**Core Concept**
Dysfunctional uterine bleeding (DUB) refers to abnormal uterine bleeding not related to any obvious structural or hormonal causes. This condition can be caused by irregularities in the menstrual cycle, leading to excessive or irregular bleeding.

**Why the Correct Answer is Right**
Option D, **Irregular shedding of endometrium**, is the correct answer. This occurs due to anovulatory cycles, where the endometrium is not shed in a synchronized manner. As a result, the endometrium undergoes irregular shedding, leading to dysfunctional uterine bleeding. This condition is often associated with hormonal imbalances, particularly an excess of estrogen without corresponding progesterone levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Uterine polyp is a structural cause of abnormal uterine bleeding, not a cause of DUB. Polyps can cause irregular bleeding, but this is a distinct entity from DUB.

**Option B:** Fibroid is another structural cause of abnormal uterine bleeding. Fibroids can cause heavy or prolonged bleeding due to their impact on the uterine lining, but this is not considered DUB.

**Option C:** Granulosa cell tumor is a type of ovarian tumor that can cause hormonal imbalances, leading to abnormal uterine bleeding. However, this is a specific cause of bleeding, not a characteristic of DUB.

**Clinical Pearl / High-Yield Fact**
It's essential to distinguish between structural and non-structural causes of abnormal uterine bleeding. Identifying the underlying cause is crucial for appropriate management and treatment.

**Correct Answer: D. Irregular shedding of endometrium**