Which one of the following is the most likely root cause of why menstruation is not taking place in regularly cycling human females?

Explanation

The correct answer is Fertilisation of the ovum, as this process leads to increased levels of hormones such as human chorionic gonadotropin (hCG), which prevent the normal menstrual cycle from occurring. The other options, while related to hormonal regulation and the reproductive cycle, do not directly address the root cause of missed periods in the context of regular cycles. Maintenance of the hypertrophical endometrial lining is a consequence of hormonal signals, not a direct cause. Maintenance of high concentration of sex hormones might occur due to various factors, including pregnancy, but is also not a direct cause of missed cycles. Retention of a well-developed corpus luteum is typically a sign of pregnancy as well, rather than a standalone cause for all cases of absent menstruation.