Which condition is characterized by a decrease in estrogen levels?

Menopause is the condition marked by a significant decline in estrogen levels. As women approach this natural phase, usually occurring in their late 40s to early 50s, their ovaries gradually produce less estrogen and progesterone. This reduction in estrogen is responsible for various symptoms associated with menopause, such as hot flashes, night sweats, mood changes, and vaginal dryness.

In contrast, conditions like Polycystic Ovary Syndrome (PCOS) are typically associated with hormonal imbalances that may increase estrogen levels rather than decrease them. Cushing's syndrome, related to excess cortisol production, and Diabetes Mellitus, characterized by issues with insulin and blood sugar regulation, do not primarily involve changes in estrogen levels. Therefore, menopause stands out as the condition directly associated with a decrease in estrogen, solidifying the correctness of identifying it as the answer.