What is the point after which an ovum is fertilized and becomes impenetrable to any other sperm?

Correct Answer: A

Rationale: The correct answer is A: The zona reaction. After fertilization, the zona pellucida undergoes a biochemical change called the zona reaction, which makes the ovum impenetrable to any other sperm. This process prevents polyspermy and ensures that only one sperm can fertilize the egg. Gametogenesis refers to the production of gametes, not the post-fertilization event. The zona pellucida is the glycoprotein layer around the ovum, and cleavage is the rapid mitotic cell division that follows fertilization, but neither of these directly relate to the ovum becoming impenetrable to other sperm.

Correct Answer: C

Rationale: The correct answer is C: Formation of a zygote that contains the diploid number of chromosomes. After fertilization, the sperm and egg combine to form a zygote with a complete set of chromosomes (diploid). This is crucial for genetic diversity and proper development. Choice A is incorrect because a zygote is diploid, not haploid. Choice B is incorrect as an embryo is formed later in development and contains diploid chromosomes. Choice D is incorrect because a morula is a stage where the zygote undergoes several rounds of cell division, but it does not yet contain a full diploid set of chromosomes.

Correct Answer: A

Rationale: The correct answer is A: IgG. IgG is the only immunoglobulin that can cross the placenta due to its small size and ability to bind to the neonatal Fc receptor. This transfer provides passive immunity to the fetus, protecting it from infections. IgA primarily functions in mucosal immunity and is found in secretions like breast milk. IgM is too large to cross the placenta and is mainly produced in response to acute infections. IgD is primarily found on the surface of B cells and functions in the activation of these cells.

Correct Answer: C

Rationale: The correct answer is C (20 weeks) because quickening, the first perception of fetal movement by the mother, typically occurs around this time. At 20 weeks, the fetus is large enough for its movements to be noticeable to the mother. Before 20 weeks, the fetus is too small and the movements are not strong enough to be felt. Choices A, B, and D are incorrect because they are too early in the pregnancy for the mother to feel fetal movement. By 20 weeks, the fetus has developed enough for the mother to perceive its movements, making option C the correct answer.

Correct Answer: B

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