Menstrual flow occurs due to lack of:

Options

A. Oxytocin released from maternal pituitary
B. Vasopressin
C. Progesterone
D. FSH

Answer

### Step-by-Step Solution:

1. **Understanding the Menstrual Cycle**: The menstrual cycle typically lasts around 28 days and consists of different phases, including the follicular phase, ovulation, the luteal phase, and menstruation.

2. **Role of Hormones**: The key hormones involved in the menstrual cycle are estrogen and progesterone. Progesterone is particularly important during the luteal phase, which follows ovulation.

3. **Function of Progesterone**: Progesterone is responsible for maintaining the endometrium (the lining of the uterus) after ovulation. It prepares the endometrium for a potential implantation of a fertilized egg.

4. **Decrease in Progesterone**: If fertilization does not occur, the levels of progesterone will decrease after the mid-secretory phase of the cycle. This drop in progesterone is crucial.

5. **Consequences of Low Progesterone**: The decrease in progesterone leads to the breakdown of the endometrial lining, causing it to shed. This shedding is what constitutes menstrual flow.

6. **Conclusion**: Therefore, menstrual flow occurs due to the lack of progesterone, which is essential for maintaining the endometrium.

### Final Answer:
Menstrual flow occurs due to a lack of progesterone.

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