**Core Concept**
Dysfunctional uterine bleeding (DUB) in women of reproductive age is often caused by anovulatory cycles, leading to unopposed estrogen stimulation of the endometrium. This results in an abnormal endometrial histology that is distinct from the normal proliferative and secretory phases seen in ovulatory cycles.

**Why the Correct Answer is Right**
In anovulatory cycles, the absence of progesterone prevents the normal transformation of the endometrium from a proliferative to a secretory phase. As a result, the endometrium continues to proliferate under the influence of estrogen, leading to an overgrowth of the endometrial glands and stroma. This results in an endometrium that is characterized by an irregular and disorganized growth pattern, with an increased number of mitoses and a lack of secretory changes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option refers to hyperplastic endometrium with atypical glandular hyperplasia, which is a precancerous condition characterized by an abnormal proliferation of endometrial glands with cellular atypia. This is not typically seen in anovulatory cycles.

**Option B:** This option refers to secretory endometrium, which is characteristic of the luteal phase of a normal menstrual cycle. This would be seen in a woman who is ovulating regularly.

**Option C:** This option refers to a decidualized endometrium, which is characteristic of early pregnancy. This would be seen in a woman who is pregnant, not anovulatory.

**Clinical Pearl / High-Yield Fact**
It's essential to note that anovulatory cycles are a common cause of DUB in women of reproductive age, and the diagnosis can often be made based on clinical and laboratory findings, including a negative pregnancy test and irregular menstrual cycles.

**Correct Answer:** C.