Correct Answer: B

Rationale: The correct answer is B. The fetus typically begins to respond to external stimulation much later in pregnancy, usually after the first trimester. During the second or third week of pregnancy, the fetus is still in the early stages of development and is not yet capable of turning or responding to external stimuli. Choices A, C, and D are incorrect because they do not accurately reflect the timeline of fetal development when it comes to responding to external stimulation.

Correct Answer: A

Rationale: Tay-Sachs disease is a genetic disorder that causes a progressive degeneration of the central nervous system, particularly in infants. Choice B, Cystic fibrosis, is a genetic disorder that primarily affects the lungs and digestive system, not the central nervous system. Choices C and D, Turner syndrome and Klinefelter syndrome, are chromosomal disorders that do not directly involve degeneration of the central nervous system.

Correct Answer: C

Rationale: Individuals with Klinefelter syndrome have an extra X chromosome (XXY) and typically have reduced testosterone levels, which can lead to gynecomastia (enlarged breasts). This condition is not usually associated with Down syndrome (choice A), XYY syndrome (choice B), or Turner syndrome (choice D), making them less likely to have gynecomastia.

Correct Answer: B

Rationale: The correct answer is B: 'The mother is Rh negative, and the father is Rh positive.' Hemolytic disease of the newborn occurs when an Rh-negative mother carries an Rh-positive fetus, leading to Rh incompatibility. In this scenario, the mother produces antibodies against the Rh antigen present in the fetus, which can result in hemolysis of the fetal red blood cells. Choices A, C, and D do not describe the Rh incompatibility that leads to hemolytic disease in newborns. Therefore, they are incorrect.

Correct Answer: C

Rationale: The correct answer is C. Mutations can occur due to environmental influences such as radiation, chemicals, or other external factors, leading to changes in the DNA sequence. Choice A is incorrect because mutations can also occur due to errors in DNA replication, not solely by chance. Choice B is incorrect as mutations affect the DNA sequence itself, not the production of specific cells. Choice D is incorrect because mutations involve changes in the DNA sequence and do not specifically involve the formation of the DNA structure.