a 16-cell mass known as a morula. This stage precedes the formation of a blastocyst, which occurs around day 5. The trophoblast refers to the outer layer of cells in the blastocyst responsible for implantation, while the embryoblast is the inner cell mass that gives rise to the embryo. Therefore, the morula stage accurately represents the 16-cell mass at this specific developmental timeline.

Correct Answer: D

Rationale: The correct answer is D: Progesterone. Progesterone is essential for maintaining the endometrium during pregnancy by promoting its growth and preventing contractions. It also plays a crucial role in preparing the mammary glands for lactation. Estrogen (A) is important in the menstrual cycle but not specifically for maintaining the endometrium during pregnancy. Human chorionic gonadotropin (hCG) (B) is produced by the placenta and is primarily involved in supporting pregnancy and detecting pregnancy tests. Prolactin (C) is responsible for milk production but is not directly related to the maintenance of the endometrium or mammary gland tissue during pregnancy.

Correct Answer: B

Rationale: The correct answer is B because the yolk sac in early pregnancy produces blood cells for the fetus. This is essential for the fetus's development as it helps in the formation of the circulatory system. The other choices are incorrect because: A: Providing antibodies to the developing fetus is typically done by the placenta, not the yolk sac. C: Hormones needed to support the pregnancy are mainly secreted by the placenta and other maternal structures, not the yolk sac. D: The yolk sac does not become the placenta; they are two distinct structures with different functions.

Correct Answer: D

Rationale: The correct answer is D (24 weeks) because alveoli formation and fetal breathing movements are typically observed during the late second trimester, around 24 weeks. At this stage, the lungs have developed sufficiently to support breathing outside the womb. Choices A (12 weeks), B (6 weeks), and C (20 weeks) are incorrect because alveoli formation and breathing movements do not occur as early as these time points in the pregnancy. Fetal breathing movements are a crucial sign of lung maturation and readiness for extrauterine life, which is why the correct answer is 24 weeks.

Correct Answer: C

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.