Which one of the following is the most likely reason of not occurring regular menstruation cycle in females?

Options

A. Fertilisation of the ovum
B. Maintenance of the hypertrophical endometrial lining
C. Maintenance of high concentration of sex- hormones in the blood stream
D. Retention of well-developed corpus luteum

Explanation

**Step-by-Step Solution:**

1. **Understanding Menstruation**: Menstruation is a regular cycle that occurs in females, typically every 28 days, involving the shedding of the uterine lining if fertilization does not occur.

2. **Identifying Irregular Menstruation**: Irregular menstruation refers to cycles that are not consistent in timing or flow. This can be caused by various factors, and the question asks for the most likely reason.

3. **Evaluating the Options**:
- **Option A: Fertilization of ovum**: When fertilization occurs, the body produces hormones that prevent the shedding of the uterine lining, leading to a temporary cessation of menstruation. This can be a sign of early pregnancy.
- **Option B: Maintenance of hypertropical endometrium lining**: This refers to the thickening of the endometrium, which occurs regardless of fertilization. It is not a direct cause of irregular menstruation.
- **Option C: Maintenance of high concentration of sex hormones in the bloodstream**: While high levels of hormones can affect menstruation, they are typically a result of fertilization rather than a cause of irregular cycles.
- **Option D: Retention of well-developed corpus luteum**: This occurs after ovulation and is maintained by hormones if fertilization happens. While it can affect menstruation, it is not the primary reason for irregular cycles.

4. **Conclusion**: The most likely reason for irregular menstruation is **Option A: Fertilization of ovum**. When fertilization occurs, menstruation is suspended, which can lead to irregular cycles.

**Final Answer**: Fertilization of ovum (Option A).

---