Which statement about a blastocyst is true?

Correct Answer: C

Rationale: The statement that is true about a blastocyst is that between the seventh and ninth days, it burrows deep into the uterine lining. This process, known as implantation, is essential for the blastocyst to establish a connection with the mother's body and receive nutrients for further development. It is a critical stage in prenatal development and sets the foundation for pregnancy. Choices A, B, and D are incorrect. A blastocyst is not a single cell but rather a structure composed of many cells. It is not a structure that feeds the developing organism; instead, it implants in the uterine lining to receive nutrients. It is not a membrane that regulates temperature but plays a crucial role in the initial stages of pregnancy by implanting in the uterus.

Correct Answer: B

Rationale: The correct answer is B. During prenatal development, the ectoderm folds over to form the neural tube, which eventually develops into the brain and spinal cord. The neural tube is the precursor to the central nervous system, including the primitive spinal cord. Choices A, C, and D are incorrect because the mesoderm does not form the neural tube, the endoderm is not involved in the formation of the central nervous system, and neuron tract is not the structure that forms the primitive spinal cord.

Correct Answer: C

Rationale: During the second trimester of pregnancy, a white, cheeselike substance called vernix emerges on the skin of the developing fetus. Vernix is a protective layer that helps prevent the skin from becoming waterlogged during the time in the amniotic fluid. Choice A, lanugo, is incorrect as lanugo refers to the fine, soft hair that covers the body of a fetus. Choice B, corpus luteum, is incorrect as it is a temporary endocrine structure involved in the menstrual cycle. Choice D, chorion, is incorrect as it is one of the membranes that surround the embryo and later the fetus.

Correct Answer: A

Rationale: A rise in the baby's heart rate during pregnancy is usually followed within five seconds by a burst of motor activity. This indicates that the baby is responding to stimuli and is an important sign of healthy fetal development. A period of sleep, a rise in Liz's heart rate, or a drop in Liz's body temperature are not typically immediate responses to changes in the baby's heart rate. Sleep is less likely to be a direct response to changes in heart rate, Liz's heart rate is independent of the baby's heart rate changes, and a drop in Liz's body temperature is not a common immediate response to fetal heart rate changes.

Correct Answer: B

Rationale: The statement that the genetic makeup of the mother and the developing organism plays an important role in the harm done by teratogens is true. Genetic factors can influence susceptibility to teratogens, and the interaction between genetic and environmental factors plays a significant role in determining the impact on the developing fetus. This highlights the importance of considering both genetic and environmental influences when assessing risks to prenatal development. Choice A is incorrect as passive smoking can indeed be harmful to the developing fetus. Choice C is incorrect as the negative impact of teratogens is often not reversible. Choice D is incorrect as serious defects can occur throughout prenatal development, not just in the germinal period.