Twins produced when a single egg is fertilized are called ______________twins. Twins produced from two eggs ovulated at the same time are called ______________ twins.
monozygotic; dizygotic
dizygotic; monozygotic
monozygotic; identical
dizygotic; nonidentical
The Correct Answer is A
A. Monozygotic; dizygotic: Monozygotic twins are identical twins that result from the division of a single fertilized egg. Dizygotic twins, or fraternal twins, result from the fertilization of two separate eggs.
B. Dizygotic; monozygotic: This is the reverse of the correct order. Dizygotic twins come from two eggs, and monozygotic twins come from one egg.
C. Monozygotic; identical: Monozygotic twins are identical, but this choice does not mention dizygotic twins, which makes it incomplete.
D. Dizygotic; nonidentical: While dizygotic twins are nonidentical, this choice does not properly define the types of twins produced by a single egg (monozygotic).
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Correct Answer is C
Explanation
A. The neural tube and yolk sac have formed: By 8 weeks, the neural tube and yolk sac have formed, but this is not the criterion for defining a fetus.
B. The amniotic cavity has formed: The formation of the amniotic cavity occurs earlier in development and is not the defining feature of a fetus.
C. All of the organ systems are present: By the end of 8 weeks, the basic structures of all major organ systems are present, which is why the embryo is referred to as a fetus from this point onwards.
D. The mesoderm has formed: The formation of the mesoderm occurs earlier and is not the defining factor for fetal status. By 8 weeks, more significant development, including the presence of organ systems, has occurred.
Correct Answer is D
Explanation
A. Estrogen is a hormone involved in the regulation of the menstrual cycle and pregnancy, but it is not the hormone detected by pregnancy tests.
B. Luteinizing hormone (LH) surges just before ovulation but is not the hormone detected by pregnancy tests.
C. Progesterone supports pregnancy by maintaining the uterine lining, but it is not the hormone detected by pregnancy tests.
D. Human chorionic gonadotropin (hCG) is the hormone produced by the placenta shortly after implantation, and it is the hormone detected by pregnancy tests.