Dichorionic-diamniotic twins result from separation of embryonic cells at the:

Correct Answer: A

Rationale: The correct answer is A: Four-cell stage. Dichorionic-diamniotic twins occur when the embryo splits into two separate embryos very early in development, typically at the four-cell stage. At this stage, each embryo will have its own chorion and amnion, leading to separate placentas and amniotic sacs. Choices B, C, and D occur later in development and do not result in the formation of two separate embryos with their own chorion and amnion, making them incorrect for the scenario of dichorionic-diamniotic twins.

Correct Answer: A

Rationale: The failure of closure of the cranial neural pore leads to anencephaly due to the incomplete development of the brain and skull. This results in the absence of a major portion of the brain, skull, and scalp. Hydrocephaly (B) is the abnormal accumulation of cerebrospinal fluid within the brain's ventricles, not caused by the cranial neural pore closure. Hydrocephalus (C) is the same as hydrocephaly. Chiari malformation (D) is a condition where brain tissue extends into the spinal canal, unrelated to the cranial neural pore closure.

Correct Answer: A

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

Correct Answer: B

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

Correct Answer: B

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