Question

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what happens if fertilization does not occur?
the egg will remain in the fallopian tube
the uterine lining will continue to thicken
the corpus luteum will eventually break down, causing a decrease in progesterone levels and leading to the shedding of the uterine lining
the menstrual cycle will stop.
the corpus luteum will continue to produce progesterone at the same level

Explanation:

If fertilization does not occur, the corpus luteum breaks down. This leads to a decrease in progesterone. Progesterone is important for maintaining the uterine lining. With its decrease, the uterine lining sheds. The egg does not remain in the fallopian tube indefinitely (it is eventually reabsorbed or expelled). The uterine lining does not continue to thicken as progesterone (which helps in its thickening maintenance) decreases. The menstrual cycle does not stop; instead, menstruation (shedding of the uterine lining) occurs. The corpus luteum does not continue to produce progesterone at the same level as it breaks down.

Answer:

The corpus luteum will eventually break down, causing a decrease in progesterone levels and leading to the shedding of the uterine lining.